Pyruvate dehydrogenase complex component B: A Gene Associated with Cuproptosis and Encoding the Beta Subunit of Pyruvate Dehydrogenase Is Involved in the Oxidative Decarboxylation Reaction

Abstract

Cuproptosis is a regulated cell death mechanism that has recently been identified and is distinct from other known cell death mechanisms (e.g., apoptosis, Ferroptosis, necrotic apoptosis, etc.). Cuproptosis causes oligomer formation through the abnormal accumulation of intracellular copper ions that target binding to lipocytosed proteins, especially those involved in the tricarboxylic acid cycle. At the same time, it can destabilize iron-containing sulfur proteins, thereby inducing proteotoxic stress, leading to triggered cell death. It has also been shown that cuproptosis is also associated with oxidative stress activation and inhibition of the ubiquitin-proteasome system. Genes linked to cuproptosis were screened, and knocking out seven genes reversed cuproptosis: reductase—ferredoxin 1; the three genes of the lipoic acid pathway—lipoyltransferase 1, lipoyl synthase, and dihydrolipoamide dehydrogenase; and the acylated protein targets—dihydrolipoyl transacetylase (DLAT), pyruvate dehydrogenase complex component A1 (PDHA1), and pyruvate dehydrogenase complex component B (PDHB). Among them, the β subunit of pyruvate dehydrogenase, encoded by the PDHB gene, can form a tetramer with the α subunit and irreversibly catalyze the physiological function of converting pyruvate to acetyl-CoA since DLAT provides structural support and also exhibits enzymatic activity within the pyruvate dehydrogenase complex (PDC). Furthermore, within the PDC, the primary target of cuproptosis is DLAT rather than PDHB or PDHA1. Consequently, the involvement of PDHB in the inactivation of PDC caused by cuproptosis is more likely a secondary consequence. In this review, the characteristics of the cuproptosis-associated gene PDHB and its role in the biological function and pathogenesis of the disease are discussed.

Introduction

Copper ions are essential trace elements in the human body, and under normal conditions, their levels are maintained at a relatively stable state through the following mechanisms. Previous studies on intracellular copper ion homeostasis have demonstrated that copper ions are transported into the cytoplasm via solute carrier family 31 member 1, a transporter located on the cell surface, by traversing the cell membrane (Zhou and Gitschier, 1997; Yu et al., 2019). Copper ions are facilitated to further penetrate the intermembrane space of mitochondria by the Cytochrome c oxidase copper chaperone (COX17) (Punter et al., 2000). Copper ions located in the intermembrane space of mitochondria entered the mitochondrial matrix through the transmembrane transport protein solute carrier family 25 member 3. Excess copper ions in the cytoplasm can exit the cell with the assistance of ATPase (ATP7A/7B) (La Fontaine et al., 2010; Zhu et al., 2021; Lutsenko et al., 2007). Any impairment in the steps aforementioned can lead to abnormal accumulation of copper ions within the cell, which can affect the interaction of components in the mitochondrial tricarboxylic acid (TCA) cycle, disrupt mitochondrial respiration, and induce cell death. Tsvetkov et al. defined this as a new death mechanism-cuproptosis. In this study, the authors found that the occurrence of cuproptosis is closely related to the presence of cellular mitochondria. Additionally, ten genes associated with cuproptosis were identified through genome-wide Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated protein 9 loss-of-function screening. Out of these, seven genes (ferredoxin 1, lipoyltransferase 1, lipoyl synthase, dihydrolipoamide dehydrogenase, dihydrolipoyl transacetylase [DLAT], pyruvate dehydrogenase complex component B [PDHB], and pyruvate dehydrogenase complex component A1 [PDHA1]) were capable of rescuing cells from cuproptosis after knockout. Among them, DLAT, PDHB, and PDHA1 support a mechanism whereby copper ions target lipoacylated proteins in cuproptosis. Furthermore, studies have confirmed that copper ions can directly bind to DLAT and cause its oligomerization, which increases cellular toxicity and leads to cuproptosis. Like DLAT, PDHB encodes a component of the pyruvate dehydrogenase complex (PDC), but there is no evidence that copper ions bind to it. Therefore, the role of PDHB in cuproptosis may be indirect, acting through DLAT (Tsvetkov et al., 2022).

Since the introduction of the concept of cuproptosis, researches in this area have certainly become a hotspot in academia. A number of public databases have been used to conduct cuproptosis-associated studies, which have revealed a strong link between cuproptosis and cancer, cardiovascular diseases, and neurodegenerative disorders. In addition, PDHB and oxidative metabolism appear frequently in the screening of cuproptosis-related genes and the search for potential regulatory pathways. Therefore, it is worthwhile to find out how PDHB plays a role in cuproptosis and cellular oxidative metabolism and how it is involved in various diseases.

