A CALMODULIN-LIKE PROTEIN AS AN EXTRACELLULAR MITOGEN FOR THE KERATINOCYTE

This study investigated the importance of extracellular calmodulin to the proliferation of the keratinocyte. Normal keratinocytes in culture produced a calmodulin-like protein in their culture media, the level of which increased abruptly and transiently during their growth. This protein was calmodulin-like, in that it specifically bound to a calmod ulin affinity column, exhibited calmodulin-like immunoreactivity in bo th an ELISA and on immunoblots when immunostained with a monoclonal an tibody against calmodulin, had an apparent M(r) between 18,000 and 20, 000, and stimulated activity in a calmodulin-dependent phosphodiestera se enzyme assay. Addition of exogenous pure calmodulin was of no furth er mitogenic benefit to the keratinocytes, and slightly reduced prolif eration under the culture conditions used. However, addition of either a neutralizing antibody to calmodulin, or W7-agarose, to the culture media of proliferating cells markedly inhibited their proliferation. A ccordingly, a calmodulin-like protein was found to satisfy all but one of the criteria for its action as an autocrine growth factor for the keratinocyte. We propose that the lack of mitogenic response to calmod ulin in vitro is due to the cell meeting its own requirement for extra cellular calmodulin.