The conventional transforming growth factor-β (TGF-β) receptor type I is not required for TGF-β1 signaling in a human prostate cancer cell line, LNCAP

LNCaP is an androgen-responsive human prostate cancer cell line that has a defective gene for ALK-5, the conventional TGF-β receptor type I. Yet, these cells respond to exogenous TGF-β1 under appropriate concentrations of dihydrotestosterone (DHT). Because a heteromeric complex composed of type I and type II receptor is required for TGF-β signaling, the expression of these receptors was investigated in LNCaP cells at following concentrations of DHT - 0, 0.1, and 100 nM. These concentrations were selected because they represent the zero DHT control in which LNCaP cells are not sensitive to TGF- β1, the proliferative dose of DHT in which these cells are sensitive to exogenous TGF-β1, and the growth-arrest dose of DHT in which LNCaP exhibits signs of TGF-β signaling but are insensitive to exogenous TGF-β1, respectively. Results of Western blot analysis showed that LNCaP cells express an increased level of type II receptor at 0.1 nM DHT, the TGF-β1- sensitive dose. However, results of competitive quantitative RT-PCR demonstrated that DHT did not significantly change the level of type II receptor mRNA, suggesting that DHT modulates the level of type II receptor at the posttranscriptional level. In contrast, ALK-5 was not detected in these cells by either Western blot analysis or RT-PCR at all concentrations of DHT used in this study. Subsequently, the expression of ALK-1, -2, and -4 in LNCaP cells was examined because these proteins have been shown to bind TGF- β1 in vitro. ALK-1 and -2 were detected in these cells. Further analysis by competitive quantitative RT-PCR and Western blot demonstrated that DHT did not affect the level of expression of ALK-1 and -2 in LNCaP cells. These observations, taken together, demonstrate that ALK-5 is not required for TGF- β1 signaling and that there may be alternative mechanism(s) for TGF-β1 signal transduction in some systems.