Oleamide,a Sleep-Inducing Compound: Effects on Ion Channels and Cancer

Abstract

Primary fatty acid amides (PFAMs) are interesting lipid-derived signaling molecules with wide-ranging actions, including bioelectric. In this perspective, we evaluate a possible triangular relationship: PFAMs—ion channels—cancer. The primary emphasis is on oleamide and its metabolic associate, oleic acid (OA). As ion channels, we have focused on voltage-gated sodium channels (VGSCs) and voltage-gated and inward rectifier potassium channels in a variety of cell types, including cancer cells. As cancers, we evaluate the evidence for breast and colorectal cancers. Cis-oleamide blocked VGSCs and potassium channels. In parallel, but not always concurrently, oleamide has been shown to produce antiproliferative and anti-invasive effects, consistent with the known control of these cellular behaviors by potassium channels and VGSCs, respectively. Importantly, there is also evidence that oleamide can potentiate chemotherapy. In contrast to oleamide, the effects of OA on the ion channels of interest and, correspondingly, cancer cell behaviors are much less consistent. The perspective is concluded with three short sections. First, we outline the actions of some other PFAMs, especially anandamide and linoleamide. Second, we give an overview of some other targets that have been associated with oleamide, especially cannabinoid receptors and gap junctions. Finally, we cover the anticancer effects of oleamide on some other neoplasms.

Introduction

Primary fatty acid amides (PFAMs) are lipid-derived signaling molecules, which include oleamide, anandamide, palmitamide, stearamide, erucamide, and linoleamide.¹ In general, they are generated from hydrolysis of fatty acids in the presence of ammonia.² Most work has been done on oleamide, especially as regards (1) ion channels as targets and (2) cancer as an associated disease state. Oleamide, cis-9-octadecenamide [CH₃(CH₂)₇CH = CH₂(CH₂)₇CONH₂], comprises an 18-carbon acyl chain with a single cis-paired link at C₉. Following the discovery by isolation from the cerebrospinal fluid of sleep-deprived cats (next paragraph), experiments showed that oleamide is present also in rat and human plasma, indicating that its biosynthesis could occur in the body.

Indeed, using rat brain microsomes, Sugiura et al. synthesized oleamide from oleic acid (OA) and ammonia as a substrate in vitro by the reversible catalytic action of fatty acid amide hydrolase (as illustrated in Fig. 1).³ OA is an omega-9 fatty acid and can be made by the body. It is also found in foods (highest levels in olive oil and other edible oils). Oleamide may also be derived from N-oleoylglycine by the action of the neuropeptide processing enzyme, peptidylglycine α-amidating monooxygenase. In addition, there are other routes for the production of oleamide.¹ All these are probably dependent on cell type and its biochemical status.

FIG. 1.

Schematic illustration of synthesis of oleamide from oleic acid and NH₃ substrates by the reversible catalytic action of FAAH. FAAH, fatty acid amide hydrolase; PAM, peptidylglycine α-amidating monooxygenase.

Oleamide was first isolated from the cerebrospinal fluid of sleep-deprived cats.^4,5 Subsequent work showed that it is involved also in a wide range of physiologies, including locomotion, angiogenesis, inflammation, thermoregulation, and vasoregulation.⁶ A range of targets and signaling mechanisms has been associated with the action of oleamide, including receptors (e.g., cannabinoid-like receptor-1) and enzymes (e.g., cyclooxygenase, metalloproteinase).⁷

There is significant evidence for involvement of oleamide and OA in the cancer process. For example, it was reported that oleamide analogs (including linoleamide) can cause apoptotic, antiproliferative, antiangiogenic, and antimetastatic effects on human bladder, urinary, and breast cancer (BCa) cells in vitro and in vivo.^8–10 Low doses (50 and 100 μM) of oleamide were found to exert antiproliferative effects and caused apoptosis in rat glioblastoma RG2 cells.¹¹

The effects of OA on cancer appeared somewhat more variable. On the whole, OA produced antitumor (antiproliferative and proapoptotic) effects.^12,13 Treatment of human lung cancer A549 cells with different doses of OA caused cytotoxicity by the release of pulmonary surfactant protein B into the tumor microenvironment.¹⁴ On the other hand, OA-induced NADPH oxidase family 4, a mitochondrial energy biosensor, and promoted the metastatic ability of human colorectal cancer (CRCa) cells by increasing the level of reactive oxygen species (ROS).¹⁵

The apparent physiological (especially neurophysiological) and pathophysiological roles of oleamide and OA raise the possibility that their molecular targets may involve ion channels. The latter are known generally to contribute to a wide range of cellular activities, from the most basic (e.g., gene expression) to whole cell (e.g., secretion, motility). Furthermore, ion channel dysfunction can give rise to range of pathologies including cancer.¹⁶

The main aim of this perspective is to evaluate the effects of oleamide (and to a lesser extent other PFAMs) on key Na⁺ and K⁺ channels and associated signaling mechanisms. Indeed, Na⁺ and K⁺ channels frequently work in concert. Furthermore, the concerted activities of these two channels have been suggested to drive the “celex” mechanism of metastasis.¹⁷ In essence, we endeavor to inquire whether there might be a basis for a possible triangular relationship, involving PFMAs (oleamide), ion (Na⁺ and K⁺) channels, and cancer, as illustrated in Figure 2.

FIG. 2.

A conceptual representation of the potential triangular relationship involving PFAMs, ion channels, and cancer. In this study, we cultivate this relationship for oleamide, Na⁺ and K⁺ channels, and breast and colorectal cancers. PFAMs, primary fatty acid amides.

Sodium Channels

There are two main types of sodium channels: voltage-gated sodium channels (VGSCs) and epithelial sodium channels. In addition, some “transient receptor potential” channels also permeate Na⁺. As regards oleamide, most work has been done on VGSCs.

