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These kinases are involved in many cellular functions, including regulating cell proliferation and cell death, as well as controlling gene transcription and translation, cell shape and regulation of cell-cell contact.
PKC enzymes were initially thought to be activated by proteolysis. A few years later, researchers discovered that PKC was activated by tumor promoting phorbol esters, which opened the door to the development of new drugs to target PKC in the treatment of cancer. This approach, however, has been complicated by the identification of 15 PKC isozymes, which have been divided into three separate groups: conventional, novel and atypical. Conventional isoforms include α, βI, βII, and γ; novel isoforms refer to δ, ε, η, and θ, and finally atypical isoforms include protein kinase Mζ and ι/λ. Despite the disparity between the isoforms, it is accepted that PKC can be used to refer to the whole family (Khalil, 2010).
Like other protein kinases, PKC contains a catalytic domain as well as a regulatory region. The catalytic domain is located in the C-terminal region, which consists of a binding site for the phospho-acceptor sequence in substrate proteins and a conserved ATP/Mg2+-binding site. The regulatory region is N-terminal and consists of C1 and C2 domains. When inactive, this region is binds to the catalytic domain and prevents its activity. Dissociation of this region from the catalytic domain is critical for the activation of the kinase.
Conventional PKCs (PKCα, βI, βII and γ) require both calcium and diacylglycerol (DAG) for their activation. Hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by phospholipase C (PLC) generates both DAG and inositol 1,4,5 -triphosphate (IP3). IP3 diffuse through the cell to bind to IP3-sensitive Ca2+ channels on the endoplasmic reticulum (ER), releasing Ca2+ ions into the cytosol. PKC binds to these Ca2+ ions and this triggers its translocation to the cell membrane where it interacts with DAG, via its C1 domain. The conformational change in the structure of PKC allows it to phosphorylate its substrates.
In contrast, novel isozymes (PKC δ, ε, θ and η) can be activated by DAG alone. As the affinity of their C1 domains for DAG is much greater than in conventional PKCs allowing them to be directly recruited to the membrane. Finally, atypical PKCs (protein kinase Mζ and ι/λ isoforms) do not require DAG nor Ca2+ for their activation, however they are dependent on different lipid metabolite second messengers (Khalil, 2010; Wu-Zhang and Newton, 2013; Mochly-Rosen et al., 2012).
As PKC play important roles in vasculogenesis and cell proliferation, it is not surprising that PKC levels are greatly perturbed in many cancer cell lines. Usually, PKCs are not considered oncogenes activated by mutations, but promote tumor growth by enhancing cellular signaling pathways (Mochly-Rosen et al., 2012). For example, studies have shown that PKCδ can potentiate angiogenesis models of prostate cancer by binding to NADPH oxidase and whereby hypoxia-inducible factor 1α ( HIF 1α) protein levels are increased (Kim et al., 2011). Targeting PKC for therapy in cancer will be archived by understanding the mechanisms of its regulation. Moreover, the development of small molecules with specificity for individual isozymes will aid the translation of this research from bench-side to bed-side.