Identification and subcellular characterization of protein kinase-C isoforms in insulinoma β-cells and whole islets

Protein kinase-C (PKC) represents a growing family of serine/threonine kinases, which include both Ca²⁺-dependent and Ca²⁺-independent members. To evaluate the expression of PKC isoforms in insulin-secreting β-cells, purified β-cells from a glucose-sensitive rat insulinoma were fractionated into cytosolic, crude membrane, and cytoskeletal/nucleoskeletal fractions. Protein samples from each fraction were resolved with sodium dodecyl sulfate- polyacrylamide gel electrophoresis and transblotted to nylon membranes. The blots were then analyzed with antibodies specific for the α, β, γ, ε, ζ, and δ isoforms. In addition, expression was analyzed in whole isolated rat islets. Expression of all except the γ isoform was detected in the insulinoma-derived β-cells. Expression of the α, β, and ε isoforms was confined predominantly to the cytosolic fractions. The δ isoform could be detected in all three of the subcellular fractions, whereas the ζ isoform was present in approximately equal amounts in both the cytosolic and crude membrane fractions. The δ isoform could be eluted from the cytoskeletal/nucleoskeletal fraction with 1% Triton X-100. All of the isoforms detected in the insulinoma-derived β-cells were also detected in whole isolated islets. It is concluded that rat insulinoma β-cells and whole islets express numerous isoforms of PKC, including both Ca²⁺-dependent and Ca²⁺-independent isoforms, which may be important in the various signal transduction processes of insulin secretion, proinsulin biosynthesis, and insulin gene expression.