LYMPHOMA MODELS FOR B-CELL ACTIVATION AND TOLERANCE - ANTIIMMUNOGLOBULIN-M TREATMENT INDUCES GROWTH ARREST BY PREVENTING THE FORMATION OF AN ACTIVE KINASE COMPLEX WHICH PHOSPHORYLATES RETINOBLASTOMA GENE-PRODUCT IN G(1)

The product of the retinoblastoma gene, RE-1, is the prototype of a cl ass of tumor suppressor genes that is expressed in most mammalian cell s. The RE protein is phosphorylated in a cell cycle-dependent manner a nd is modulated during cellular differentiation. We have shown previou sly that anti-immunoglobulin M (anti-mu) treatment of WEHI-231 and CH3 1 B-lymphoma cells caused cell cycle blockade and apoptosis. In such a rrested cells, pRB was predominantly in the underphosphorylated (activ e) form, in contrast to hyperphosphorylated pRB in control log phase c ells. Herein we examine the modulation of pRB phosphorylation by anti- mu and its effect on a cyclin:kinase complex that can act on pRB in mu rine B-lymphoma cells. In unsynchronized B-lymphoma cells, anti-mu cro ss-linking of membrane immunoglobulin M leads to an accumulation of th e hypophosphorylated form of pRB, a decrease in the abundance of one f orm of cyclin A, and inhibition of cyclin A and cdk2-associated kinase activity. Using centrifugal elutriation, we also show that anti-mu tr eatment prevents the phosphorylation of the retinoblastoma gene produc t only when added in early G(1). In addition, there is a critical poin t after which membrane immunoglobulin M crosslinking is no longer effe ctive at preventing this process. We suggest that anti-mu-mediated gro wth arrest is due to the direct or indirect inactivation of an active kinase complex capable of pRB phosphorylation.