VGSCs are composed of one α-subunit (VGSCα) and 1–2 β-subunit(s) (VGSCβ). VGSCαs (220–260 kD) form the Na⁺ pore region and also include the various posttranslational modification sites.¹⁸ The β-subunits (30–40 kDa) are auxiliary and contribute to the channel's intracellular trafficking and stability in plasma membrane. VGSCαs comprise a multigene family with nine functional subtypes (Nav1.1–1.9) with varying tissue expression patterns.¹⁹ Functional VGSCs are expressed in classically “excitable” cells, such as nerves and muscles. However, increasing evidence suggests that VGSC expression also occurs in so-called “nonexcitable” cells, for example, fibroblasts, glia, and cells of the immune system.^19,20 Importantly, cancer, including carcinoma, cells also express functional VGSCs, which play a significant role in driving, possibly even initiating, metastasis.^21–23 The basic functional mode of VGSCs is generation of all-or-none action potentials (APs) although nonconducting roles also occur.²⁴ VGSC activity can be blocked selectively by the natural toxin, tetrodotoxin (TTX). Indeed, TTX-sensitive APs have been recorded in human strongly metastatic BCa cells in vitro and in vivo.^25,26

Behavioral effects of oleamide were initially studied in mice.²⁷ Thus, intraperitoneally applied oleamide was found to significantly suppress a range of motor functions, including rearing, grooming, and locomotion. These effects were dose dependent with some gender dependence. Interestingly, oleamide also demonstrated an antidepressant-like property. These results were confirmed, and it was also shown that oleamide potentiated the inhibitory effect of diazepam and antagonized the stimulatory effects of ethanol, methamphetamine, and caffeine.²⁸ From such diverse neurophysiological/pharmacological effects, it was suggested that oleamide could interact with a multiplicity of neurotransmitter receptors and ion channels in the central nervous system (CNS).²⁷ The molecular basis of such effects of oleamide was studied on cultured pyramidal neurons.²⁹ Cis-(but not trans-) oleamide reversibly suppressed the sustained repetitive firing but had no effect on the amplitude or time course of APs (Fig. 3a).

FIG. 3.

Effects of oleamide and oleic acid on VGSCs in different cell types. (a) Pyramidal neurons of rat. (b) Suppression of SRF by cis-oleamide (64 μM), consistent with state-dependent block of VGSCs. SRF was elicited by intracellular injection of current (3 pA). (b) Dose–response relationship for cis-oleamide-induced SRF. Each data point denotes mean ± SEM (n = 4–10). (c) Acute inhibition of inward VGSC (neonatal Nav1.5) current in MDA-MB-231 human breast cancer cells by 33.3 μM oleamide. Currents were elicited under voltage clamp conditions by ramping the membrane potential from −100 to 0 mV. The effect was largely reversible. Scale bars, 40 mV and 200 msec (a) and 800 pA and 7 msec (c). (a, b), modified from Verdon et al.²⁹ SEM, standard error of the mean; SRF, sustained repetitive firing; VGSCs, voltage-gated sodium channels.

This effect was dose dependent with an average EC50 value of 4.12 μM (Fig. 3b). The mode of action was consistent with a frequency- and/or state-dependent block of VGSCs. Such a modulatory effect seemed similar to the actions of anticonvulsant barbiturates and a variety of anesthetics, which also serve as blockers of VGSCs.²⁹ Further experiments were undertaken with radiolabeled batrachotoxin A 20-alpha-benzoate (BTX-B), a VGSC ligand.³⁰ Thus, Nicholson et al. confirmed that oleamide blocked the binding of BTX-B to VGSCs in mouse CNS synaptosomes and cultured NIE115 murine neuroblastoma cells.³¹ More recently, leaf extracts of several species of Ipomoea and Dillenia, containing high levels of oleamide, similarly inhibited several subtypes of recombinant VGSCs.³² In human, strongly metastatic BCa MDA-MB-231 cells, VGSC (neonatal Nav1.5) activity was also inhibited strongly by oleamide (Fig. 3c).

In contrast, from the available evidence, OA appeared to produce inconsistent effects. On VGSCs expressed in cultured human bronchial smooth muscle cells, OA was inhibitory.³³ On the other hand, intracellularly applied OA potentiated VGSC (presumed Nav1.4) currents recorded from adult human skeletal muscle biopsies.³⁴

In conclusion, oleamide (synthetic or natural) is consistent in inhibiting VGSC activity in a variety of cellular preparations, but further work is required to understand the action of OA.

Potassium Channels

There are three main types of K⁺ channel: (i) voltage-gated K⁺ (Kv) channels; (ii) inward-rectifier K⁺ (Kir) channels; and (iii) two-pore K⁺ channels.³⁵ These subserve a range of cellular functions dependent on the physiology of the tissue of origin. Studies on Caenorhabditis elegans suggested the expression of over 100 potassium channel genes. More than 80 genes of potassium channels were found in human genome.³⁶ As regards oleamide, most work has been done on Kv and Kir channels.

Kv channels

In human genome, there are some 40 different genes encoding for Kv channel subunits and these are subdivided into 12 subfamilies (Kv1–Kv12).³⁷ The molecular structure of given Kv channel subunits comprises six transmembrane (TM) helices (S1–S6) and one pore-forming P-region. Functional Kvs occur as tetramers mainly of given subfamily members. The different channel subtypes vary in their voltage sensitivity and kinetic profiles.³⁸ Their cellular functions are varied, including generation of the membrane potential (V_m) and patterned signal transmission. Kvs contribute to signal generation in “excitable” cells but can also occur extensively in epithelial cells.³⁹

Effects of oleamide (as the main hexane extract from leaves of Ipomoea and Dillenia species) were studied on three different Kv channels expressed in Xenopus oocytes (Fig. 4a). Thus, activities of Kv1.1 and Kv10.1 channel were shown to be inhibited while the Kv11.1 (hERG) channel was not affected. These effects were linked to an anti-inflammatory property of oleamide.³²

FIG. 4.

