The expression of retinoblastoma and Sp1 is increased by low concentrations of cyclin‐dependent kinase inhibitors

We examined the effect of suboptimal concentrations of cyclin‐dependent kinase inhibitors, which do not interfere with cell proliferation, on retinoblastoma expression in hamster (Chinese hamster ovary K1) and human (K562 and HeLa) cells. To achieve this, we used the chemical inhibitors roscovitine and olomoucine (which inhibit CDK2 preferentially), UCN‐01 (which also inhibits CDK4/6) and p21 (as an intrinsic inhibitor). All chemical inhibitors and overexpression of p21 strongly induced retinoblastoma protein expression. UCN‐01‐mediated retinoblastoma expression was caused by an increase in both the levels of retinoblastoma mRNA and the stability of the protein. The expression of the transcription factor Sp1, a retinoblastoma‐interacting protein, was also enhanced by all the cyclin‐dependent kinase inhibitors tested. However, Sp1 expression was caused by an increase in the levels of Sp1 mRNA without modification in the stability of the protein. By using luciferase experiments, the transcriptional activation of both retinoblastoma and Sp1 promoters by UCN‐01 was confirmed. Bisindolylmaleimide I, at concentrations causing a similar or higher inhibition of protein kinase C than UCN‐01, provoked a lower activation of retinoblastoma and Sp1 expression. Finally, the effects of cyclin‐dependent kinase inhibitors on dihydrofolate reductase gene expression were evaluated. Treatment with UCN‐01 increased cellular dihydrofolate reductase mRNA levels, and dihydrofolate reductase enzymatic activity was enhanced by UCN‐01, roscovitine, olomoucine and p21, in transient transfection experiments. These results support a mechanism for the self‐regulation of retinoblastoma expression, and point to the need to establish the appropriate dose of cyclin‐dependent kinase inhibitors as antiproliferative agents in anticancer treatments.