Dynamics of glucose-induced localization of PKC isoenzymes in pancreatic β-cells: Diabetes-related changes in the GK rat

Glucose metabolism affects most major signal pathways in pancreatic β-cells. Multiple protein kinases, including protein kinase C (PKC) isoenzymes, are involved in these effects; however, their role is poorly defined. Moreover, the dynamics of kinase isoenzyme activation in reference to the biphasic insulin secretion is unknown. In perfused pancreas of Wistar rats, PKCα staining was strongly associated with insulin staining, jointly accumulating in the vicinity of the plasma membrane during early first-phase insulin response. The signal declined before the onset of second phase and reappeared during second-phase insulin release as foci, only weekly associated with insulin staining; this signal persisted for at least 15 min after glucose stimulation. In the GK rat, glucose had minimal effect on β-cell PKCα. In control β-cells, PKCδ stained as granulated foci with partial association with insulin staining; however, no glucose-dependent translocation was observed. In the GK rat, only minimal staining for PKCδ was observed, increasing exclusively during early first-phase secretion. In Wistar β-cells, PKCε concentrated near the nucleus, strongly associated with insulin staining, with dynamics resembling that of biphasic insulin response, but persisting for 15 min after cessation of stimulation. In GK rats, PKCε staining lacked glucose-dependent changes or association with insulin. PKCζ exhibited bimodal dynamics in control β-cells: during early first phase, accumulation near the cell membrane was observed, dispersing thereafter. This was followed by a gradual accumulation near the nucleus; 15 min after glucose stimulus, clear PKCζ staining was observed within the nucleus. In the GK rat, a similar response was only occasionally observed. In control β-cells, glucose stimulation led to a transient recruitment of PKCθ associated with first-phase insulin release, not seen in GK β-cell. Data from this and related studies support a role for PKCα in glucose-induced insulin granule recruitment for exocytosis; a role for PKCε in activation of insulin granules for exocytosis and/or in the glucose-generated time-dependent potentiation signal for insulin release; and a dual function for PKCζ in initiating insulin release and in a regulatory role in the transcriptional machinery. Furthermore, diminished levels and/or activation of PKCα, PKCε, PKCθ, and PKCζ could be part of the defective signals downstream to glucose metabolism responsible for the deranged insulin secretion in the GK rat.