Comparison of different tumour promoters and bryostatin 1 on protein kinase C activation and down-regulation in rat renal mesangial cells

Abstract

The effects of a series of protein kinase C (PKC) activators with different spectra of biological activities and reportedly different patterns of PKC isoenzyme activation were examined in renal mesangial cells. Treatment of mesangial cells with the tumor promoters phorbol 12-myristate 13-acetate (PMA), debromoaplysiatoxin, dihydroteleocidin and thymeleatoxin, as well as with the marine natural product bryostatin 1, caused translocation and at least partial down-regulation of the PKC-α,-δ and -ε isoenzymes as assessed by immunoblot analysis. Bryostatin 1 mediates a faster depletion of PKC-α isoform than any of the other PKC activators. Thymeleatoxin, which has been reported to selectively activate PKC-α,-β and -γ, but not PKC-δ or -ε isoenzymes in vitro, turned out to exert the most potent effect on PKC-δ and -ε in mesangial cells and down-regulated these isotypes within 8–24 hr. None of the compounds tested affected cellular distribution or amount of PKC-ζ in mesangial cells. Thus, all of the PKC activators tested are able to translocate and down-regulate three of the four PKC isoenzymes present in mesangial cells although with different kinetics. All PKC activators stimulated a phospholipase A 2-mediated arachidonic acid release, a phospholipase D-mediated phosphatidylcholine hydrolysis, a comparable small proliferative response and an inhibition of phospholipase C-mediated inositol trisphosphate generation. These results suggest: (i) that the PKC activators investigated in this study do not display any type of isotype-speciflcity that could be used to selectively activate or down-regulate PKC isoenzymes in intact cell-systems; (ii) that thymeleatoxin has a different isoenzyme selectivity in intact cells as compared to in vitro enzyme inhibition data; and (iii) PKC-ζ is resistent to all PKC activators investigated in this study.