Involvement of protein kinase C delta in contact-dependent inhibition of growth in human and murine fibroblasts

There is evidence that protein kinase C delta (PKC delta) is a tumor suppre ssor, although its physiological role has not been elucidated so far. Since important anti-proliferative signals are mediated by cell-cell contacts we studied whether PKC delta is involved in contact-dependent inhibition of g rowth in human (FH109) and murine (NIH3T3) fibroblasts. Cell-cell contacts were imitated by the addition of glutardialdehyde-fixed cells to sparsely s eeded fibroblasts. Downregulation of the PKC isoforms alpha, delta, epsilon , and mu after prolonged treatment with 12-O-tetradecanoylphorbol-13-acetat e (TPA, 0.1 mum) resulted in a significant release from contact-inhibition in FH109 cells. Bryostatin I selectively prevented TPA-induced PKC delta -d ownregulation and reversed TPA-induced release from contact-inhibition argu ing for a role of PKC delta in contact-inhibition. In accordance, the PKC d elta specific inhibitor Rottlerin (1 mum) totally abolished contact-inhibit ion. Interestingly, immunofluorescence revealed a rapid translocation of PK C delta to the nucleus when cultures reached confluence with a peak in earl y-mid G1 phase. Nuclear translocation of PKC delta in response to cell-cell contacts could also be demonstrated after subcellular fractionation by Wes tern blotting and by measuring PKC delta -activity after immunoprecipitatio n. Transient transfection of NIH3T3 cells with a dominant negative mutant o f PKC delta induced a transformed phenotype. We conclude that PKC delta is involved in contact-dependent inhibition of growth.