Abstract
C
Cellular proliferation is defined as an increase in cell number by cell division, whereas cell migration implies movement of a population of cells from one site to another. Cell proliferation produces two cells from one, and it requires cell growth followed by cell division. Uncontrolled cell proliferation is a hallmark of cancer. Multiple mutations that accumulate in somatic cells over many years eventually remove an elaborate set of controls that would otherwise prevent cancer cells from dividing unchecked. The normal mechanisms that allow nearly all of the billions of cells to proliferate are subverted in cancer cells, and it is impossible to understand cancer without first understanding the controls that keep the vast majority of the cells that form the human body from misbehaving. Cell proliferation is usually limited to cells that replenish tissue in normal, unstressed organs and tissue. In addition, because most tissue contains stem cells that fulfil this function as a result of their self-renewing and asymmetrical division characteristics, thereby producing new stem cells and progenitor cells that further divide, causing terminal differentiation and resulting in specialized cells and function, division will then terminate. Most tissues are made up of such non-dividing cells. Thus proliferation is normally tightly controlled so that only particular cells in the body are dividing. Cell number is dependent not only on cell proliferation, but also on cell death. Programmed cell death, or apoptosis, is the process by which excess or damaged cells in the body are removed. It is the balance between the production of new cells and cell death that maintains the appropriate number of cells in a tissue. Apoptosis is a key mechanism by which cancer-prone cells are eliminated. Both normal apoptotic processes and normal cell mechanisms that control proliferation usually need to be altered to produce enough abnormal cell proliferation to cause cancer. 1
Fundamental to the proliferation process is cell division, which occurs in defined stages that together comprise the cell cycle. It is broken up into four stages: G1, S, G2, and M. DNA replication occurs during the S (synthesis) phase. DNA packaging, chromosome segregation, and cell division (cytokinesis) occur in M (mitosis). S phase and M phase are separated by Gap phases. G1 is the gap between M and S. Cell growth is one of the important events of G1. The transition from G1 to S is the critical control point in the cell cycle. G2 is the gap between S and M, and provides time for proofreading to ensure that DNA is properly replicated and packaged prior to cell division. G0 or quiescence occurs when cells exit the cell cycle because of the absence of growth-promoting signals or presence of pro-differentiation signals. Ordered progression through each phase is intricately regulated through both positive and negative regulatory signaling molecules. The cell cycle is controlled as follows. The mechanisms of regulation consist of two parts: regulation of the different phases to occur in the correct order and regulation of extracellular signals that activate or inhibit the cell cycle. The core activators of the cell cycle control system are the cyclin dependent kinases (Cdk) that turn specific proteins on and off at appropriate times in the cell cycle by phosphorylating them. Cdk inhibitors include the Rb protein, and Cdk inhibitors p16, p21, and p27. These act by binding directly to Cdk-cyclin complexes and blocking their protein kinase activity. Cellular proliferation is controlled by mitogens, antimitogens and cell signaling. Cell signaling is the transmission of information from the extracellular environment into the cell so that it can respond appropriately. Signaling pathways are built from a limited set of molecules and molecular mechanisms that allow for communication within and between cells with a number of common properties. In particular, they allow signaling proteins to undergo switch-like activation from an inactive to an active state, and they can also be readily reversed. 1
The biophysiological functions of the four wound healing phases overlap and are integrated. Hemostasis, inflammation, proliferation, and tissue remodeling or resolution must proceed in the correct order and are time and duration specific while occurring at the optimal intensity. 2 Impaired wound healing implies interference with this highly orchestrated sequence of events caused by specific factors that contribute to improper tissue repair. These factors interrupt progress of wound healing through the normal stages, causing pathologic inflammation and uncontrolled, uncoordinated healing. Re-epithelialization refers to the proliferation and migration of epithelial cells within the wound with the aim of covering the matrix. This occurs during the proliferative stage of wound healing. Fibroblasts and endothelial cells are the main role players during this stage in the dermis, because it enhances capillary growth, collagen formation, and synthesis of granulation tissue at the site of injury. The most important components of the extracellular matrix (ECM); namely, collagen, glycosaminoglycans, and proteoglycans, are produced by the fibroblasts. The final remodeling phase can continue for years, depending upon the severity of the injury. Complexity and coordination of the injury and the healing process contribute to the importance of the therapeutic approach and application. 3
LILI, low-level laser therapy (LLLT), or photobiomodulation refer to the application of low-power light to injuries to stimulate healing. Photobiomodulation is currently a popular treatment modality for several medical conditions, which include pain management, skin traumas and subsequent wound healing, soft tissue injuries, arthritis, and many more. Photobiomodulation has a wavelength- and fluence-dependent capability to alter cellular and molecular function in the absence of heating. 4 The general theoretical explanation for the mechanism of action is that light or photonic energy incident to a cell is absorbed by cellular chromophores containing organelles such as the mitochondria. Several cellular functions are affected, including cellular proliferation and migration. 5,6 The initiation of cell proliferation and migration is critical to the repair mechanism in the process of wound healing. Stressed cells respond more favorably to photobiomodulation by recovering to their normal functional capability. In vitro studies have shown an increase in cellular migration, proliferation, viability, collagen production, ATP, cyclic adenosine monophosphate (cAMP), cytochrome c oxidase activity, nitric oxide, growth factors, mitochondrial membrane potential and gene regulation, and a decrease in matrix metalloproteinases (MMPs) apoptosis and pro-inflammatory cytokines, to name but a few. 4,5 Cellular proliferation studies using new, well-researched and established assays including ATP luminescence, MTT assays, trypan blue staining, and a variety of additional measures, have now become a hallmark of analyzing the cellular effects of photobiomodulation, and rightfully so, considering the intricate mechanisms involved in this process. As a first-level monitoring method to determine the cellular effects of photobiomodulation, viability and proliferation analysis seems perfectly sound. However, caution should be exercised in the inference of this outcome. Proliferation and viability studies should never be used as the only method of monitoring photobiomodulation, and must be accompanied by more sensitive and specific investigation that includes the molecular mechanisms associated with cell growth.