Decreased expression of proliferating cell nuclear antigen is associated with dexamethasone inhibition of the proliferation of rat tendon fibroblasts

Objective. To investigate the effects of dexamethasone (Dex) on the proliferation of cultured rat Achilles tendon fibroblasts at concentrations typically used for local injection treatment. Methods. Fibroblasts cultured from rat Achilles tendons were treated with Dex at concentrations of 0, 10^-4, 3 × 10^-4, and 10^-3 M. [3H]thymidine incorporation was used to measure the rate of cell proliferation. mRNA expression of proliferating cell nuclear antigen (PCNA) and cyclin kinase inhibitor p21^CIP1 was determined by reverse transcription-polymerase chain reaction (RT-PCR). The protein levels of PCNA and p21^CIP1 were investigated by Western blot analysis. Results. An initial inhibitory effect on tendon fibroblast proliferation was observed at a concentration of 10^-4 M. Further, a significant decline in [3H]thymidine incorporation as a function of Dex concentration was noted (p = 0.019). RT-PCR results revealed that PCNA mRNA expression was inhibited after Dex treatment. Western blot analysis of PCNA protein also revealed Dex downregulation. Gradual declines in the levels of PCNA mRNA expression and PCNA protein as a function of Dex concentration were noted. The expression of p21^CIP1 both at mRNA and the protein levels remained constant. Conclusion. These results suggest that Dex inhibition of the proliferation of rat tendon fibroblasts is associated with a p21^CIP1 independent decrease of the PCNA gene expression.