1α,25-dihydroxyvitamin D3 inhibits C4-2 prostate cancer cell growth via a retinoblastoma protein (Rb)-independent G1 arrest.
BACKGROUND
The active metabolite of vitamin D, 1α,25-dihydroxyvitamin D(3) (1,25D) reduces the growth of several prostate cancer cell lines, most commonly by inducing a cell-cycle arrest in G(1). This is mediated, in part, through down-regulation of c-Myc, a positive regulator of the transcription factor, E2F. There is evidence that prostate cancer cells lacking functional retinoblastoma protein (Rb), a negative regulator of E2F activity, are poorly responsive to 1,25D treatment. Since up to 60% of prostate cancers demonstrate a loss of heterozygosity for Rb, we sought to determine whether Rb is required for the growth inhibitory effects of 1,25D.


METHODS
Using siRNA, Rb was reduced in C4-2 prostate cancer cells, and the response of cells to 1,25D treatment or depletion of c-myc measured by [(3)H]-thymidine incorporation and flow cytometry. The effects of 1,25D treatment on E2F levels and activity, and E2F target gene expression were also measured.


RESULTS
1,25D treatment and c-Myc depletion both cause a G(1) arrest inhibiting C4-2 cell proliferation independently of Rb. 1,25D reduces c-Myc expression and causes a decrease in E2F and E2F target genes. Bcl-2, an E2F target and positive regulator of C4-2 cell growth, also is down-regulated by 1,25D independently of Rb.


CONCLUSIONS
Redundant growth inhibitory pathways compensate for the loss of Rb, and tumors lacking functional Rb may be responsive to 1,25D.
Washington
Michele N
MN
Department of Molecular and Cellular Biology, Baylor College of Medicine, Houston, Texas 77030 USA
Kim
Jung-Sun
JS
Weigel
Nancy L
NL
D002117:Calcitriol / Q000494:pharmacology*; D002277:Carcinoma / Q000378:metabolism*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D015536:Down-Regulation / Q000187:drug effects; D050684:E2F Transcription Factors / Q000032:analysis; D016193:G1 Phase / Q000187:drug effects*; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D019254:Genes, bcl-2; D016259:Genes, myc; D006801:Humans; D008297:Male; D011471:Prostatic Neoplasms / Q000378:metabolism*; D016160:Retinoblastoma Protein / Q000235:genetics / Q000378:metabolism*
D050684:E2F Transcription Factors; D016160:Retinoblastoma Protein; D002117:Calcitriol