Overview of PDHB

Discovery of PDHB

Lap Ho and Koike et al. isolated complementary DNA (cDNA) clones encoding the β-subunit of pyruvate dehydrogenase from human hepatocytes and human Hela cell γgt11 cDNA library respectively, and named them PDHB (Ho et al., 1988; Koike et al., 1988). Southern analysis of hybrid cell DNA by DNA probes located the PDHB gene on human chromosome 3 and narrowed down the position to the 3pl3-q23 region by excluding the possibilities that it was located distal to the short arm or distal to the long arm of the chromosome (Olson et al., 1990). With the rise of cuproptosis research in recent years, it has been found that there is an association between PDHB gene expression and cuproptosis occurrence (Tsvetkov et al., 2022).

Genetic structure of PDHB

The full length of the PDHB cDNA gene consists of 1,557 nucleotides, including a 41-nucleotide Poly A+ tail and an open reading frame that extends from position 1 to position 1,077. The PDHB-encoded precursor protein consists of 359 amino acid residues (Relative Molecular Mass [Mr] of 39,046), which contains a lead sequence of 30 amino acid residues. The mature protein after excision of the leading sequence contains 329 residues (Mr of 35,911) (Koike et al., 1988; Koike et al., 1989). The polyadenylation signals were found on the isolated PDHB cDNA sequence isolated by Ho, L at both the 3' terminal 1417 and 1463 sites: The former was a variant signal (ATTAAA), while the latter was a consensus signal (AATAAA) (Ho and Patel, 1990). It was confirmed that the pyruvate dehydrogenase-E1 (PDC E1) β subunit is a single gene product of PDHB, so further analysis of the DNA transcriptional structure showed that the PDHB gene is 18 kb in size and consists of 10 exons and 9 introns in its coding region. The exons range in size from 36 bases (exon 5) to 550 bases (exon 10), and the introns range in size from 82 bases (intron 1) to 1,645 bases (intron 2). Exon 1 contains the ATG, the initiation site for transcription and translation, and transcription begins at an adenine residue 132 positions upstream of it. There is also a CAAT box structure in the −82 to −78 position region. Exon 2 contains a sequence encoding the terminal region of the mature PDC E1 β. In addition, the PDHB gene has an Alu repeat in intron 2 and an Alu repeat in intron 8 (Huh et al., 1990; Koike et al., 1992; Koike et al., 1990).

Physiological function of PDHB-related proteins

The PDC is a mitochondrial enzyme that catalyzes the irreversible decarboxylation of pyruvate to acetyl coenzyme A (Ho et al., 1988). The pyruvate dehydrogenase beta subunit encoded by the PDHB gene is one of the components that make up PDC E1. In turn, PDC E1, together with dihydrolipoyl transacetylase (DLAT) and dihydrolipoyl dehydrogenase (PDC E3), makes up the PDC. PDC E1 is a tetramer consisting of two alpha subunits and two beta subunits, which are involved in catalyzing the first step reaction of the TCA cycle (oxidative decarboxylation of pyruvate): its carbon number one (C-1), together with two oxygen atoms linked to it, results in the formation of Carbon dioxide is released; its second carbon, C-2, combines with thiamine pyrophosphate (TPP) in E1 to form hydroxyethyl TPP (Koike et al., 1988). During this phase of the reaction, the β-subunit encoded by the PDHB gene binds to the thiazolium ring of the TPP and provides the key catalytic histidine residue for the active site (Robinson and Chun, 1993).

The researchers cultured the cells with glucose-containing (involved in glycolysis) and galactose-containing (inhibits glycolysis and promotes mitochondrial respiration) cultures, and found that mitochondrial respiration-dependent cells were nearly 1,000-fold more sensitive to Cu-elesclomol (which triggers cuproptosis) than glycolysis-dependent cells. Moreover, treatment with mitochondrial antioxidants, inhibitors of mitochondrial function, inhibitors of the electron transport chain, and inhibitors of pyruvate uptake reduced the sensitivity of cells to Cu-elesclomol, confirming that the occurrence of cuproptosis is associated with mitochondrial respiration (i.e., the TCA cycle). Previous studies have suggested that one of the key mechanisms of cuproptosis is the targeted binding of the lipoylated component of the TCA cycle. Protein lipoylation, a highly conserved post-translational modification of lysine that links lipoate to protein lysine residues via an amide bond, can occur in the DLAT of the TCA cycle (Tsvetkov et al., 2022). PDC E1 β can bind to PDC E1 α to form a tetrameric structure on the one hand; on the other hand, it can also provide amino acid residue sites from Ile-52 to Tyr-112 and Phe-269 to Val-300 to bind to DLAT to form the PDC (Urata et al., 1991; Jeng et al., 1994).

Taken together, the development of cuproptosis is not only dependent on aerobic respiration but also related to the targeting of copper ions to DLAT, and pyruvate dehydrogenase is the hub linking between glycolysis and the TCA cycle, and is complexed with DLAT. Therefore, it is worthwhile to further explore whether there is a direct link between PDHB and its encoded products and the mechanism of cuproptosis, or whether there is an indirect mechanism through DLAT. Therefore, it is worth exploring whether there is a direct link between PDHB and the mechanism of cuproptosis or an indirect mechanism through DLAT.

