STAUROSPORINE ACTIVATES A 60,000 MR PROTEIN-KINASE IN BOVINE CHROMAFFIN CELLS THAT PHOSPHORYLATES MYELIN BASIC-PROTEIN IN-VITRO

Bovine chromaffin cells contain a family of renaturable protein kinase s. One of these, a 60,000 M(r) kinase (PK60) that phosphorylated myeli n basic protein in vitro, was activated fourfold when cells were treat ed with the protein kinase inhibitor staurosporine. Because staurospor ine inhibits protein kinase C, the role of this kinase in the regulati on of PK60 activity was investigated. Fifty nanomolar staurosporine pr oduced half-maximal inhibition of protein kinase C activity in chromaf fin cells, whereas approximately 225 nM staurosporine was required to induce half-maximal activation of PK60. Other protein kinase C inhibit ors, H-7 and K-252a, did not mimic the effect of staurosporine on PK60 activity. Chromaffin cells have three protein kinase C isoforms: alph a, epsilon, and zeta. Prolonged treatment with phorbol esters depleted the cells of protein kinase C alpha and epsilon, but not zeta. Neithe r activation nor depletion of protein kinase C affected the basal acti vity of PK60. Moreover, staurosporine activated PK60 in cells depleted of protein kinase C alpha and epsilon; thus, staurosporine appeared t o activate PK60 by a mechanism that does not require these protein kin ase C isoforms. Incubation of cell extracts with staurosporine in vitr o did not activate PK60. Incubation of these extracts with adenosine 5 '-O-(3-thiotriphosphate), however, caused a twofold activation of PK60 . Although this suggests that PK60 activity is regulated by phosphoryl ation, the mechanism by which staurosporine activates PK60 is not know n. Staurosporine has been reported to promote neurite outgrowth from c hromaffin cells. The role of PK60 in mediating the effects of staurosp orine on chromaffin cell function remains to be determined.