Lymphoma models for B-cell activation and tolerance: Anti-immunoglobulin M treatment induces growth arrest by preventing the formation of an active kinase complex which phosphorylates retinoblastoma gene product in G₁

The product of the retinoblastoma gene, RB-1, is the prototype of a class of tumor suppressor genes that is expressed in most mammalian cells. The RB protein is phosphorylated in a cell cycle-dependent manner and is modulated during cellular differentiation. We have shown previously that anti- immunoglobulin M (anti-μ) treatment of WEHI-231 and CH31 B-lymphoma cells caused cell cycle blockade and apoptosis. In such arrested cells, pRB was predominantly in the underphosphorylated (active) form, in contrast to hyperphosphorylated pRB in control log phase cells. Herein we examine the modulation of pRB phosphorylation by anti-μ and its effect on a cyclin:kinase complex that can act on pRB in murine B-lymphoma cells. In unsynchronized B-lymphoma cells, anti-μ cross-linking of membrane immunoglobulin M leads to an accumulation of the hypophosphorylated form of pRB, a decrease in the abundance of one form of cyclin A, and inhibition of cyclin A and cdk2-associated kinase activity. Using centrifugal elutriation, we also show that anti-μ treatment prevents the phosphorylation of the retinoblastoma gene product only when added in early G₁. In addition, there is a critical point after which membrane immunoglobulin M cross-linking is no longer effective at preventing this process. We suggest that anti-μ-mediated growth arrest is due to the direct or indirect inactivation of an active kinase complex capable of pRB phosphorylation.