PDHB variants

Among the reported cases, most of the PDC defects are caused by PDHA1 gene mutation, while the number of PDHB gene mutation cases is rare. In this review, seven clinical cases of PDHB gene mutation were collected, as shown in Table 1, including patient gender, clinical manifestations, imaging manifestations, abnormal biochemical examination, and prognosis of the children up to the time reported in the literature. (Quintana et al., 2009; Okajima et al., 2008; Hiramatsu et al., 2022).

Table 1.

Clinical Cases Associated with PDHB Mutations

	NO	Gender	Age	Clinical feature/biochemical index/MRI	Mutation site	Parental close relatives	Prognostic outcome	Reference
Homozygous point mutation	A	Male	Neonatal period	Dyspnea, decreased muscle tone, convulsive encephalopathy;	c.302T>C (p.M101T)	—	Died at 5 months	(Quintana et al., 2009)
				Metabolic acidosis, high blood/urine lactate, high blood alanine;
				Hypoplasia of the corpus callosum
	B	Male	—	Moderate spastic paralysis, mild dystonia, eyeball disturbance	c.106C>T (p.R36C)	Y	Alive	(Okajima et al., 2008)
				Lactic acid acidosis, normal lactate/pyruvate ratio—
	C	Female	Neonatal period	Respiratory insufficiency, developmental delay, epilepsy	c.395A>G (p.Y132C)	Y	Dead
				Lactic acidosis, normal lactate/pyruvate ratio
				Hypoplasia of the corpus callosum、Lateral ventricular dilatation, basal ganglia ischemia
	D	Female	2 years	Epilepsy, gait disorders, strabismus;—	c.880G>A (p.G294R)	N	Alive	(Hiramatsu et al., 2022)
				Cerebral atrophy without white matter lesions
Heterozygous point mutation	E	Male	16 months	Hypotonia, transient mild paralysis of limbs, ptosis, strabismus;	c.301A>G (p.M101V)c.313G>A (p.R105Q)	—	—	(Quintana et al., 2009)
				Lactic acidosis;
				Abnormally high signal in bilateral pallidum, midbrain, and cerebral peduncles
	F	Female	Neonatal period	Feeding difficulties, dyspnea neurological impairment;	c.916T>C (p.C306R)c.956A>T (p.D319V)	N	Died at 1 month	(Okajima et al., 2008)
				Lactic acidosis, normal lactate/pyruvate ratio, high blood alanine;
				Partial hypoplasia of the corpus callosum
	G	Female	1 year	Moderate developmental delay, epilepsy, mild ataxia, nystagmus, mild hypotonia；	c.426A>G (p.I142M)c.494G>C (p.W165S)	N	Alive
				Lactic acidosis;
				Cerebellar and brainstem atrophy, small lesions in the splenium of the corpus callosum

Although the cases related to PDHB mutations are scarce, in both homozygous and heterozygous mutations, we can observe a clinical manifestation of multisystemic impairment, including respiratory function, locomotion, growth, and development. Most of the symptoms are present immediately after birth. The common biochemical features are acidosis and hyperlactatemia, which may be related to the pyruvate reaction catalyzed by the PDHB-encoded protein. Magnetic Resonance Imaging (MRI) images showed abnormal development and foci of abnormality at multiple sites, including the corpus callosum, lateral ventricles, and pallidum. Therefore, we hypothesize that mutations in the PDHB gene may affect intrauterine energy metabolism, leading to multisystemic dysfunction and developmental abnormalities in the neonatal period.

Mechanism of targeting PDHB to regulate pyruvate dehydrogenase activity

The prevailing view is that pyruvate dehydrogenase activity is regulated by two main mechanisms: the phosphorylation/dephosphorylation cycle and product inhibition regulation. The former regulates enzyme activity through a cycle of phosphorylation/dephosphorylation of the serine site on the PDC E1 alpha subunit by pyruvate dehydrogenase kinase and pyruvate dehydrogenase phosphatase (Yeaman et al., 1978; Linn et al., 1969). The doctrine of product inhibition regulation suggests that pyruvate oxidation products (acetyl coenzyme A and nicotinamide adenine dinucleotide [NADH]) inhibit pyruvate dehydrogenase activity, and that coenzyme A and NAD+ competitively reverse this inhibitory effect (Tsai et al., 1973; Reed, 1981). Phenylbutyric acid can also indirectly regulate pyruvate dehydrogenase (PDH) activity by inhibiting pyruvate dehydrogenase kinase activity (Ferriero et al., 2014). In addition, other researchers have found that the pyruvate dehydrogenase of Escherichia coli is also regulated by nucleotides: the enzyme activity is inhibited by guanosine triphosphate (GTP) and reversed by guanosine diphosphate (GDP). In other words, pyruvate dehydrogenase activity was regulated by the GTP/GDP ratio (Schwartz and Reed, 1970).

