Differential regulation of surface Ig- and Lyb2-mediated B cell activation by cyclic AMP I. Evidence for alternative regulation of signaling through two different receptors linked to phosphatidylinositol hydrolysis in murine B cells

The differential effect of cAMP on the regulation of early biochemical and cellular functions mediated through two different receptors on murine B cells are reported here. Surface IgM, the Ag receptor, and Lyb2, a 45-kDa differentiation Ag are concomittantly expressed on mature murine B lymphocytes. Triggering of B cells through these molecules, independently, resulted in inositol 1,4,5-triphosphate (IP₃) generation, increase in intracellular Ca²⁺ levels, and cell enlargement associated with progression of cells from G₀ to G₁ ultimately resulting in DNA synthesis. Pretreatment of resting B cells with cholera toxin as well as other agents that raise the intracellular cAMP [(cAMP)_i] such as forskolin, N⁶,2′-O-dibutyryl cyclic AMP, and 3-isobutyl-1 methyl xanthine inhibited the Ag receptor but not Lyb2-mediated DNA synthesis. The elevation of (cAMP)_i inhibited the surface IgM but not Lyb2-mediated IP₃ generation, Ca²⁺ response, and progression from G₀ to G₁ phase of the cell cycle. Failure of forskolin or N⁶,2′-O-dibutyryl cyclic AMP to inhibit Lyb2-mediated responses did not appear to be due to induction of cAMP-specific phosphodiesterase activity. Concentrations of H8 [N-(2-(methylamino)-ethyl)-5-isoquinoline sulfonamide, diHCl] inhibitory to cAMP dependent PKA prevented the inhibitory effect of forskolin on surface IgM-mediated Ca²⁺ response, suggesting that cAMP exerted its effects through PKA. These findings suggest that distinct PLC-coupled receptors, such as sIgM and Lyb2 molecules in B cells, may use either alternative mechanisms for phosphatidylinositol 4,5-bisphosphate hydrolysis or may use different intermediary transducer molecules that differ in their sensitivity to increased (CAMP)_i levels. Thus "cross-talk" among cAMP and phosphatidylinositol signaling pathways was demonstrated for IgM but not Lyb2- mediated B cell activation.