Insulin regulates serine/threonine phosphorylation in activated human B lymphocytes

Activation of either dense tonsilar B lymphocytes or the B lymphoblastoid cell line T5-1 with Staphylococcus aureus, Cowan strain I, induced surface expression of insulin receptors. Addition of insulin to these activated cells resulted in subsequent phosphorylation of the B cell surface protein CD20, the functions to regulate B cell activation. The cytoplasmic domains of CD20 contain multiple serine and threonine residues, of which at least two are followed by acidic sequences typical of substrate sites favored by casein kinase II. Tryptic mapping of CD20 isolated from intact cells treated with insulin showed increased phosphorylation on peptides having similar migration to those phosphorylated by casein kinase II in vitro. Treatment of tonsilar B cells or T5-1 cells with phorbol esters or in vitro phosphorylation by purified protein kinase C did not result in phosphorylation of peptides phosphorylated by casein kinase II, suggesting that protein kinase C is not directly involved in CD20 phosphorylation in the response to insulin. Phosphorylation of CD20 cannot be triggered by insulin in resting B cells, as the insulin receptor is expressed only after entry into the G1 phase of the cell cycle. Thus, distinct regulation of activation pathways are available to resting as opposed to activated B lymphocytes.