Effects of oleamide and oleic acid on depolarization-induced potassium currents. (a) Inhibition of hexane extract of Ipomoea and Dillenia leaves (mainly oleamide) on Kv1.1 and Kv10.1 channels expressed in Xenopus oocytes. Outward currents were elicited by ramping the membrane voltage from −50 to 65 mV. Modified from Ameamsri et al.³² (b) Time-dependent inhibitory effect of oleic acid on outward K⁺ currents in NG 108-15 neuroblastoma cells. The effect is time dependent (downward arrow), currents decreasing steadily after control level (top). The subsequent current recordings are shown after 0.5, 2, and 5 min. Modified from Rouzaire-Dubois et al.⁴⁰ Scale bars: 0.2 μA, 250 msec (Kv1.1); 500 μA, 250 msec (Kv10.1); 1 nA, 50 msec (b).

OA also inhibited Kv (“delayed rectifier”) currents measured in NG 108-15 neuroblastoma cells (Fig. 4b). Two effects of OA were apparent: (i) time-dependent suppression of peak current and (ii) acceleration of the current inactivation.⁴⁰

Some work has also been done on calcium-activated K⁺ (K_Ca) channels. These can be considered as a member of the Kv family due to their molecular similarity, although K_Ca channels are opened primarily by a rise in intracellular Ca²⁺ and are only weakly voltage sensitive. Effects of some unsaturated fatty acids, including OA, on K_Ca channels were studied on gastric myocytes of guinea pigs. OA significantly increased the K_Ca current dose dependently by up to 13%.⁴¹ This could be due to OA mobilizing intracellular Ca²⁺.⁴²

Kir channels

These channels also occur as tetramers, each subunit being formed from two TM helices and one P-region.⁴³ As well as contributing to membrane electrogenesis in some cells, their classic role is in tissue K⁺ homeostasis as in neuro–glia interactions in the CNS⁴⁴ and vasodilatory effects in brain.⁴⁵

Liu and Wu showed originally that oleamide inhibited an E-4031-sensitive presumed Kir current in pituitary GH₃ cells and in neuroblastoma IMR-32 cells in a dose-dependent manner.⁴⁶

A variant of Kir is the ATP-gated K⁺ (K_ATP) channel. This channel opens (and V_m hyperpolarizes) when the cellular level of ATP falls. Kir (in particular Kir 6.1/2) is an integral part of the K_ATP channel complex. OA was shown to inhibit the K_ATP channels in pro-opiomelanocortin (POMC) neurons and cause excitation in hypothalamus.⁴⁷ Such an effect could be associated with metabolic aspects of food intake and insulin signaling.⁴⁸ Interestingly, in another study, OA was found to suppress the expression of Kir6.1 protein in human umbilical artery smooth muscle cells.⁴⁹ This raises the possibility of long-term effects of oleamide/OA with possible feedback interactions inherent in the metabolism (Fig. 1).

Effects on Cancer Cells

In this section, we review some highlights of the effects of oleamide and OA on cancer cells and attempt to link these to possible involvement of ion channels, especially sodium and potassium channels, covered in Sodium Channels and Potassium Channels sections, respectively. While most experiments have been carried out using these agents exogenously, we should note that cancer cells themselves can also synthesize oleamide.⁵⁰ Furthermore, oleamide may be released from cells in “extracellular vesicles” and thus mediate cell–cell interactions within the tumor microenvironment.⁵¹ Overall, our intention is not to give an exhaustive account of all cancers. Instead, we highlight the best understood cases, as follows.

Breast cancer

There is significant experimental evidence showing that oleamide can suppress various aspects of BCa cell behavior, including proliferation and invasion. Indeed, using the MDA-MB-231 cell model of triple-negative human BCa, Zibara et al. showed that oleamide inhibited proliferation (cell cycle arrest) as well as invasiveness in vitro.⁵² Similarly, oleamide inhibited the proliferation of human BCa EFM-19 cells.⁵⁰ Furthermore, Wisitpongpun et al. showed that oleamide-rich leaf extracts of Moringa oleifera showed anticancer activities by inducing cell cycle arrest and triggering apoptosis in MDA-MB-231 cells.⁵³ The underlying mechanism was suggested to be suppression of Bcl-2 expression and subsequent activation of caspase 3.⁵³ Importantly, oleamide also reduced metastasis in a murine xenograft model in vivo.⁵² These effects were generally dose and time dependent.

In conclusion, oleamide could suppress both primary and secondary tumorigenesis. These effects of oleamide may be explained by its blocking actions on VGSCs and potassium channels, which are well known, respectively, to control such behaviors in cancer cells. While these observations would support, at least initially, the proposed triangular relationship “oleamide—ion (Na⁺ and K⁺) channels—cancer,” much more work remains to be done to verify it more directly.

Importantly, oleamide has also been shown to enhance the anticancer effects of chemotherapy. Thus, Jiang et al. demonstrated that oleamide caused enhanced antiproliferative effects on MCF-7 BCa cells pretreated with doxorubicin (Adriamycin). This effect was suggested to be mediated by uncoupling of gap junctions (GJs).⁵⁴ Indeed, oleamide is also a strong inhibitor of GJs (see section on Additional Targets of Oleamide) and the role of cytochrome c in biosynthesis of oleamide could contribute to initiating the apoptotic pathway from mitochondria. Thus, GJ communication could be blocked by mitochondrial metabolic elements activated by oleamide.¹ Nevertheless, there may be an ionic component since GJs are well known to be controlled by Ca²⁺ and pH. A nanotechnological study also showed that oleamide could increase the antiviability effects of cytotoxic drugs on MCF-7 cells.⁵⁵ Further details of this approach are given in Colorectal Cancer section.

