1,25-Dihydroxyvitamin D3 and PI3K/AKT Inhibitors Synergistically Inhibit Growth and Induce Senescence in Prostate Cancer Cells

BACKGROUND. 1-Alpha, 25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) inhibits proliferation of multiple cancer cell types including prostate cells and upregulates p21 and/or p27, while loss of Pten and PI3K/AKT activation stimulates survival and downregulates p21 and p27. We hypothesized that inhibition of the PI3K/AKT pathway synergizes with the antiproliferative signaling of 1,25(OH)(2)D-3. METHODS. Viability, cell cycle and senescence of cells were evaluated upon combinational treatment with 1,25(OH)(2)D-3 and pharmacological PI3K/AKT inhibitors. RESULTS. Pharmacological inhibitors of PI3K or Akt and 1,25(OH)(2)D-3 synergistically inhibited growth of DU145, LNCaP, primary human prostate cancer cell strains and Pten null mouse prostatic epithelial cells (MPEC). The inhibitors used included API-2 (Triciribine) and GSK690693 which are currently in clinical trials for treatment of cancer. A novel mechanism for antiproliferative effects of 1,25(OH)(2)D-3 in prostate cells, induction of senescence, was discovered. Combination of 1,25(OH)(2)D-3 and AKT inhibitor cooperated to induce G(1) arrest, senescence, and p21 levels in prostate cancer cells. As AKT is commonly activated by PTEN loss, we evaluated the role of Pten in responsiveness to 1,25(OH)(2)D-3 using shRNA knockdown and by in vitro knockout of Pten. MPEC that lost Pten expression remained sensitive to the antiproliferative action of 1,25(OH)(2)D-3, and showed higher degree of synergism between AKT inhibitor and 1,25(OH)(2)D-3 compared to Pten-expressing counterparts. CONCLUSIONS. These findings provide the rationale for the development of therapies utilizing 1,25(OH)(2)D-3 or its analogs combined with inhibition of PI3K/AKT for the treatment of prostate cancer. Prostate 70: 1658-1671, 2010. (C) 2010 Wiley-Liss. Inc.