PKC potentiates insulin release in beta cells however it

PKC potentiates insulin release in beta cells; however, it is currently not clear how this is mechanistically accomplished in living A939572 [15]. First, we discuss PKC structure, regulation, and activation in beta cells. Then, we address two fundamental questions: (1) what is the specific effect of PKC on insulin release, and (2) how does PKC achieve these effects? In the context of these questions, we discuss the current models for PKC action on glucose-stimulated insulin release and further examine three exocytotic proteins that are likely targets of PKC and control vesicle fusion at the plasma membrane.
PKC structure and activation PKC isozymes are ubiquitous serine-threonine kinases with importance to human health and physiology, as evidenced by their role in many diseases including heart disease, cancer, and diabetes [16]. PKC isozymes are highly regulated; the conventional isozymes require the regulatory lipid diacylglycerol (DAG), negatively charged phospholipids phosphatidylserine and phosphatidylinositide-4,5-bisphosphate (PIP2), and calcium [17]. PKC activity was initially identified in pancreatic islets by Tanigawa et al., and this activity arises from expression of many PKC isoforms in beta cells, including α, βII, ε, λ, and ζ and possibly βI, θ, and η [18], [19]. In beta cells, PKC is activated during glucose stimulated insulin secretion (GSIS) when DAG is produced during calcium influx [20]. DAG production occurs when glucose-induced calcium influx first activates phospholipase C (PLC), which hydrolyzes some of the PIP2 in the membrane to yield DAG and inositol triphosphate (IP3) [21]. In addition to calcium, many PLC isoforms are activated via G-protein coupled receptor pathways, some of which contribute to the regulation of insulin secretion and secretion amplification [22], [23]. Glucose alone can induce both DAG production in islets and subsequent PKC translocation to the plasma membrane, a hallmark of enzyme activation [20], [24], [25], [26]. These findings mechanistically link PKC activation to the physiological stimulus that activates beta cells. There are eight PKC isozymes divided into three classes based on the molecules that activate them [17]. These are (1) conventional PKCs, activated by calcium and DAG; (2) novel PKCs, activated by DAG alone; and (3) the atypical PKCs, activated by neither calcium nor DAG [17]. At the structural level, PKCs have two common domains: the kinase domain, composed of an ATP dependent active site and region for binding the substrate consensus sequence, and the regulatory domain, composed of conserved C1 (binds DAG molecules) and C2 (binds PIP2 via calcium-dependent mechanism) and pseudo-substrate domains [17] (Fig. 2A). In its inactive state, the regulatory domain auto-inactivates the kinase domain by positioning the pseudo-substrate sequence in the active site of the kinase domain (Fig. 2C-1). Recent structural work on PKC-β suggests a second mechanism of allosteric regulation whereby the C1 domains clamp a critical helix in the kinase domain, which must be released for kinase activation [27], [28] (Fig. 2B). In the presence of calcium, the C2 domain binds PIP2 and PS headgroups, tethering the enzyme to the plasma membrane (Fig. 2C-2). After membrane association, the C1 domains can interact with DAG. DAG binding then activates the kinase by inducing a conformational change which removes the pseudo-substrate from the active site and releases the C1 clamp. (Fig. 2C-3) For conventional PKC, tripartite regulatory interactions provide spatiotemporal control over enzyme activity in the cell by restricting PKC activation to only organelles and regions that simultaneously contain all three of these regulatory modules.
What is the effect of PKC on insulin release? For close to 40 years PKC has been known to potentiate insulin release from beta cells [29], [30]. The foundational work on PKC’s effects on exocytosis relied on the application of phorbol esters (phorbol 12-myristate 13-acetate (PMA or also abbreviated TPA)). Phorbol esters act as analogs to the lipid DAG that, together with calcium, activate conventional PKCs [31], [32], [33]. Consequently, enhanced insulin release upon PMA treatment was taken as evidence that PKC stimulates insulin secretion [34], [35], [36], [37]. The use of PKC inhibitors supported this conclusion [38], [39], [40], [41]. Additionally, in several cell types, the application of exogenous PKC to permeabilized cells enhanced exocytosis, providing further evidence for a direct effect of the enzyme on secretion [42], [43]. PKC activation and translocation to the membrane are linked, and thus in addition to correlating enhanced secretion with PMA, the necessary translocation of PKC to the plasma membrane upon PMA treatment has also been observed [24], [25], [38], [44], [45], [46], [47], [48], [49]. These experiments first took the form of enzymatic assays of PKC activity that was associated with the plasma membrane [38]. Further biochemical and imaging studies showed that PKC isozymes α, β, ε, and θ all translocate to the plasma membrane or insulin granules upon activation with several compounds (glucose, PMA, or potassium) [24], [25], [44], [45], [46], [47], [48], [49]. Additionally, there is evidence that atypical PKC isozymes may play a role in secretion [50]. Overall, there is strong consensus that PKC activation enhances insulin release.