Androgen stimulated cellular proliferation in the human prostate cancer cell line LNCaP is associated with reduced retinoblastoma protein expression.
To elucidate the mechanism of androgen-dependent cellular proliferation in prostate cancer, androgen-dependent alterations of individual cell cycle regulatory proteins in the androgen-sensitive prostate cancer cell line LNCaP were evaluated. LNCaP cells were deprived of androgens by culture in steroid-depleted media for 5 days, which resulted in the maximal accumulation of cells in G(0)/G(1) phase of the cell cycle. The mitogenic concentration of the synthetic androgen R1881 was established as 0.1 nM using cell proliferation assay. Protein and mRNA levels of particular cyclins, cyclin-dependent kinases (Cdks), cyclin-dependent kinase inhibitors (Ckis), and the retinoblastoma proteins (Rb) were assessed. Androgen stimulation resulted in a post-transcriptional reduction in Rb protein levels, an increase in Rb phosphorylation at serine 780 and an accumulation of high molecular weight Rb protein species. Androgen stimulation also induced the expression of the Cdk2 and Cdk1 as well as their regulatory partners, cyclin A and cyclin B, resulting in a corresponding increase in cyclin A/Cdk2 activity in vitro. Pulse-chase showed decreased Rb protein stability in androgen-treated LNCaP cells. Collectively, our findings suggest a novel mechanism of androgen-dependent prostate cancer growth in which androgen stimulation results in decreased Rb protein expression in LNCaP cells. The observation of decreased Rb protein stability in the setting of increased phosphorylation supports the concept of phosphorylation mediated protein degradation. We propose that the observed reduction in Rb protein level occurs through Rb degradation via the ubiquitin/proteasome pathway, and is preceded by selective Rb phosphorylation by cyclin A/Cdk2 and cyclin B/Cdk1.
Taneja
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Department of Urology, The Kaplan Comprehensive Cancer Center, New York University School of Medicine, New York, New York 10016, USA. Samir.Taneja@med.nyu.edu
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Garabedian
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D000728:Androgens / Q000494:pharmacology*; D042846:CDC2-CDC28 Kinases; D002384:Catalysis / Q000187:drug effects; D002455:Cell Division / Q000187:drug effects / Q000502:physiology; D019925:Cyclin A / Q000096:biosynthesis; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases / Q000096:biosynthesis / Q000187:drug effects / Q000378:metabolism; D016213:Cyclins / Q000187:drug effects / Q000378:metabolism; D015870:Gene Expression / Q000187:drug effects; D006207:Half-Life; D006801:Humans; D046911:Macromolecular Substances; D008297:Male; D011471:Prostatic Neoplasms / Q000378:metabolism / Q000473:pathology*; D017346:Protein Serine-Threonine Kinases / Q000096:biosynthesis; D016160:Retinoblastoma Protein / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D014407:Tumor Cells, Cultured
D000728:Androgens; D019925:Cyclin A; D016213:Cyclins; D046911:Macromolecular Substances; D016160:Retinoblastoma Protein; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases