Cyclic AMP modulation of human B cell proliferative responses: Role of cAMP-dependent protein kinases in enhancing B cell responses to phorbol diesters and lonomycin

Abstract

The ability of cyclic AMP (cAMP) to modulate human B cell proliferative responses and the possible role of cAMP-dependent kinases (PKA) in cAMP modulation of proliferative responses were investigated. The addition of dibutyl cAMP (Bt 2 cAMP) or the cAMP-elevating agent forskolin to B cells stimulated by crosslinking surface immunoglobulins (sIg) resulted in a concentration-dependent inhibition of proliferative responses. By contrast, Bt 2 cAMP or forskolin enhanced the proliferative responses of B cells after direct stimulation by phorbol myristate acetate (PMA) and the calcium ionophore ionomycin. The inhibition and enhancement of B cell proliferative responses by Bt 2 cAMP were observed at different incubation intervals and were not due to temporal shifts of optimal responses. Also, Bt 2 cAMP caused only small changes in B cell RNA synthesis compared to modulation of proliferative responses. Exposure of B cells to Bt 2 cAMP rapidly activated PKA. Blocking Bt 2 cAMP activation of PKA with the kinase inhibitor HA 1004 prevented Bt 2 cAMP enhancement of B cell responses after direct stimulation by PMA and ionomycin. In reciprocal experiments, the kinase inhibitor H7 resulted in some inhibition of PKC activation but did not inhibit Bt 2 cAMP activation of PKA or Bt 2 cAMP enhancement of proliferative responses. Other experiments demonstrated that B cells treated with Bt 2 cAMP had selective increases in the de novo phosphorylations of two endogenous substrates which reflected PKA activation. Furthermore, concentrations of HA1004 or H8 which inhibited Bt 2 cAMP enhancement of proliferative responses also inhibited PKA phosphorylations of these substrates whereas H7 did not. Thus, elevations of cAMP can enhance or inhibit human B cell proliferative responses to different stimuli and the activation of PKA is important for cAMP enhancement of certain responses.