Risk factors for prostate cancer (PCa) include age, hormones, race, family history and diet. Recently, epidemiologic evidence has indicated that history of diabetes mellitus (DM) is inversely associated with risk of PCa. However, epidemiological investigations have yielded inconsistent results. Hence, the exact mechanism of DM-induced reduction in the incidence of PCa has yet to be fully elucidated. The aim of this study was to investigate the effects of DM factors, including glucose, insulin and insulin-like growth factor-1 (IGF-1), on the proliferation of PCa cell lines in vitro. Cell proliferation and expression of hormone receptors was examined in MTT assay and Western blot analysis, respectively. The results showed that DM factors did not affect the viability of androgen receptor (AR)-expressing PCa cell lines. However, cell proliferation increased after treatment with DM factors in androgen-independent PCa cell lines. On PCa tissue arrays, intensities of total AR and nuclear IGF-1R were higher in malignant tissues than in normal prostate glands. In terms of hormonal receptors, androgen-dependent LNCaP cells treated with insulin and IGF-1 in a low-serum medium showed decreased expression of insulin receptor beta (IRβ) and elevated expression of IGF-1 receptor beta (IGF-1Rβ). Moreover, expression of AR was upregulated after insulin and IGF-1 treatment in LNCaP cells, but not in the other PCa cell lines. Most of the studied antidiabetic drugs promoted the viability of PCa cells. However, metformin decreased the viability of AR-expressing PCa cells. These results suggest that diabetic factors modify the expression of AR, IR and IGF-1R to increase cancer cell proliferation. Moreover, the growth suppressing effects of metformin on PCa may be via the regulation of the AR signaling pathway.