The roles of specific isoforms of protein kinase C in growth control and human colon cancer

Abstract

Protein kinase C (PKC) consists of a family of lipid-regulated enzymes which play a pivotal role in signal transduction. Studies with the cell line R6-PKC3, a derivative of R6 rat fibroblasts that overexpresses PKC beta 1, provide direct evidence that this isoform of PKC influences cell morphology, growth control, the production of an autocrine growth factor, and the action of an activated H-ras oncogene. Analysis of 32P-labelled phosphoproteins indicates that R6-PKC cells display increased phosphorylation of a 80/87 kDa protein (designated MARCKS), and after treatment with TPA they display a dramatic prolongation in the phosphorylation and in the cytosolic accumulation of this protein. These alterations in MARCKS may be responsible, at least in part, for the altered growth properties of R6-PKC3 cells. We have also examined the expression of endogenous isoforms of PKC in R6 cells and oncogene-transformed derivatives. Normal R6 cells express four isoforms of PKC, cPKC alpha, nPKC epsilon, nPKC delta, and nPKC zeta; nPKC delta and nPKC epsilon are the most abundant. nPKC epsilon and nPKC delta have an unusual distribution since 60-80% is membrane-bound. In response to TPA, the cytosolic levels of all four PKC isozymes were recruited to the membrane fraction. Prolonged treatment of R6 cells with TPA caused total loss of cPKC alpha, nPKC delta and nPKC zeta but only a 60% reduction in nPKC epsilon.(ABSTRACT TRUNCATED AT 250 WORDS)