Modulation of Ets-1 Expression in B Lymphocytes is Dependent on the Antigen Receptor-mediated Activation Signals and Cell Cycle Status

In this report, we tested the hypothesis that Ets-1 transcription factor is modulated at the mRNA level during B cell antigen receptor (BCR)-induced cell-signalling events. Quiescent B cells express high levels of Ets-1 mRNA. Stimulation through the BCR results in time-dependent inhibition of Ets-1 mRNA expression in primary splenic B cells with maximal inhibition observed by 16-h post-stimulation. Inhibition of Ets-1 expression is specific to antigen receptor but not CD40-mediated activation. Antigen receptor-induced inhibition of Ets-1 mRNA can be mimicked by phorbol myristate acetate (PMA) and/or ionomycin. PMA but not ionomycin-induced inhibition of Ets-1 expression is rescued by the inhibitors of protein kinase C and MEK. Extended time-course analysis revealed a time-dependent cyclical pattern in the re-expression of Ets-1 mRNA. While resting cells revealed maximal Ets-1 mRNA expression, activation events that induced exit from G(0)/G(1) or cells blocked in early S phase exhibited decreased Ets-1 mRNA levels. Interestingly, cells arrested at late G2 or M phase of the cell cycle failed to down modulate Ets-1 mRNA expression. Overexpression of Ets-1 in 70Z/3 B cell line caused abnormal accumulation of cells in S phase associated with increased cyclin A expression. Consistent with a requirement for Ets-1 in BCR-induced cell cycle entry, splenic B cells from mice deficient in Ets-1 showed defective antigen receptor-induced DNA synthesis and S phase entry. These results suggest a critical role for Ets-1 regulation during B cell activation and cell cycle entry.