Vivo/ex vivo experiments have confirmed that prolyl hydroxylase (PHD3), as an enzyme that induces HIF-α degradation, interacts with the PDC E1β subunit. The activity of PDH decreases in the absence of PHD3 (the decrease is more pronounced in hypoxia). It is hypothesized that PHD3 regulates PDH activity by interacting with PDH-E1β. In addition, cellular experiments confirmed that in PHD3-deficient cells, PDH activity was decreased, mitochondrial respiration was inhibited, cells consumed less oxygen, less ROS were produced, and cells were more able to survive under hypoxic conditions (Kikuchi et al., 2014).

Immunoprecipitation and mass spectrometry analyses show that PDH-E1β in the retina interacts with the renin (pro)receptor ATP6AP2. ATP6AP2 siRNA experiments demonstrated that ATP6AP2 deficiency led to a decrease in PDH activity as well as a decrease in acetyl coenzyme A and an increase in lactate; whereas no significant changes were detected with ATP6AP2 overexpression.

Further studies revealed that ATP6AP2 deficiency resulted in a reduction in PDHB protein expression, but no change in mRNA levels, as well as elevated levels of tyrosine phosphorylation of PDHB. In the presence of a broad-spectrum tyrosine kinase inhibitor, PDHB protein levels were then restored. In summary, in the absence of ATP6AP2, PDHB undergoes tyrosine phosphorylation modification and induces degradation. At the same time, ATP6AP2 acts as a stabilizer of PDHB and protects it from post-translational modifications (Kanda et al., 2015).

In resting Treg cells, Parkinson’s disease protein 7 (DJ-1, encoded by the gene PARK7) can bind to PDHB. The combination of the two not only promotes PDH enzyme activity by inhibiting the level of Ser293 phosphorylation of PDC E1α, but also regulates iTreg cell differentiation (Danileviciute et al., 2022).

These studies indicate that PDC activity is regulated by various substances, partly through PDHB, but there is no strong evidence to link them or associate them with cuproptosis directly.

PDHB and Disease

In this review, the author compiled and summarized the literatures related to PDHB and its associated diseases as shown in Table 2. Among these, evidence suggests that changes in the expression level of PDHB may be directly related to certain diseases. The expression of PDHB in clear cell renal cell carcinoma (ccRCC) tissues from the Cancer Genome Atlas (TCGA) database was significantly decreased, and high expression of PDHB was significantly associated with better survival in ccRCC, indicating its role as a tumor suppressor (Bian et al., 2022). The Gene Expression Omnibus and International Cancer Genome Consortium datasets, along with immunohistochemical detection, further supported this perspective. Additionally, it was discovered that the expression of PDHB correlated with the infiltration of immune checkpoints and immune cells (Wang et al., 2023). Immune cell Proportion (IPS) scores of patients with varying levels of PDHB indicated that IPS scores of Cytotoxic T-Lymphocyte-Associated Protein 4 (CTLA-4) blockade and CTLA4+programmed death-1 combined blockade were significantly higher in patients exhibiting high PDHB expression. Additionally, patients with low PDHB expression may derive greater benefit from these two treatment regimens. PDHB may influence tumor immune escape, immune-mediated carcinogenesis, and cancer prognosis. PDHB expression analysis across different cohorts (TCGA-Kidney Renal Cell Carcinoma cohort and GSE40435 cohort), along with Receiver Operating Characteristic curve and univariate Cox regression analysis, identified PDHB as the key gene for cuproptosis (Wu et al., 2023b). After treating the 786-O cell line with copper chloride, researchers found that the expression level of PDHB increased with the rising concentration of copper chloride, while the growth of cancer cells was inhibited, and the apoptosis rate significantly increased. Therefore, ccRCC cells treated with copper may promote cell death through cuproptosis. A PDHB overexpression model was constructed in the 786-O cell line, resulting in a significant reduction in the migration and invasion abilities of the cells, and most of the cell proliferation was inhibited in the S phase (Wang et al., 2023). Compared to normal nasopharyngeal epithelial cells, PDHB mRNA and protein levels were decreased in multiple nasopharyngeal carcinoma (NPC) cell lines. In these NPC cell lines, overexpression of PDHB inhibited cell growth and migration. Simultaneously, it affected cell apoptosis and the cell cycle by up-regulating cleaved poly ADP-ribose polymerase (a DNA damage repair enzyme and the cleavage substrate of the core member of apoptosis, caspase) while down-regulating the expression of proliferating cell nuclear antigen and Cyclin D1. Additionally, overexpression of PDHB inhibited RAS-induced NPC cell growth by negatively regulating the phosphorylation state of extracellular regulated protein kinases, a downstream target of RAS, thus inhibiting RAS-driven malignant transformation. Conversely, knockdown of PDHB expression promoted the growth and invasion of NPC cells. In vivo studies further validated that the deletion of PDHB expression increased tumor formation. In summary, PDHB inhibits the proliferation and differentiation of NPC cells, and reinstating PDHB expression could serve as a novel approach for the treatment of NPC (Tang et al., 2016). After peripheral nerve injury, PDHB expression in dorsal root ganglion (DRG) neurons varied: it rose significantly at 3 h post-injury, then decreased and stabilized by 1 day, and significantly decreased again by 7 days. Experiments showed PDHB knockdown inhibited axon outgrowth, while overexpression increased it after nerve injury, confirming PDHB’s role in axon regeneration. However, the effect of PDHB overexpression was negated by knocking down MCT2, a major lactate transporter, indicating that lactate may mediate PDHB’s promotion of axonal regeneration through energy metabolism. Additionally, a small amount of PDHB in the nucleus regulates the RAS signaling pathway and arachidonic acid metabolism by promoting RSA-14-44 expression and inhibiting Pla2g4a expression via histone acetylation, influencing axonal regeneration injury (Jiang et al., 2023a). PDHB is a key gene that is differentially expressed in patients with sarcopenia, and the FoxP1/Arih2 axis may be a downstream target of PDHB. The expression levels of the PDHB gene were significantly decreased in skeletal muscle samples from older adults compared to younger adults. After PDHB was knocked down through Si-PDHB transfection, the mRNA levels of myogenic differentiation marker genes declined, suggesting that myogenic differentiation was impaired. Conversely, overexpression of PDHB in the mouse model increased mean fiber cross-sectional area, improved the characteristic manifestations of sarcopenia, and restored the expression levels of myogenic markers (Jiang et al., 2023b; Zhu et al., 2023). Finally, a homozygous PDHB variant c856A>G/p.Tr286ala was identified through whole-exome sequencing in an Indian patient with paroxysmal non-motor dyskinesia, indicating that PDHB mutations might also be the genetic cause of paroxysmal nonkinesigenic dyskinesia (Agarwal et al., 2022).