In contrast to oleamide, OA and most of its other derivatives appear to have mixed effects and could promote BCa cell behavior.⁵⁶ Thus, treatment of MCF-7 cells with OA increased their viability and increased proliferative activity.⁵⁷ It is not clear if these effects involve Kv and/or Kir channels, but other fatty acids are known to promote K⁺ channel activity.⁵⁸ In another study, on MDA-MB-231 cells, oleate, a derivative of OA, caused an increase in the level of cytosolic Ca²⁺ and induced phosphorylation of protein kinase B.⁵⁹ Calcium signaling and protein kinase modulators contribute to the proliferation capacity of BCa cells. Furthermore, in media containing OA, adhesiveness of MDA-MB-435 BCa cells increased while in the presence of calphostin C, an inhibitor of Ca²⁺-dependent protein kinase C (PKC), cellular adhesiveness decreased. It was concluded that the effects of OAs on cellular invasiveness and adhesion were PKC dependent.⁶⁰

In conclusion, oleamide consistently suppressed while OA could (also) enhance BCa cell behaviors. Further work is required, however, to substantiate these findings and determine the degrees to which ionic mechanisms may contribute to such effects.

Colorectal cancer

Effects of oleamide on CRCa have been studied mainly through plant extracts. In one study, oleamide-rich fruit of Ziziphus jujuba was shown to delay development and progression of colon cancer along with suppression of tumor inflammation in a mouse model.⁶¹ The effect of oleamide on CRCa has also been investigated in relation to ulcerative colitis (UC), which is known to have a positive association with CRCa.⁶² Acetic acid-induced UC in a rat model indeed was associated with decreased levels of oleamide.⁶³ Treatment with a nanoformulation of a butanol extract of Acacia saligna restored metabolite levels in the inflammatory cascades to normal.⁶³ In an in vivo rat model, conjugated linOA inhibited the growth of CRCa (Caco-2) cells and this was suggested to occur, at least in part, through increased production of oleamide.⁶⁴

Some work has also been done with OA and has produced mixed effects. A high level of OA was observed in plasma membranes of cells from CRCa patients and this could be related to diet.⁶⁵ OA increased proliferation and mitogenic activity of CRCa Caco-2 cells.⁶⁶ In contrast, another study suggested that OA could inhibit the proliferation of CRCa cells by increasing both release of Ca²⁺ from intracellular stores and intracellular ROS production or caspase 3 activity.⁶⁷ An antiapoptotic effect has also been described.⁶⁸ An earlier study had also shown that OA could inhibit development of colon cancer in a rat carcinogenic (1,2-dimethylhydrazine) model.⁶⁹

OA has also been used in combination with cancer drugs. In an interesting nanotechnological approach, OA nanoparticles (NPs) were coated with hyaluronic acid (HA), which could then be coupled to two drugs, AZD6244 (a mitogen activated protein kinase [MAPK] inhibitor) and cisplatin (a DNA damaging drug). Release of the drugs from the NPs was much greater in an acidic environment (pH = 5.5), compatible with the low pH of the tumor microenvironment. Importantly, application of OA–HA NPs to human colon cancer (HTC-116) cells with and without the drugs suppressed cell viability. However, compared with the drug cocktail alone, the potency of the combination involving OA was more than 10-fold greater.⁵⁵

From the available evidence, similar to BCa, we conclude that oleamide produced a range of inhibitory effects on CRCa cells. On the other hand, the effects of OA, again, were mixed.

Broader Aspects

In this section, we outline a number of issues related to oleamide, ion channels, and cancer, for further awareness.

Other PFAMs

As noted in the Introduction, there are several other PFAMs. Of these, the “cannabinoid” anandamide is the best known. Independent of cannabinoid receptor-1 (CB1) and cannabinoid receptor-2 (CB2) receptors, anandamide acts on a variety of targets, including transient receptor potential vanilloid channel subfamily 1, peroxisome proliferator-activated receptor gamma, K⁺ channels, VGSCs, and Ca²⁺ channels.⁷⁰ Cellular effects included inhibitions of adhesion and migration of BCa cells⁷¹ and proliferation of CRCa cells.⁷² Anandamide can also associate with oleamide functionally as in sleep.⁷³ In addition, the apoptotic effect of oleamide on neural crest cells was enhanced by anandamide.⁷⁴ Linoleamide, a structural analog of oleamide, was shown to produce apoptotic effects through activation of Bax and DNA fragmentation in MDA-MB-231 BCa cells.¹⁰

In addition, linoleamide, together with oleamide, showed potent inhibition of sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) in primate kidney (COS-1) cells.⁵ Finally, as already noted, natural sources have contributed significantly to the understanding of PFAM effects. Thus, palmitamide, erucamide, linoleamide, and oleamide were identified in extracts of Ericaria amentacea and Ericaria crinita plants and shown to have strong antioxidant properties, including synergistically “entourage effect”.⁷⁵

Additional targets of oleamide

In addition to sodium and potassium channels, oleamide has been shown to act on a variety of other targets including, importantly, G-protein coupled (CB1 and CB1) receptors for cis-oleamide.⁷⁶ Downstream signaling pathways included adenylyl cyclase, focal adhesion kinase, MAPK, and nitric oxide synthase.⁷⁷ Biological responses to oleamide also could involve GJs, serotonergic receptors, and gamma aminobutyric acid type A receptors.⁷⁸ Interestingly, as mentioned in Potassium Channels section, in contrast to oleamide, OA stimulated POMC neurons in the hypothalamus and decreased food intake through K_ATP channel inhibition. Such opposite effects of OA and oleamide may be explained, at least in part, by impact on ion channels and association with contrasting activities of cis- and trans-oleamide. Another well-known target of oleamide is SERCA and downstream Ca²⁺ signaling.⁵ In RG2 glioblastoma cells, oleamide produced antiproliferative/apoptotic effects, which were suppressed by Ca²⁺ chelators.¹¹ In glial cells, on the other hand, the effect of oleamide was independent of Ca²⁺ and appeared, instead, to involve change in membrane fluidity.⁷⁹

