Distinct PKC isozymes regulate bufalin-induced differentiation and apoptosis in human monocytic cells

Bufalin, an Na+-K+-ATP-ase inhibitor, simultaneously induced cell differentiation and apoptosis in human monocytic leukemia THP-1 cells. In this study, we investigated the regulatory role of protein kinase C (PKC) isozymes in bufalin-induced cell differentiation and apoptosis. A PKC-specific but isozyme-nonselective inhibitor, Ro-31-8220, and a cPKC selective inhibitor, Go-6976, caused significant attenuation of bufalin-induced interleukin-1 beta (IL-1 beta) gene expression, a mature monocytic marker, indicating that cPKC participates in the bufalin-induced cell differentiation. On the other hand, cPKC beta- and nPKC delta -defective THP1/TPA cells displayed strong resistance to the bufalin-induced DNA ladder formation. Rottlerin, an nPKC delta -specific inhibitor, partially attenuated preapoptotic effects of bufalin, such as the limited proteolysis of nPKC delta and poly(ADP-ribose) polymerase and the cell staining by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, suggesting that nPKC delta is involved, at least in part, in bufalin-induced apoptosis. In contrast, Go-6976 and rottlerin significantly augmented bufalin-induced apoptosis and differentiation, respectively. The findings suggest that bufalin-induced cell differentiation and apoptosis are interlinked and that distinct PKC isozymes are involved in the fate of the cell.