Osteocalcin (OC) is a small (6 kDa) polypeptide whose expression was thought to be limited to mature osteoblasts. The discovery of OC expression in prostate cancer specimens led us to study the regulation of OC gene in androgen-independent (AI) metastatic human prostate PC3 cells. An 800 bp human OC (hOC) promoter-luciferase construct exhibited strong basal and vitamin D-induced activity in OC-positive human prostate and osteosarcoma cell lines. Through deletion analysis of the hOC promoter, the functional hierarchy of the cis-acting elements, OSE1, OSE2 and AP-1/VDRE, was established in PC3 cells (OSE1>AP-1/VDRE>OSE2). By juxtaposing dimers of these 3 cis-elements, we produced a minimal hOC promoter capable of displaying high tissue-specific activity in prostate cancer cells. Our study demonstrated three groups of transcription factors, Runx2, JunD/Fra-2 and Sp-1, responsible for the high hOC promoter activity in PC3 cells by binding to the OSE2, AP-1/VDRE and OSE1 elements, respectively. Among the three groups of transcription factors, the expression levels of Runx2 and Fra-2 are higher in the OC-positive PC3 cells and osteoblasts, compared to the OC-negative LNCaP cells. Interestingly, unlike the mouse OC promoter, the OSE1 site in hOC promoter is regulated by members of Sp-1 family instead of the osteoblast specific factor Osf1. The molecular basis for AI prostate cancer cells behaving like mature osteoblasts may be explained by the interplay and coordination of these transcription factors under the tight regulation of autocrine and paracrine mediators.