Table 2.

PDHB Expression Levels and Potential Mechanisms in Diseases

Evidence grade	Disease	PDHB expression	Potential mechanisms and pathways	Reference
Direct evidence★★★	Clear cell renal cell carcinoma	Downregulated	Tumor immune evasion and anti-tumor immunity; Cuproptosis	(Wu et al., 2023b)
Direct evidence★★★	Nasopharyngeal carcinoma	Downregulated	a. Promote cell growth and inhibit cell migration;	(Tang et al., 2016)
			b. Upregulated lysed PARP and downregulated proliferating cell nuclear antigen and Cyclin D1, affecting cell apoptosis and the cell cycle;
			c. Negatively regulate the phosphorylation of ERK (the downstream target of RAS) and inhibit RAS-driven malignant transformation.
	Peripheral nerve injury	Upregulated and then downregulated	a. PDHB enhances axonal growth following nerve injury via lactate metabolism.b. Enhancing RSA-14-44 expression and suppressing Pla2g4a expression by modulating neuronal histone acetylation influence the RAS signaling pathway and arachidonic acid metabolism, respectively, thereby regulating axonal regeneration after an injury.	(Chen et al., 2023a)
	Sarcopenia	Downregulated	With age, the downregulation of PDHB expression can inhibit the FoxP1/Arih2 axis and impair myogenic differentiation, leading to muscle loss symptoms.	(Jiang et al., 2023b)
	Paroxysmal nonkinesigenic dyskinesia		Homozygous PDHB variant c856A>G/p.Tr286ala	(Agarwal et al., 2022)
Indirect evidence★★	Gastric cardia cancer	Downregulated	Affect the aggregation of tumor cells by influencing the metabolism of pyruvate.	(Cai et al., 2010)
	Hepatocellular carcinoma		In instances of glutamine deficiency, the aggregation of tumor cells is adversely affected due to its influence on the metabolism of pyruvate.	(Yang et al., 2022)
	Colorectal cancer（CRC）	Downregulated	a. Elevated levels of miR-146b-5p and miR-103a-3p can reduce the expression of PDHB protein, leading to increased proliferation, invasion, and glycolysis of CRC cells.b. Biological Clock (Time Rhythm)	(Zhu, Wu et al. 2017)(Wang, Ye et al., 2021)(Fuhr, El-Athman et al., 2018)
	Ovarian carcinoma	Disputable	a. miR-203, has-miR-186-5p, and has-miR-590-3p regulate PDHB expression and affect tumor cells;	(Xiaohong, Lichun et al., 2016)
			b. Anisomycin induces cuproptosis in ovarian cancer stem cells by inhibiting YY1 and down-regulating PDHB expression;	(Nie, Chen et al., 2022)
			c. The antiparasitic drug Ivermectin inhibits the proliferation of epithelial ovarian cancer cells by down-regulating PDHB protein levels.	(Li et al., 2018)(Li et al., 2020)
	Gliomas	Downregulated	a. PDHB overexpression can eliminate miR-363-3p’s carcinogenic effect;	(Xu et al., 2018)
			b. ELN1 regulates glucose/glutamate metabolism in glioma cells via PDHB;	(Shatokhina et al., 2022)
			c. Pseurotin A up-regulates PDHB expression and inhibits glioma cell proliferation.	(Anjum, Bi et al., 2018)。
	Melanoma	Downregulated	Upregulated miR-370 may inhibit PDHB expression, exerting oncogenic effects.	(Wei and Ma, 2017)
	Gestational hypertension/preeclampsia		Elevated levels of soluble prorenin receptors may be one of the mechanisms that protect the level and activity of PDHB protein under hypoxia conditions, thereby supporting continued fetal growth under placental hypoxia.	(Suda et al., 2020)
	Mycoplasma pneumoniae infection		PDHB aids Mycoplasma pneumoniae in invading and colonizing the respiratory tract while evading immune responses by inhibiting complement activation, degrading human fibrinogen, and binding to fibronectin.	(Dallo et al., 2002; Thomas et al., 2013; Gründel et al., 2015a; Gründel et al., 2015b; Yu, Wang et al., 2020)
	Parkinson’s Disease	Downregulated	a. Down-regulation of miR-409-3p caused an up-regulation of PDHB expression;	(Ahmed et al., 2009)
			b. The mitochondrial protein OMI (HtrA2) may interact with PDHB.	(Wang and Gao, 2023)
				(Leverenz et al., 2007; Kawamoto et al., 2008; Johnson and Kaplitt, 2009)

