Conventional protein kinase C plays a critical role in negative regulation of CD98-induced homotypic aggregation

CD98, a heterodimeric type II transmembrane protein, is involved in many different cellular events, ranging from amino acid transport to cell-cell adhesion. Little is known about the positive and negative signalling pathways involved in these responses. Therefore, we examined the role of conventional protein kinase C (PKC) isoforms during CD98-induced intracellular signalling and homotypic aggregation of U937 cells. The CD98-induced aggregation was enhanced by the general protein kinase inhibitors GF109203X and staurosporin, and by specific PKC-alpha/-beta peptide inhibitor 19-27, but inhibited by PKC activators such as phorbol 12-myristate 13-acetate (PMA). PMA-inhibition was reversed by PKC inhibitors recognising the ATP-binding site in PKC (e.g. staurosporin, GF109203X and Go6983). Inhibitors which bind to diacylglycerol (DAG) or Ca(2+)-binding sites of PKC (calphostin C and Go6967) had no effect. PMA-induced translocation of conventional PKC (cPKC) isozymes (alpha, beta and gamma), but decreased the expression of PKC-delta, which plays an important role in CD98-induced homotypic aggregation. PMA treatment also suppressed the surface level of CD98 but not CD29, CD18 and CD147, dose- and time-dependently. These data provide evidence that PMA-responsive cPKC isoforms (alpha, beta and gamma) play a key role in negative regulation of CD98 signalling and homotypic aggregation.