Modulation of apoptosis of proliferating and differentiating HL-60 cells by protein kinase inhibitors: suppression of PKC or PKA differently affects cell differentiation and apoptosis.

The relationship between RA- or dbcaMP-mediated differentiation and subsequent apoptosis in HL-60 cells was assessed by modulating the levels of differentiation suppressing the activity of PKC and PKA with calphostin C or GF 109203X and H89, respectively. Results demonstrated that (1) RA and dbcAMP caused a dose-dependent increase in apoptosis concomitant with progressive differentiation; (2) the suppression of PKC activity resulted in an increase of apoptosis unrelated to the modulated levels of differentiation; (3) the inhibition of PKA decreased granulocytic differentiation, but did not significantly affect apoptosis; (4) the pretreatment of cells with dbcAMP strongly potentiated RA-mediated differentiation without apparent changes in apoptosis; (5) cell differentiation and apoptosis were associated with cell cycle arrest in G1 phase and G2/M phases, respectively. Our findings indicate that the functional maturity of differentiating cells is not directly related to the apoptotic programme, and suggest that induction of cell differentiation and apoptosis are regulated by separate mechanisms in which PKC and PKA are involved.