Relevant evidence★	Pancreatic ductal adenocarcinoma		Loss of heterozygosity and methylation of DNA	(Chen et al., 2023b)
	Cervical carcinoma		PDHB-AS could enhance Dkk1 by competitively binding to miR-582-5p while recruiting PABPC1 to further inhibit cell proliferation and the endothelial transformation process through the Wnt/β-catenin pathway. However, the expression of PDHB-AS, upstream of PDHB-AS, was down-regulated by the interference of miR-4536-5P in cervical cancer.	(Chi et al., 2023)
	Nonalcoholic fatty liver disease	Upregulated	Pyruvate and nicotinamide adenine dinucleotide (NADH) can target PDHB, and supplementation with pyruvate and NADH may inhibit PDHB and halt lesion progression.	(Wu et al., 2023a)
	Hypertension	Downregulated	Hypoxia may affect de novo lipogenesis by modulating PDHB expression.	(García-Fuentes et al., 2015)
	Pulmonary hypertension	Downregulated	Downregulation of circSMOC1 increases the release of its bound PTBP1, which increases the PKM2/PKM1 ratio and induces enhanced glycolysis; its downregulation also decreases PDHB expression by decreasing adsorption of miR-329-3p.	(Wu et al., 2021)
	Head and Neck Squamous Cell Carcinoma	Downregulated	PDHB inhibits the phosphorylation of ERK, a target downstream of RAS, thereby blocking RAS-driven oncogenic transformation.	(Tang et al., 2016)
	Lung cancer	Disputable	Tumor suppressor miRNAs (let-7 and miR-26 family) play a role in regulating PDHB; the FOXA2 gene, which inhibits EMT and metastasis in cancer cells, may act on PDHB through miR-200a-3p and miR-141-3p.	(Sheng et al., 2018; Giannos et al., 2021)

PDHB, pyruvate dehydrogenase complex component B.

