The cAMP response element-binding protein (CREB) is generally considered to be responsive to elevation of cAMP through the activity of protein kinase A (PKA). Although it is well known that cAMP-raising agents can strongly influence B cell stimulation, the regulation of CREB has been little studied. Recently, cross-linking of surface Ig (sIg) was shown to result in trans-activation of a cAMP response element (CRE)-dependent promoter to which bound B cell CREB. In this study, we explored the mechanism underlying this unexpected linkage between sIg and CREB. We found that sIg cross-linking results in phosphorylation of CREB at Ser133. Although this phosphorylation step is mediated by PKA in pheochromocytoma cells, it depends on protein kinase C (PKC) in B lymphocytes. This conclusion is based on abrogation of sIg-induced CREB Ser133 phosphorylation by long-term phorbol-ester treatment to deplete PKC, and mimicking of sIg-induced CREB phosphorylation and CRE-dependent gene expression by short-term PKC agonism. Furthermore, CD40 ligand (CD40L) and LPS, two PKC-independent forms of B cell stimulation, failed to induce phosphorylation of CREB Ser133. These results suggest that CREB responds to specific surface-receptor signals in B cells and that this response is mediated by PKC. Interestingly, forskolin failed to induce phosphorylation of CREB Ser133 in B cells, although it did so in PC12 pheochromocytoma cells. Taken together with PKC mediation of CREB Ser133 phosphorylation in B cells, these results suggest that the dominant mode of CREB regulation is cell-type specific.

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