B Cell Antigen Receptor-Mediated Activation of Cyclin-Dependent Retinoblastoma Protein Kinases and Inhibition by Co-Cross-Linking with Fcγ Receptors

Abstract

Cross-linking the B cell Ag receptor (BCR) to surface Fc receptors for IgG (FcγR) inhibits G1-to-S progression; the mechanism by which this occurs is not completely known. We investigated the regulation of three key cell cycle regulatory components by BCR-FcγR co-cross-linking: G1-cyclins, cyclin-dependent kinases (Cdks), and the retinoblastoma gene product (Rb). Rb functions to suppress G1-to-S progression in mammalian cells. Rb undergoes cell-cycle-dependent phosphorylation, leading to its inactivation and thereby promoting S phase entry. We demonstrate in this paper for the first time that BCR-induced Rb phosphorylation is abrogated by co-cross-linking with FcγR. The activation of Cdk4/6- and Cdk2-dependent Rb protein kinases is concomitantly blocked. FcγR-mediated inhibition of Cdk2 activity results in part from an apparent failure to express Cdk2 protein. By contrast, inhibition of Cdk4/6 activities is not due to suppression of Cdk4/6 or cyclins D2/D3 expression or inhibition of Cdk-activating kinase activity. Cdk4- and Cdk6-immune complexes recovered from B cells following BCR-FcγR co-cross-linking are devoid of coprecipitated D-type cyclins, indicating that inhibition of their Rb protein kinase activities is due in part to the absence of bound D-type cyclin. Thus, BCR-derived activation signals that up-regulate D-type cyclin and Cdk4/6 protein expression remain intact; however, FcγR-mediated signals block cyclin D-Cdk4/6 assembly or stabilization. These results suggest that assembly or stabilization of D-type cyclin holoenzyme complexes 1) is an important step in the activation of Cdk4/6 by BCR signals, and 2) suffice in providing a mechanism to account for inhibition of BCR-stimulated Rb protein phosphorylation by FcγR.