Current research indicates that PDHB’s impact on diseases is primarily indirect. Changes in PDHB expression predominantly influence diseases by affecting cellular energy metabolism, gene regulation, and other related factors. In gastric cardia cancer, the expression of PDHB was significantly decreased, while its overexpression could redirect more pyruvate to the TCA cycle. This shift leads to decreased lactate production, inhibited cell growth, and a reduced ability of cells to form colonies AGAR (Cai et al., 2010). In hepatocellular carcinoma (HCC), glutamine deficiency forces HCC cells to depend more on pyruvate metabolism for energy. PDHB gene knockout significantly inhibits HCC cell proliferation (Yang et al., 2022). The miR-146b-5p up-regulated in colorectal cancer targets PDHB mRNA, decreases PDHB protein expression, and enhances the proliferation, invasion, and glycolysis of colorectal cancer cells. However, PDHB overexpression can significantly reduce cell proliferation, alleviate apoptosis inhibition, and diminish cell invasion glycolysis (Wang et al., 2021). Finally, changes in the circadian clock (time rhythm) can alter the metabolic pattern of tumor tissue by influencing the expression of PDHB (Fuhr et al., 2018). In ovarian cancer, boosted miR-203 expression fosters the growth, migration, and glycolysis of cancer cells, possibly linked to the inhibition of PDHB (Xiaohong et al., 2016). Anisomycin can induce cuproptosis in ovarian cancer stem cells by inhibiting the expression and activity of YY1, a transcription factor that can bind to PHDB, and downregulating the expression of PDHB (Nie et al., 2022). Moreover, lncRNA SNHG3 is linked to the energy metabolism and viability of ovarian cancer tissues. Additionally, SNHG3 regulates the expression of PDHB via has-miR-186-5p has-miR-590-3p (Li et al., 2018). The anti-parasitic drug Ivermectin can inhibit the proliferation of epithelial ovarian cancer (EOC) cells in vitro and reverse the expression of the PDHB protein in EOC cells (Li et al., 2020). In glioma, up-regulated miR-363-3p promotes cell growth, inhibits apoptosis, enhances tumor cell invasion, and negatively regulates PDHB. However, PDHB overexpression abolishes this cancer-promoting effect of miR-363-3p (Xu et al., 2018). In glucose deficiency, ERN1 knockout increases PDHB expression. Glutamine deficiency reduces PDHB expression in glioma cells, and ERN1 knockout partially reverses this effect, enhancing PDHB tolerance to glutamine deficiency. Thus, ERN1 may regulate glucose/glutamate metabolism in glioma cells (Shatokhina et al., 2022). Pseurotin A (from Lepas anatifera) inhibits the proliferation of four glioma cell lines and up-regulates the expression of PDHB (Anjum et al., 2018). In melanoma, the significantly upregulated miR-370 promoted cell proliferation, invasion, and glycolysis, and it was significantly negatively correlated with the tumor’s TNM stage. miR-370 could inhibit the expression of PDHB, and the overexpression of PDHB could eliminate the tumor-promoting effect of miR-370 (Wei and Ma, 2017). Under hypoxic conditions, siRNA knockout of soluble prorenin receptor s(P)RR in trophoblasts significantly decreased the expression and activity of PDHB while increasing the phosphorylation level of PDHB, which may be related to gestational hypertension/pre-eclampsia (Suda et al., 2020). PDHB can inhibit the activation of complement by binding to factor H, helping Mycoplasma pneumoniae evades the attack of complement (Yu et al., 2020). At the same time, the binding of the PDHB subunit of Mycoplasma pneumoniae to plasminogen can activate plasminogen, and the PDHB-plasminogen complex can induce the degradation of human fibrinogen. PDH-E1β protein subunits on the surface of Mycoplasma pneumoniae can also bind to the extracellular matrix component fibronectin, suggesting that PDHB may be associated with Mycoplasma pneumoniae invasion, colonization of the respiratory tract, and evasion of the immune response attack (Thomas et al., 2013; Gründel et al., 2015a; Gründel et al., 2015b; Dallo et al., 2002). The expression of PDHB is down-regulated in the blood of PD patients (Ahmed et al., 2009). In the PD mouse model, receiving repetitive transcranial electrical stimulation, downregulation of miR-409-3p expression leads to upregulation of PDHB expression (Wang and Gao, 2023). Additionally, the mitochondrial protein high-temperature requirement protein A2 (OMI/HtrA2) might engage with PDHB binding, potentially influencing the metabolism of PD neurons (Kawamoto et al., 2008; Leverenz et al., 2007; Johnson and Kaplitt, 2009).

The final section of the study can only indicate that PDHB levels vary in certain diseases but lacks additional evidence to clarify the specific mechanism or the significance of these changes. PDHB expression is notably altered in endometrial cancer (down-regulated), rheumatoid arthritis (down-regulated), osteoarthritis (up-regulated), osteonecrosis of the femoral head (up-regulated), dilated cardiomyopathy (up-regulated), lung adenocarcinoma (up-regulated), type 2 diabetes mellitus (down-regulated), and various other diseases. Of these, PDHB shows a correlation with immune cells in osteoarthritis (Chen, 2022; Wu et al., 2021; Chang et al., 2023; Qi et al., 2023; Colak et al., 2016; Liu et al., 2022; Bosi et al., 2022). PDHB gene alterations in pancreatic ductal adenocarcinoma are more frequently associated with loss of heterozygosity CNV, and the level of methylation changes (Chen et al., 2023b). The downregulation of PDHB antisense lncRNA-Pyruvate Dehydrogenase E1 Subunit Beta Antisense RNA (PDHB-AS) in cervical cancer promotes a malignant phenotype (cell proliferation, invasion, and metastasis) in cells (Chi et al., 2023). PDHB plays different roles in breast cancer: higher PDHB expression correlates with longer recurrence-free survival (RFS) in estrogen-receptor-positive early breast cancer (Fan et al., 2023). Nevertheless, elevated levels of PDHB in LumA and LumB tumor cells correlated with poorer recurrence-free survival (Carlini et al., 2018). PDHB is a differentially expressed gene linked to non-small cell lung cancer, with low expression associated with reduced survival rates in patients (Sheng et al., 2018; Giannos et al., 2021). Furthermore, TCGA transcriptional data indicated a reduction in PDHB expression in samples of head and neck squamous cell carcinoma, and there was a positive correlation between PDHB expression levels and patient prognosis (Jiang et al., 2023c). PDHB is significantly upregulated in nonalcoholic fatty liver disease, which may promote liver steatosis and trigger liver inflammation through Cuprotosis. Targeting PDHB with pyruvate and NADH may offer a method to block this process (Wu et al., 2023a). PDHB expression is lower in obese subjects than in nonobese subjects, and there is a significant correlation between PDHB and HIF-α mRNA expression, which may be related to morbid obesity (García-Fuentes et al., 2015). The expression of PDHB in children with active pulmonary tuberculosis (ATB) is lower than in children with latent pulmonary tuberculosis (LTBI) (Chen et al., 2023a). The next-generation sequencing results of 219 patients diagnosed with Leigh syndrome suggested that the PDHB mutation was one of the pathogenic gene variants associated with the disease phenotype (Kistol et al., 2023). In the rat model of pulmonary hypertension, the downregulated circSMOC1 could bind to PDHB mRNA through miR-329-3P, interfering with its translation and leading to the impairment of mitochondrial oxidative phosphorylation (Lu et al., 2022).

