1α,25-Dihydroxyvitamin D3-induced down-regulation of the checkpoint proteins, Chk1 and Claspin, is mediated by the pocket proteins p107 and p130

A previous cDNA microarray analysis in murine MC3T3-E1 osteoblasts revealed a cluster of genes involved in cell cycle progression that was significantly down-regulated after a single treatment with 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] [L. Verlinden, G. Eelen, I. Beullens, M. Van Camp, P. Van Hummelen, K. Engelen, R. Van Hellemont, K. Marchal, B. De Moor, F. Foijer, H. Te Riele, M. Beullens, M. Bollen, C. Mathieu, R. Bouillon, A. Verstuyf, Characterization of the condensin component Cnap1 and protein kinase Melk as novel E2F target genes down-regulated by 1,25-dihydroxyvitamin D3, J. Biol. Chem. 280 (45) (2005) 37319–37330]. Among those genes were the DNA replication and DNA damage checkpoint proteins, Chk1 and Claspin, of which the human homologues were recently shown to be E2F-responsive. Quantitative real-time PCR experiments in 1,25(OH)₂D₃-treated MC3T3-E1 cells confirmed the down-regulation observed in the microarray experiment. Moreover, Chk1 and Claspin promoter activities were also reduced after incubation with 1,25(OH)₂D₃, and this reduction was mediated through the E2F recognition motifs within their promoters because mutation of these motifs almost completely abolished the repressive effect of 1,25(OH)₂D₃. The antiproliferative effect of 1,25(OH)₂D₃ as well as its potential to down-regulate the expression of Chk1 and Claspin depended on the pocket proteins p107 and p130 because 1,25(OH)₂D₃ lost its antiproliferative action and failed to repress these E2F-target genes in p107^−/−;p130^−/−-cells, but not in pRb^−/−-cells.