Human cyclin D1 encodes a labile nuclear protein whose synthesis is directly induced by growth factors and suppressed by cyclic AMP.

We show that the cyclin D1 gene is regulated by a variety of growth factors in human diploid fibroblasts (WI-38). Expression of cyclin D1 mRNA is low in quiescent WI-38 cells and reaches a maximum around 10 hours after serum stimulation, i.e. approximately 8 hours prior to the onset of DNA synthesis. A cyclin D1-specific antiserum raised against a bacterially expressed fusion protein detected a 39 kDa polypeptide in WI-38 cells. In agreement with the RNA expression data, cyclin D1 protein synthesis is also serum-inducible, reaching a maximum around 9 hours post-stimulation. The results obtained by pulse-chase experiments, cell fractionation and immunostaining techniques strongly suggest that cyclin D1 is a labile protein (t1/2 approximately 38 min), which is located in the nucleus. Cyclin D1 is directly induced by growth factors, i.e. in the presence of cycloheximide, and its expression does not significantly fluctuate during the cell cycle in synchronized cells. Cyclin D1 therefore fundamentally differs from "classical" cyclins, such as the mitotic cyclin B, whose expression is clearly cell cycle-dependent. Cyclin D1 may rather establish a direct link between growth control mechanisms and the cell cycle. Interestingly, cyclin D1 expression is stimulated by the protein kinase C activator TPA, but suppressed by dibutyryl-cAMP and the adenylate cyclase inducer forskolin, pointing to multiple regulatory pathways controlling cyclin D1 expression.