Effects of selective protein kinase C inhibitors on the proteolytic down-regulation of L-selectin from chemoattractant-activated neutrophils

The signaling factors that direct the rapid shedding of L-selectin from neu trophils upon chemoattractant stimulation are poorly understood. Protein ki nase C (PKC) has been implicated, yet previous studies have relied on the u se of phorbol esters and nonselective kinase inhibitors, We treated neutrop hils with various selective kinase inhibitors to evaluate their effects on N-formylmethionyl-leucyl-phenylalanine (fMLP)-induced L-selectin shedding. We found that three selective inhibitors of PKC, structurally related to st aurosporine, largely blocked both fMLP- and phorbol 12-myristate 13-acetate (PMA)-induced L-selectin shedding; however; these inhibitors did not affec t fMLP-induced up-regulation of Mac-1 (CD11b/CD18) expression, which has be en shown not to involve PKC. Other selective serine, threonine, and tyrosin e kinase inhibitors were found not to block fMLP-induced L-selectin sheddin g. These findings provide more definitive evidence for the role of PKC in c hemoattractant-induced L-selectin proteolysis. It is interesting that certa in highly selective PKC inhibitors, not structurally related to staurospori ne, were found to directly induce L-selectin shedding from neutrophils.