Biochemical events associated with inhibition of B-cell proliferation by phorbol diesters

Phorbol myristate acelate (PMA), a potent tumor promoter, has a variety of effects on cells of the immune system resulting in altered patterns of cell proliferation and differentiation. Although PMA is mitogenic or co-mitogenic for human lymphocytes and murine T-cells, it inhibits proliferation of murine B-cells stimulated by LPS or anti-Ig. PMA, however, does not inhibit the ability of LPS or anti-Ig to activate B-cells, as evidenced by increased Ia antigen expression and RNA synthesis. In the present studies it was shown that inhibition of DNA synthesis by PMA coincided with qualitative and quantitative changes in phosphorylated proteins. In particular, PMA treatment resulted in a unique profile of phosphoproteins independent of LPS or anti-Ig treatment. Inhibition of DNA synthesis occurred over a wide range of PMA concentrations. At concentrations up to 10(-9) M, inhibition of proliferation correlated with decreased phosphatidylinositol turnover and decreased intracellular Ca2+ levels, suggesting that PMA affects the phosphoinositide signal transduction pathway. However, at PMA concentrations less than 10(-10) M, inhibition of anti-Ig- and LPS-mediated proliferation occurred without inhibition of the phosphoinositide transduction signal. At these concentrations, PMA-induced inhibition of DNA synthesis was highly sensitive to recombinant IL-2. These data suggest that the antiproliferative effects of PMA on B-cells stimulated by LPS or anti-Ig may be mediated by two mechanisms. At high concentrations, PMA causes a feedback regulation of the phosphoinositide-dependent messenger system, while at lower concentrations, PMA alters the response to specific growth factors. Since PMA induces unique phosphoproteins and both of these events can be regulated by protein phosphorylation, it is possible that these unique phosphoproteins are responsible for the antiproliferative effects of PMA.