GJs have also received considerable attention as potential targets of oleamide, several studies showing inhibitory effects. GJs can contribute to the cancer process in a wide range of ways, including controlling cellular proliferation, epithelial–mesenchymal transition and apoptosis, including in relation to chemotherapy.^53,80 Finally, nitric oxide could be an intermediary in the action of oleamide, as in vasorelaxation.⁸¹ In conclusion, oleamide, and probably other PFAMs, are associated with a wide range of targets and intracellular signaling mechanisms. It is highly likely that these are connected by common factors like V_m and Ca²⁺.

Effects on other cancers

Earlier (Effects on Cancer Cells section), we described in some detail the effects of oleamide on breast and CRCa. Various effects of oleamide have also been described on other cancers. For example, in pheochromocytoma and T24 bladder cancer cells, oleamide (and linoleamide, a structural analog) increased the level of intracellular Ca²⁺ by release of Ca²⁺ from internal stores as well as influx from outside.⁸ However, the possible impact of these events on the cancer process was not studied. Two oleamide derivatives (MI-18 and MI-22) suppressed viability of metastatic Bl6 melanoma cells by blocking their GJ coupling.⁹ Interestingly, oleamide also enhanced significantly the sensitivity of p53-mutant lung cancer cells to irradiation and increased antiproliferative and apoptotic effects through caspase activation.⁸² Thus, oleamide and probably other PFAMs can affect a range of cancers and cell behaviors associated with the cancer process, but more work is required to broaden this knowledge.

Conclusions and Future Perspectives

Our overall conclusion is that there is indeed a possible broad basis for a functional triangular relationship involving PFAMs, ion (Na⁺ and K⁺) channels, and cancer cell behavior (Fig. 2). As regards PFAMs, we covered mainly oleamide for which most evidence relates to its acute effects. These depended on dose and exposure time as well as the nature of the cell under investigation. Thus, VGSC and potassium channels were shown to be significant targets, and these could be linked to the anticancer effects of oleamide.

Importantly, and not surprisingly, long-term effects are also possible. Indeed, application of oleamide to mouse neurons caused changes in levels of transcription factors, resulting in induction of c-fos messenger RNA.⁸³ Expression of c-fos in neurons is known to regulate synaptic activity, including during sleep.^84,85 In addition, ion channel (including VGSC) activity itself can lead to gene expression changes, including autoregulatory effects.^86,87 As well as determining the possible long-term effects of oleamide, more work is needed to elucidate its effects in vivo.

As noted in the Introduction, oleamide can be found in diet as a derivative of OA and also occurs in high amounts in various plants. Its anticancer effects can occur at least partially through modulation of ion channels, in common with several other natural agents.^88,89 More broadly, dietary and nutraceutical compounds are important components of integrated management of difficult-to-treat cancers, for example, pancreatic cancer.^90–93 Accordingly, understanding their mechanisms of action, including in relation to possible interaction with existing clinical treatment modalities, could ultimately be of significant benefit to human health.

Footnotes

Author Disclosure Statement

The authors declare no conflicts of interest.

Funding Information

No funding was received for this article.

References

Mueller

, Driscoll

. Biosynthesis of oleamide. Vitam Horm, 2009; 81:55–78.

Farrell

, Merkler

. Biosynthesis, degradation and pharmacological importance of the fatty acid amides. Drug Discov Today. 2008; 13:558–568.

Sugiura

, Kondo

, Kodaka

et al. Enzymatic synthesis of oleamide (cis-9, 10-octadecenoamide), an endogenous sleep-inducing lipid, by rat brain microsomes. IUBMB Life, 1996; 40:931–938.

Arafat

, Trimble

, Andersen

RN et al

. Identification of fatty acid amides in human plasma. Life Sci, 1989; 45:1679–1687.

Yamamoto

, Takehara

, Ushimaru

. Inhibitory action of linoleamide and oleamide toward sarco/endoplasmic reticulum Ca2+-ATPase. Biochim Biophys Acta Gen Subj, 2017; 1861:3399–3405.

Moon

S-M

, Lee

, Hong

, et al. Oleamide suppresses inflammatory responses in LPS-induced RAW264.7 murine macrophages and alleviates paw edema in a carrageenan-induced inflammatory rat model. Int Immunopharmacol, 2018; 56:179–185.

Hernández-Díaz

, Juárez-Oropeza

, Mascher

, et al. Effects of oleamide on the vasomotor responses in the rat. Cannabis Cannabinoid Res, 2020; 5:42–50.

Y-K

, Tang

K-Y

, Chang

W-N

, et al. Effect of oleamide on Ca2+ signaling in human bladder cancer cells. Biochem Pharmacol, 2001; 62:1363–1369.

Nojima

, Ohba

, Kita

. Oleamide derivatives are prototypical anti-metastasis drugs that act by inhibiting Connexin 26. Curr Drug Safety, 2007; 2:204–211.

10.

Yerlikaya

, Baloglu

, Diuzheva

, et al. Investigation of chemical profile, biological properties of Lotus corniculatus L. extracts and their apoptotic-autophagic effects on breast cancer cells. J Pharm Biomed Analysis, 2019; 174:286–299.

11.

Torres-Román

, García-Hernández

, Rangel-López

, et al. Oleamide induces cell death in glioblastoma RG2 cells by a cannabinoid receptor–independent mechanism. Neurotox Res, 2020; 38:941–956.

