B cell activation. VIII. Membrane immunoglobulins transduce signals via activation of phosphatidylinositol hydrolysis.

Considerable evidence indicates that cross-linking of B cell surface Ig results in a "first signal" in B cell activation. We have shown that transduction of this signal is manifest by changes in plasma membrane potential leading to increased expression of surface I-A antigen. In previous studies, we have provided evidence that suggests that this signal is transduced via phosphatidylinositol (PI) hydrolysis liberating diacylglycerol (DAG), which subsequently activates protein kinase C. These biochemical events are aspects of a transmembrane signal transduction mechanism that is common in nature and utilizes the PI metabolic cycle for generation of "second messenger" diacylglycerol. Here we report direct evidence that treatment of B cells with various antibodies to surface Ig results in activation of the PI cycle. Results suggest that the increased phospholipid metabolism that occurs in B cells in response to anti-Ig involves only those phospholipids in the PI cycle and is a consequence of turnover of existing lipid rather than de novo synthesis. Furthermore, we show that PI cycle activation requires cross-linking of membrane Ig and is inhibitable by increased intracellular cyclic AMP. These findings are particularly important in view of previous studies that have shown identical requirements for and inhibitability of induction of B cell membrane depolarization and increased I-A expression. Thus, these results are consistent with our previous hypothesis that early B cell activation events initiated by receptor Ig occupancy are mediated via PI hydrolysis, diacylglycerol generation, and protein kinase C activation.