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 G0/G1 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.