The growth and progression of prostate cancer are dependent on androgens and androgen receptor (AR), which act by modulating gene expression. Utilizing a gene microarray approach, we have identified the a1-subunit gene of soluble guanylyl cyclase (sGC) as a novel androgen-regulated gene. A heterodimeric cytoplasmic protein composed of one a and one b subunit, sGC mediates the widespread cellular effects of nitric oxide (NO). We report here that, in prostate cancer cells, androgens stimulate the expression of sGCa1. A cloned human sGCa1 promoter is activated by androgen in an AR-dependent manner, suggesting that sGCa1 may be a direct AR target gene. Disruption of sGCa1 expression severely compromises the growth of both androgendependent and androgen-independent AR-positive prostate cancer cells. Overexpression of sGCa1 alone is sufficient for stimulating prostate cancer cell proliferation. Interestingly, the major growth effect of sGCa1 is independent of NO and cyclic guanosine monophosphate, a major mediator of the sGC enzyme. These data strongly suggest that sGCa1 acts in prostate cancer via a novel pathway that does not depend on sGCb1. Tissue studies show that sGCa1 expression is significantly elevated in advanced prostate cancer. Thus, sGCa1 may be an important mediator of the procarcinogenic effects of androgens.