12.

Carrillo Pérez

, Cavia Camarero

MdM

, Alonso de la Torre

. Antitumor effect of oleic acid; mechanisms of action: A review. Nutr Hosp, 2012; 27:1860–1865.

13.

Giulitti

, Petrungaro

, Mandatori

, et al. Anti-tumor effect of oleic acid in hepatocellular carcinoma cell lines via autophagy reduction. Front Cell Dev Biol, 2021; 9:629182.

14.

Zhou

, Chen

, Zhang

, et al. Effects of oleic acid on SP-B expression and release in A549 cells. Eur Rev Med Pharmacol Sci, 2015; 19:3438–3443.

15.

Shen

C-J

, Chang

K-Y

, Lin

B-W

, et al. Oleic acid-induced NOX4 is dependent on ANGPTL4 expression to promote human colorectal cancer metastasis. Theranostics, 2020; 10:7083.

16.

Prevarskaya

, Skryma

, Shuba

. Ion channels in cancer: Are cancer hallmarks oncochannelopathies?. Physiol Rev, 2018; 98:559–621.

17.

Djamgoz

MBA

. Biophysics of cancer: Cellular excitability (“CELEX”) hypothesis of metastasis. J Clin Exp Oncol, 2014; S1:005.

18.

Catterall

. Voltage-gated sodium channels at 60: Structure, function and pathophysiology. J Physiol, 2012; 590:2577–2589.

19.

Diss

, Fraser

, Djamgoz

. Voltage-gated Na+ channels: Multiplicity of expression, plasticity, functional implications and pathophysiological aspects. Eur Biophys J, 2004; 33:180–193.

20.

Djamgoz

, Firmenich

. Novel immunotherapeutic approaches to cancer: Voltage-gated sodium channel expression in immune cells and tumors. In: Amiji MM and Milane LS, eds. Cancer Immunology and Immunotherapy. London: Elsevier, 2022: 83–109.

21.

Bennett

, Smith

, Harper

. Voltage-gated Na+ channels confer invasive properties on human prostate cancer cells. Pflügers Archiv, 2004; 447:908–914.

22.

Djamgoz

, Fraser

, Brackenbury

. In vivo evidence for voltage-gated sodium channel expression in carcinomas and potentiation of metastasis. Cancers, 2019; 11:1675.

23.

Yildirim

, Altun

, Gumushan

, et al. Voltage-gated sodium channel activity promotes prostate cancer metastasis in vivo. Cancer Lett, 2012; 323:58–61.

24.

Djamgoz

MBA

. Ion transporting proteins and cancer: Progress and perspectives. Rev Physiol Biochem Pharmacol, 2022; 183:251–277.

25.

McCallum

, Shiralkar

, Suciu

, et al. Chronic neural activity recorded within breast tumors. Sci Rep, 2020; 10:1–13.

26.

Ribeiro

, Elghajiji

, Fraser

, et al. Human breast cancer cells demonstrate electrical excitability. Front Neurosci, 2020; 14:404.

27.

Akanmu

, Adeosun

, Ilesanmi

. Neuropharmacological effects of oleamide in male and female mice. Behav Brain Res, 2007; 182:88–94.

28.

Yang

J-Y

, Wu

C-F

, Song

H-R

. Studies on the sedative and hypnotic effects of oleamide in mice. Arzneimittelforschung, 1999; 49:663–667.

29.

Verdon

, Zheng

, Nicholson

, et al. Stereoselective modulatory actions of oleamide on GABAA receptors and voltage-gated Na+ channels in vitro: A putative endogenous ligand for depressant drug sites in CNS. Br J Pharmacol, 2000; 129:283–290.

30.

Catterall

, Morrow

, Daly

, et al. Binding of batrachotoxinin A 20-alpha-benzoate to a receptor site associated with sodium channels in synaptic nerve ending particles. J Biol Chem, 1981; 256:8922–8927.

31.

Nicholson

, Zheng

, Robin Ganellin

, et al. Anesthetic-like interaction of the sleep-inducing lipid oleamide with voltage-gated sodium channels in mammalian brain. J Am Soc Anesthesiol, 2001; 94:120–128.

32.

Ameamsri

, Chaveerach

, Sudmoon

, et al. Oleamide in Ipomoea and Dillenia species and inflammatory activity investigated through ion channel inhibition. Curr Pharm Biotechnol, 2021; 22:254–261.

33.

, Iida

, Kishida

, et al. Acute and chronic effects of eicosapentaenoic acid on voltage-gated sodium channel expressed in cultured human bronchial smooth muscle cells. Biochem Biophys Res Commun, 2005; 331:1452–1459.

34.

Wieland

, Gong

, Fletcher

, et al. Altered sodium current response to intracellular fatty acids in halothane-hypersensitive skeletal muscle. Am J Physiol Cell Physiol, 1996; 271:C347–C353.

35.

Biggin

, Roosild

, Choe

. Potassium channel structure: Domain by domain. Curr Opin Struct Biol, 2000; 10:456–461.

36.

Lawson

, McKay

. Modulation of potassium channels as a therapeutic approach. Curr Pharm Design, 2006; 12:459–470.

37.

Gutman

, Chandy

, Grissmer

, et al. International Union of Pharmacology. LIII. Nomenclature and molecular relationships of voltage-gated potassium channels. Pharmacol Rev, 2005; 57:473–508.

38.

Villa

, Combi

. Potassium channels and human epileptic phenotypes: An updated overview. Front Cell Neurosci, 2016; 10:81.

39.

Nieves-Cintrón

, Syed

, Nystoriak

, et al. Regulation of voltage-gated potassium channels in vascular smooth muscle during hypertension and metabolic disorders. Microcirculation, 2018; 25:e12423.

