Intracellular signaling events associated with the induction of DNA synthesis in human B lymphocytes. I. Stimulation of PKC-dependent and -independent pathways by LMW-BCGF.

The low molecular weight B cell growth factor (LMW-BCGF) induces the G1 --> S transition in human B lymphocytes activated by a first signal, Staphylococcus aureus Cowan (SAC) or anti-mu antibody. It also stimulates proliferation of normal long-term B cell lines and some B cell tumors. We have previously reported that LMW-BCGF induces the hydrolysis of polyphosphoinositides (PI) and a rise in intracellular free calcium concentration, through the generation of inositol trisphosphate (InsP3) (Renard et al., J. Immunol. 18, 1705, 1988). In the present work we have analyzed the possible association between early signaling events elicited by LMW-BCGF in SAC-activated B cells and its ability to provoke DNA synthesis, notably at the level of phospholipase C (PLC) and protein kinase C (PK-C) activation. Inhibitors of PLC and of InsP3-induced calcium release were found to block LMW-BCGF-dependent DNA synthesis. An increase in membrane-associated protein kinase C (PK-C) activity was detected after the addition of the growth factor and the mitogenic effect of LMW-BCGF was partially suppressed when B cell blasts were incubated with staurosporine or H-3, two inhibitors of PK-C activity. In addition, the mitogenic effect due to the addition of LMW-BCGF was not modified by the incubation of B cell blasts with high concentrations of TPA, even if this treatment inhibited cellular response to a low concentration of TPA. LMW-BCGF also increased intracellular pH in B cell blasts and lymphokine-induced mitogenic activity was reduced when the Na+/H+ amiloride or ethylisopropyl amiloride (EIPA) antiport blockers were added. These results suggest that (i) LMW-BCGF-induced PI breakdown and CA2+ mobilization and cell alkalinization are associated with the induction of cell proliferation, and (ii) the activation of PK-C does not appear to be the sole pathway activated by LMW-BCGF.