ACTIVATION OF PP60(C-SRC) DEPENDING ON CELL-DENSITY IN PC12H CELLS

The Src family tyrosine kinases and their substrates are involved in c ell-cell and cell-matrix interactions. We found that in PC12h cells, a n increase of cell density enhanced the tyrosine phosphorylation level s of several intracellular proteins including p130(cas). Because it is a possible substrate for Src family kinases, we measured pp60(c-src) activity and found that it was higher in high density cultures than in low density cultures. This phenomenon was also observed in PC12 (the parental cell line of the PC12h subclone), Balb/c 3T3, Swiss 3T3, and Hela cells, One of the possible mechanisms regulating the kinase activ ity of pp60(c-src) is the phosphorylation and dephosphorylation of its negative regulatory site located at its C terminus. However, the tyro sine phosphorylation level of the regulatory site did not change depen ding on cell density. Subcellular fractionation showed that in high de nsity culture, pp60(c-src) was translocated from detergent-soluble to detergent-insoluble fractions, These results suggest that cell-cell in teraction might induce the activation of pp60(c-src) without changing its tyrosine phosphorylation levels.