ROLE FOR CA2-CYCLE-REGULATED GENES IN PAF-STIMULATED CELLS( IN EXPRESSION OF CELL)

Platelet-activating factor (PAF) is a powerful stimulator of a wide va riety of cells. In transformed human B-lymphoblastoid cell lines, PAF increases intracellular Ca2+ concentrations ([Ca2+](i)) and induces th e expression of the proto-oncogenes c-fos and early growth response ge ne-2 (EGR2). Here, we present data that evaluates the role of Ca2+ in the PAF-dependent induction of these cell-cycle activated genes. PAF ( 10(-7) M) increased c-fos and EGR2 mRNA levels in cells suspended in C a2+-containing medium by 6-10-fold. In PAF-stimulated cells suspended in medium depleted of Ca2+, eliminating Ca2+ influx but not intracellu lar store release of Ca2+, the induction of gene expression was reduce d by approx. 50%. In contrast, buffering of Ca2+ released from intrace llular stores but maintaining transmembrane Ca2+ uptake had little eff ect on gene expression. When both sources of Ca2+ were eliminated, PAF -stimulated expression of these genes was completely prevented. This w as not due to any toxicity to the cells since the response to phorbol ester under identical conditions was unaffected. The regulation of c-f os mRNA expression was paralleled by changes in levels of FOS protein. These data indicate that changes in [Ca2+](i), primarily from stimula ted entry across the plasma membrane and to a lesser extent release of Ca2+ from sequestered intracellular stores, play an essential role in PAF-dependent triggering of c-fos and EGR2 mRNA expression.