Cell cycle-independent induction of D1 and D2 cyclin expression, but not cyclin-Cdk complex formation or Rb phosphorylation, by IFNgamma in macrophages

Abstract

During G1 to S phase transition, D-type cyclins form complexes with cyclin-dependent kinases (Cdk), which in turn phosphorylate retinoblastoma gene product (Rb) and inhibit its growth-inhibitory function, leading ultimately to cell proliferation. We report here a novel finding that D1 and D2 cyclins are induced in macrophages by antiproliferative factor gamma interferon (IFNgamma). The induction appears to be transcriptional activation of the D cyclin genes, since indirect events such as IFNgamma-induced colony-stimulating factor-1 (CSF-1) autocrine stimulation, alteration of D1 and D2 mRNA stability and lipopolysaccharide contamination in commercial IFNgamma preparations play no roles. In contrast to CSF-1, IFNgamma neither induces D1-Cdk4 complex formation and Rb hyperphosphorylation nor interferes with CSF-1-stimulated D1-Cdk4 interaction and Rb phosphorylation, while it completely blocks CSF-1-stimulated cell proliferation. This study suggests that induction of D1 and D2 cyclins is not necessarily associated with cell cycle progression, and D cyclins may have cell cycle-independent functions in response to IFNgamma.