40.

Rouzaire-Dubois

, Gérard

, Dubois

J-M

. Modification of K+ channel properties induced by fatty acids in neuroblastoma cells. Pflügers Archiv, 1991; 419:467–471.

41.

Zheng

H-F

, Li

X-L

, Jin

Z-Y

, et al. Effects of unsaturated fatty acids on calcium-activated potassium current in gastric myocytes of guinea pigs. World J Gastroenterol WJG, 2005; 11:672.

42.

Hidalgo

, Nahuelpan

, Manosalva

, et al. Oleic acid induces intracellular calcium mobilization, MAPK phosphorylation, superoxide production and granule release in bovine neutrophils. Biochem Biophys Res Commun, 2011; 409:280–286.

43.

Zhang

, Zheng

, Xie

, et al. Potassium channels and their emerging role in parkinson's disease. Brain Res Bull, 2020; 160:1–7.

44.

Kofuji

, Newman

. Potassium buffering in the central nervous system. Neuroscience, 2004; 129:1043–1054.

45.

González

, Baez-Nieto

, Valencia

, et al. K+ channels: Function-structural overview. Compr Physiol, 2012; 2:2087–2149.

46.

Liu

Y-C

, Wu

S-N

. Block of erg current by linoleoylamide, a sleep-inducing agent, in pituitary GH3 cells. Eur J Pharmacol, 2003; 458:37–47.

47.

Y-H

, Su

, Gutierrez-Juarez

, et al. Oleic acid directly regulates POMC neuron excitability in the hypothalamus. J Neurophysiol, 2009; 101:2305–2316.

48.

Toda

, Santoro

, Kim

, et al. POMC neurons: From birth to death. Ann Rev Physiol, 2017; 79:209–236.

49.

Bai

X-J

, Tian

H-Y

, Wang

T-Z

, et al. Oleic acid inhibits the KATP channel subunit Kir6. 1 and the KATP current in human umbilical artery smooth muscle cells. Am J Med Sci, 2013; 346:204–210.

50.

Bisogno

, Katayama

, Melck

, et al. Biosynthesis and degradation of bioactive fatty acid amides in human breast cancer and rat pheochromocytoma cells: Implications for cell proliferation and differentiation. Eur J Biochem, 1998; 254:634–642.

51.

Dominiak

, Chełstowska

, Olejarz

, et al. Communication in the cancer microenvironment as a target for therapeutic interventions. Cancers, 2020; 12:1232.

52.

Zibara

, Awada

, Dib

, et al. Anti-angiogenesis therapy and gap junction inhibition reduce MDA-MB-231 breast cancer cell invasion and metastasis in vitro and in vivo. Sci Rep, 2015; 5:1–16.

53.

Wisitpongpun

, Suphrom

, Potup

, et al. In vitro bioassay-guided identification of anticancer properties from Moringa oleifera Lam. leaf against the MDA-MB-231 cell line. Pharmaceuticals, 2020; 13:464.

54.

Jiang

, Liu

, Zhao

, et al. Effect of gap junction modulation on antitumor effects of adriamycin in estrogen receptor-positive breast cancer cells. Nan Fang Yi Ke Da Xue Xue Bao, 2018; 38:780–786.

55.

Palvai

, Kuman

, Basu

. Hyaluronic acid cloaked oleic acid nanoparticles inhibit MAPK signaling with sub-cellular DNA damage in colon cancer cells. J Mater Chem B, 2017; 5:3658–3666.

56.

Donovan

, Selmin

, Stillwater

, et al. Do olive and fish oils of the mediterranean diet have a role in triple negative breast cancer prevention and therapy? An exploration of evidence in cells and animal models. Front Nutr, 2020; 7:197.

57.

Repossi Marquez

, Pasqualini

, Das

, et al. Polyunsaturated fatty acids differentially modulate cell proliferation and endocannabinoid system in two human cancer lines. Arch Med Res, 2017; 48:46–54.

58.

Elinder

, Liin

. Actions and mechanisms of polyunsaturated fatty acids on voltage-gated ion channels. Front Physiol, 2017; 8:43.

59.

Hardy

. St-Onge GG, Joly E, et al. Oleate promotes the proliferation of breast cancer cells via the G protein-coupled receptor GPR40J Biol Chem, 2005; 280:13285–13291.

60.

Palmantier

, George

, Akiyama

, et al. cis-Polyunsaturated fatty acids stimulate β1 integrin-mediated adhesion of human breast carcinoma cells to type IV collagen by activating protein kinases C-ɛ and-μ. Cancer Res, 2001; 61:2445–2452.

61.

Periasamy

, Liu

C-T

, Wu

W-H

, et al. Dietary Ziziphus jujuba fruit influence on aberrant crypt formation and blood cells in colitis-associated colorectal cancer mice. Asian Pac J Cancer Prev, 2015; 16:7561–7566.

62.

Lakatos

, Lakatos

. Risk for colorectal cancer in ulcerative colitis: Changes, causes and management strategies. World J Gastroenterol WJG, 2008; 14:3937.

63.

Abdallah

, Ammar

, Abdelhameed

, et al. Protective mechanism of Acacia saligna butanol extract and its nano-formulations against ulcerative colitis in rats as revealed via biochemical and metabolomic assays. Biology, 2020; 9:195.

64.

Kim

, Jun

J-G

, Park

, et al. Conjugated linoleic acid (CLA) inhibits growth of Caco-2 colon cancer cells: Possible mediation by oleamide. Anticancer Res, 2002; 22:2193–2197.

65.

Szachowicz-Petelska

, Sulkowski

, Figaszewski

. Altered membrane free unsaturated fatty acid composition in human colorectal cancer tissue. Mol Cell Biochem, 2007; 294:237–242.

66.

