Members of Shc family conventionally serve as critical adaptors in tyrosine phosphorylation signal transduction pathways. p66Shc protein, a member of Shc family, is predominantly expressed in epithelial cells, whereas the regulation of its expression remains an enigma. We describe the effect of steroid hormones on the protein level of p66Shc and growth stimulation in hormone-sensitive human prostate, testicular and breast cancer cells. In DHT-treated androgen-sensitive prostate cancer LNCaP C-33 cells, the protein level of p66Shc was elevated by approximately 3-fold, correlating with increased cell growth. This DHT effect on p66Shc protein level and growth regulation was also observed in another androgen-sensitive prostate cancer cell line MDA PCa2b as well as 2 testicular cancer cell lines, Tera-1 and Tera-2 cells. Similarly, the female sex hormone estrogen had a stimulating effect on p66Shc protein level and proliferation in estrogen-sensitive MCF-7 breast cancer cells. The upregulation of p66Shc protein level by DHT was competitively abolished by Casodex, an androgen antagonist used to treat prostate cancer. Moreover, immunohistochemical analyses showed that the p66Shc protein level was significantly higher in primary prostate tumors than in adjacent non-cancerous cells (p < 0.05). The data collectively indicate that p66Shc protein levels correlate with steroid hormone-stimulated cell growth and prostate carcinogenesis. © 2003 Wiley-Liss, Inc.