Inactivation of Id-1 increases the sensitivity to TGF-β1-induced growth arrest in prostate epithelial cells

Abstract

Id-1 is a member of the Id (inhibitor of differentiation or DNA binding) protein family, which belongs to the helix-loop-helix (HLH) transcription factors. A growing body of evidence indicates that Id-1 is not only essential for cell cycle progression, but it may also be involved in tumorigenesis. TGF-β1, meanwhile, plays dual roles during carcinogenesis: on one hand, it inhibits the growth of most non-malignant cells, especially those of epithelial origin; on the other hand, TGF-β1 contributes to tumor aggressiveness and cells become resistant to the anti-proliferative effects of TGF-β1 during tumorigenesis. To date, several lines of evidence have suggested that TGF-β1 may be one of the negative regulators of Id-1. In addition, down-regulation of Id-1 expression by TGF-β1 is accompanied by up-regulation of p21WAF1/Cip1 and p27KIP1, both of which are associated with TGF-β1-induced growth arrest in the prostate epithelia. Hence, we have hypothesized that the presence of Id-1 in epithelial cells might be responsible for the heightened cellular resistance to the growth inhibitory effect of TGF-β1, which in turn may promote tumorigenesis. Using an immortalized prostate epithelial cell line, NPTX, we have suppressed endogenous Id-1 levels through transfection of an antisense Id-1 construct. We have found that inactivation of Id-1 down-regulated NPTX cell proliferation, and more cells were arrested at G2/M phase of cell cycle. In addition, inactivation of Id-1 would make cells more vulnerable to TGF-β1-induced growth arrest. Furthermore, the sensitization effect to TGF-β1 was associated with up-regulation of p21WAF1/Cip1 and p27KIP1, at both mRNA and protein levels. These results have suggested that down-regulation of Id-1 levels may lead to increased sensitivity to TGF-β1, which may be mediated by up-regulation of p21WAF1/Cip1 and p27KIP1. Hence, the endogenous Id-1 levels may be important in modulating the cellular response to TGF-β1-induced growth inhibitory effect. The findings from the Id-1 knock down studies likely could contribute to the design of a therapeutic strategy in which inactivation of endogenous Id-1 might sensitize malignant cells to a host of therapeutic drugs, such as TGF-β1.