PTHrP contributes to the anti-proliferative and integrin α6β4-regulating effects of 1,25-dihydroxyvitamin D3

Parathyroid hormone-related protein (PTHrP) increases the growth and metastatic potential of prostate cancer cells, making it important to control PTHrP expression in these cells. 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] suppresses PTHrP expression and exerts an anti-proliferative effect in prostate carcinoma cells. We used the human prostate cancer cell line C4-2 as a model system to ask whether down-regulation of PTHrP expression by 1,25(OH)2D3 plays a role in the anti-proliferative effects of 1,25(OH)2D3. Since PTHrP increases the expression of the pro-invasive integrin α6β4, we also asked whether 1,25(OH)2D3 decreases integrin α6β4 expression in C4-2 cells, and whether modulation of PTHrP expression by 1,25(OH)2D3 plays a role in the effects of 1,25(OH)2D3 on integrin α6β4 expression. Two strategies were utilized to modulate PTHrP levels: overexpression of PTHrP (−36 to +139) and suppression of endogenous PTHrP expression using siRNAs. We report a direct correlation between PTHrP expression, C4-2 cell proliferation and integrin α6β4 expression at the mRNA and cell surface protein level. Treatment of parental C4-2 cells with 1,25(OH)2D3 decreased cell proliferation and integrin α6 and β4 expression. These 1,25(OH)2D3 effects were significantly attenuated in cells with suppressed PTHrP expression. 1,25(OH)2D3 regulates PTHrP expression via a negative vitamin D response element (nVDRE) within the noncoding region of the PTHrP gene. The effects of 1,25(OH)2D3 on cell proliferation and integrin α6β4 expression were significantly attenuated in cells overexpressing PTHrP (−36 to +139), which lacks the nVDRE. These findings suggest that one of the pathways via which 1,25(OH)2D3 exerts its anti-proliferative effects is through down-regulation of PTHrP expression.