AbstractThe RRR‐α‐tocopheryl succinate derivative of vitamin E, referred to as vitamin E succinate (VES), inhibits the proliferation of three metastatic human prostatic cancer cell lines, LNCaP, PC‐3, and DU‐145. LNCaP is a lymph node‐derived androgen‐sensitive prostate cell line; these cells are defective for response to transforming growth factor‐β(TGF‐β) but are normal for cell cycle‐related tumor suppressor genes: p53 and retinoblastoma (Rb). PC‐3 is a bone marrow‐derived androgen‐insensitive prostate cell line; these cells are defective for both p53 alleles but normal for both Rb alleles. DU‐145 is a brain‐derived androgen‐insensitive prostate cell line; these cells are defective for both p53 and both Rb alleles. VES at 5, 10, and 20μg/ml inhibited DNA synthesis in the three cell lines in a dose‐dependent manner. Purified TGF‐β1at 1 ng/ml inhibited DNA synthesis of PC‐3 cells within 24–72 hours and DU‐145 cells at 72 hours but did not inhibit DNA synthesis of LNCaP cells. Previous studies in our laboratory showed that VES growth‐inhibited tumor cells secrete biologically active antiproliferative factor TGF‐βs, suggesting that VES's mechanism of growth inhibition may involve the TGF‐βsystem of growth control. The significance of the studies reported here is that VES can inhibit the proliferation of 1) cells that have lost the ability to respond to TGF‐βs (i.e., LNCaP), 2) cells that are defective for Rb and p53 (i.e., DU‐145), and 3) cells that are defective for p53 (i.e., PC‐3), critical cell cycle regulatory molecules. Taken together, these studies show that VES most likely inhibits the proliferation of tumor cells via mechanism(s) in addition to and independent of TGF‐β, as well as the functional status of Rb and p53.