TGF-β1 Induces the Cyclin-Dependent Kinase Inhibitor p27<i>Kip1</i> mRNA and Protein in Murine B Cells

Abstract

Abstract TGF-β1 inhibits the cell cycle progression of many types of cells by arresting them in the G1 phase. This cell cycle arrest has been attributed to the regulatory effects of TGF-β1 on both the levels and the activities of the G1 cyclins and their kinase partners. The activities of these kinases are negatively regulated by a number of proteins, such as p15INK4b, p21WAF1/Cip1, and p27Kip1, that physically associate with cyclins, cyclin-dependent kinases (Cdk), or cyclin-Cdk complexes. In epithelial cell lines, TGF-β1 was previously shown to inhibit cell cycle progression through down-regulation of Cdk4 and/or up-regulation of p15INK4b and/or p21WAF1/Cip1. However, TGF-β1 had little or no effect on the p27Kip1 mRNA and protein levels. In this report, we show that, in contrast to observations in epithelial cell lines, TGF-β1 increased the p27Kip1 mRNA and protein levels in the murine B cell lines CH31 and WEHI231. This TGF-β1-mediated induction of p27Kip1 also resulted in an increased association of p27Kip1 with Cdk2 and a decreased Cdk2 kinase activity. In contrast to epithelial cells, however, TGF-β1 had little or no effect on the Cdk4 and p21WAF1/Cip1 protein levels in these B cells. Finally, although several studies suggested a direct role of p53 in TGF-β1-mediated cell cycle arrest in epithelial cells, TGF-β1 inhibited cell cycle progression in CH31 even in the absence of wild-type p53. Taken together, these results suggest that TGF-β1 induces G1 arrest in B cells primarily through a p53-independent up-regulation of p27Kip1 protein.