Discussion

The PDHB-encoded protein is crucial for cellular energy metabolism, especially in the TCA cycle. Pyruvate dehydrogenase initiates pyruvate’s entry into the TCA cycle, facilitating energy production and mitochondrial oxidative phosphorylation to generate ATP for cellular needs. PDHB is also a newly identified gene associated with cuproptosis, warranting further investigation into its role in cellular cuproptosis and its links to various diseases.

Based on available studies, evidence of changes in PDHB gene expression or protein levels has been found in various disease states. First, we collected case reports of primary PDC deficiency, where PDHB deficiency is one of the causes. The disease caused by PDHB gene variation is often characterized by growth retardation, abnormal nervous system manifestations, and metabolic changes, and most children do not survive. However, we are also concerned that this is roughly the same disease state as that caused by PDHA mutations, which is associated with more reported cases. It seems that mutations in the PDHA gene are the most important and common cause. Additionally, when summarizing the relationship between other diseases and PDHB, we categorized the available studies into three different levels of evidence: (1) Direct evidence: The same disease contains different types of related research, including database bioinformatics analysis, in vitro and in vivo experiments, and clinical studies, among others. The research conclusions support that changes in PDHB expression (overexpression/knockdown expression) will affect the occurrence, progression, and response to treatment of the disease and establish a link between PDHB and the disease. (2) Indirect evidence: It also suggests a relationship between PDHB and diseases, but most are reflected in the indirect effects of energy metabolism or gene regulatory axes, revealing some possible pathogenic mechanisms. However, due to the limited number of studies and the singular research type, the evidence supporting this conclusion is weak. (3) Relevant evidence: It indicates that the conclusions of this part of the research only suggest that the expression of PDHB changes in the disease state but cannot clarify its pathogenic mechanism. Most of these findings are based on biological data research, resulting in insufficient credibility.

In conclusion, we believe that PDHB is indeed associated with various diseases, and it is useful to explain the pathogenesis, predict development, and assess the prognosis of these diseases. In research related to PDHB, particularly regarding tumor diseases, PDHB is involved in numerous gene regulatory axes, which may be significant for the future development of precision medicine. However, whether it concerns primary PDC deficiency or other disease types, such as tumors, cardiovascular diseases, motor system diseases, and nervous system diseases, the role of PDHB remains controversial. Our perspective is that, based on current evidence, PDHB primarily functions as a bridging factor. In other words, its pathogenic mechanism tends to be an indirect effect. Many studies have failed to ascertain whether the onset of disease is linked to changes in PDHB expression. Furthermore, in some studies, alterations in PDHB expression levels exhibit inconsistent polarization. Importantly, there is limited evidence connecting PDHB, disease, and cuproptosis.

Conclusion

Cuproptosis is a novel concept introduced in 2022. Following ferroptosis, it represents a new mechanism that links transition metal ions with diseases, indicating a distinct form of cell death. PDHB encodes a protein that contributes to TCA cycle enzymes but is not directly affected by cuproptosis. Our research implies that PDHB’s role in disease is also indirect. Consequently, future investigations should aim to determine whether there is a more direct connection between PDHB and diseases, as well as further examine how PDHB is involved in the onset and progression of diseases via cuproptosis.

Footnotes

Authors’ Contributions

R.L. and D.X.: Conceptualization. R.L., F.T., X.Z., J.F., and D.X.: Software, validation, investigation, resources, and writing—review and editing. R.L. and J.F.: Writing—original draft preparation. D.X.: Visualization and supervision. R.L. and D.X.: Funding acquisition. All the authors have read and approved the final article.

Funding Information

This study was supported by the National Science Foundation of China (grant nos. 82001593).

Disclosure Statement

The authors declare no conflict of interest.

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