A role for the cell cycle phosphatase Cdc25a in vitamin D-dependent inhibition of adult rat vascular smooth muscle cell proliferation

We have explored the mechanism(s) underlying 1,25 dihydroxyvitamin D's (1,25(OH)2D) suppression of agonist-induced vascular smooth muscle cell (VSMC) proliferation. Quiescent cultured adult rat VSMC were treated with 1,25(OH)2D for 48 h and endothelin (ET) or angiotensin II (AII) for the final 24 h. We show that VSMC responded to 1,25(OH)2D or its less hypercalcemic analogue RO 25-6760 with ∼70% inhibition of ET-dependent 3H-thymidine incorporation. The inhibition was linked to a comparable reduction in ET-stimulated cyclin-dependent kinase 2 (Cdk2) activity and suppression of an ET-induced Cdk2 activator, cell division cycle 25 homolog A (Cdc25A). Both 1,25(OH)2D and RO 25-6760 completely inhibited the ET-dependent increase in Cdc25A mRNA and protein levels, phosphatase and promoter activities. 1,25(OH)2D also suppressed AII-induced DNA synthesis, Cdk2 activity and Cdc25A gene transcription. Inhibition of Cdc25A gene expression using a siRNA approach resulted in significant inhibition of ET or AII-dependent Cdk2 activity and 3H-thymidine incorporation. The Cdc25A siRNA-mediated inhibition of ET or AII-induced Cdk2 activity and DNA synthesis was not additive with that produced by 1,25(OH)2D treatment. These data demonstrate that 1,25(OH)2D inhibits VSMC proliferation through a Cdc25A-dependent mechanism and suggest that this hormone may prove useful in the management of disorders characterized by aberrant proliferation of VSMC in the vascular wall.