Storniolo

, Martínez-Hovelman

, Martínez-Huélamo

, et al. Extra virgin olive oil minor compounds modulate mitogenic action of oleic acid on colon cancer cell line. J Agric Food Chem, 2019; 67:11420–11427.

67.

Carrillo

, del Mar Cavia

, Alonso-Torre

. Oleic acid inhibits store-operated calcium entry in human colorectal adenocarcinoma cells. Eur J Nutr, 2012; 51:677–684.

68.

Liang

Y-S

, Qi

W-T

, Guo

, et al. Genistein and daidzein induce apoptosis of colon cancer cells by inhibiting the accumulation of lipid droplets. Food Nutr Res, 2018; 62. doi: 10.29219/fnr.v62.1384.

69.

Nelson

, Samelson

. Inability of the mutagen-blocking agent oleic acid to protect against colon carcinogenesis in the rat. Mutat Res Lett, 1984; 140:155–157.

70.

Muller

, Paula

, Reggio

. Cannabinoid ligands targeting TRP channels. Front Mol Neurosci, 2019; 11:487.

71.

Grimaldi

, Pisanti

, Laezza

, et al. Anandamide inhibits adhesion and migration of breast cancer cells. Exp Cell Res, 2006; 312:363–373.

72.

Liao

Y-S

, Wu

, Wang

, et al. Anandamide inhibits the growth of colorectal cancer cells through CB1 and lipid rafts. Zhonghua Zhong Liu Za Zhi [Chinese J Oncol], 2011; 33:256–259.

73.

Méndez-Díaz

, Ruiz-Contreras

, Cortés-Morelos

, et al. Cannabinoids and sleep/wake control. Adv Exp Med Biol. 2021; 1297:83–95.

74.

Bannerman

, Nichols

, Puhalla

, et al. Early migratory rat neural crest cells express functional gap junctions: Evidence that neural crest cell survival requires gap junction function. J Neurosci Res, 2000; 61:605–615.

75.

Radman

, Čižmek

, Babić

, et al. Bioprospecting of less-polar fractions of Ericaria crinita and Ericaria amentacea: Developmental toxicity and antioxidant activity. Marine Drugs, 2022; 20:57.

76.

Pertwee

. Endocannabinoids and their pharmacological actions. Handb Exp Pharmacol, 2015; 231:1–37.

77.

Howlett

, Mukhopadhyay

. Cellular signal transduction by anandamide and 2-arachidonoylglycerol. Chem Phys Lipids, 2000; 108:53–70.

78.

Hiley

, Hoi

. Oleamide: A fatty acid amide signaling molecule in the cardiovascular system?. Cardiovasc Drug Rev, 2007; 25:46–60.

79.

Boger

, Patterson

, Guan

, et al. Chemical requirements for inhibition of gap junction communication by the biologically active lipid oleamide. Proc Natl Acad Sci USA, 1998; 95:4810–4815.

80.

Jindal

, Chockalingam

, Ghosh

, et al. Connexin and gap junctions: Perspectives from biology to nanotechnology based therapeutics. Transl Res, 2021; 235:144–167.

81.

Tseng

H-L

, Hoi

. Vasorelaxation of two isoflavonoids, calycosin and formononetin, in rat isolated mesenteric artery and their potential relationship to cannabinoid CB1 receptor. Int J Mol Med, 2011; 28:S74–S74.

82.

Lee

Y-J

, Lee

S-J

, Jhon

, et al. Enhanced radiosensitization of p53 mutant cells by oleamide. Int J Radiat Oncol Biol Phys, 2006; 64:1466–1474.

83.

Thomas

, Cravatt

, Sutcliffe

. The endogenous lipid oleamide activates serotonin 5-HT7 neurons in mouse thalamus and hypothalamus. J Neurochem, 1999; 72:2370–2378.

84.

Lugaresi

. The thalamus and insomnia. Neurology, 1992; 42:28–33.

85.

Steriade

, McCormick

, Sejnowski

. Thalamocortical oscillations in the sleeping and aroused brain. Science, 1993; 262:679–685.

86.

Brackenbury

, Djamgoz

. Activity-dependent regulation of voltage-gated Na+ channel expression in Mat-LyLu rat prostate cancer cell line. J Physiol, 2006; 573:343–356.

87.

Fraser

, Ozerlat-Gunduz

, Brackenbury

, et al. Regulation of voltage-gated sodium channel expression in cancer: Hormones, growth factors and auto-regulation. Philos Trans R Soc Lond B Biol Sci, 2014; 369:20130105.

88.

Djamgoz

, Isbilen

. Dietary compounds as anti-cancer agents: A preliminary evaluation of ion channels and membrane excitability as possible target mechanisms. Turkish Biochem, 2006; 31:57–68.

89.

Lopez-Charcas

, Pukkanasut

, Velu

, et al. Pharmacological and nutritional targeting of voltage-gated sodium channels in the treatment of cancers. Iscience, 2021; 24:102270.

90.

Aktas

, Akgun

. Naringenin inhibits prostate cancer metastasis by blocking voltage-gated sodium channels. Biomed Pharmacother, 2018; 106:770–775.

91.

Aktas

, Ayan

. Oleuropein: A potential inhibitor for prostate cancer cell motility by blocking voltage-gated sodium channels. Nutr Cancer, 2021; 73:1758–1767.

92.

Fraser

, Hemsley

, Djamgoz

. Caffeic acid phenethyl ester: Inhibition of metastatic cell behaviours via voltage-gated sodium channel in human breast cancer in vitro. Int J Biochem Cell Biol, 2016; 71:111–118.

93.

Jentzsch

, Davis

, Djamgoz

. Pancreatic Cancer (PDAC): Introduction of evidence-based complementary measures into integrative clinical management. Cancers, 2020; 12:3096.