1α,25-dihydroxyvitamin D3 inhibits in vitro invasiveness through the extracellular matrix and in vivo pulmonary metastasis of B16 mouse melanoma

We investigated the role of 1 alpha,25-dihydroxyvitamin D-3 (1 alpha,25(OH)(2)D-3) in modulating tumor cell invasiveness through the extracellular matrix (ECM) and pulmonary metastasis in B16 mouse melanoma. The pretreatment of B16 cells for 48 hours with 1 alpha,25(OH)(2)D-3 significantly inhibited in vitro invasiveness through the ECM by a mechanism that is not directly correlated with the inhibition of cell proliferation. When cells were treated with 1 alpha,25(OH)(2)D-3 for only 8 hours during the assay, no inhibitory effect was observed, suggesting that pretreatment with the hormone for more than 8 hours is necessary to inhibit the invasive potential of B16 cells. The activity of B16 cells to adhere to reconstituted basement membrane (Matrigel) and type IV collagenolysis was inhibited by pretreatment of the cells with 1 alpha,25(OH)(2)D-3 for 48 hours. Cell motility was not influenced by the hormone. Mice were inoculated subcutaneously with 3 x 10(6) B16 cells and were given 1 alpha,25(OH)(2)D-3 (0.5 mu g/kg) or vehicle daily for 28 days, beginning 1 day after tumor inoculation. In the 1 alpha,25(OH)(2)D-3-treated group, no significant inhibition in exponential tumor growth, body weight, and serum level of calcium was observed until the twenty-eighth day. The mean serum concentration of the hormone was about 50 ng/mL, and there were no significant changes in its concentration during the treatment period. In both spontaneous and experimental metastasis models of tumor-bearing mice, treatment with 1 alpha,25(OH)(2)D-3 inhibited pulmonary metastasis. These findings suggest that 1 alpha 25(OH)(2)D-3 acts on B16 cells, inhibiting invasiveness through the ECM that is caused by the inhibition of cell adhesion to the ECM and the degradation of the ECM by the cells. 1 alpha 25(OH)(2)D-3 may have the potential to inhibit metastasis by a mechanism that is not exclusively based on its anti-cell proliferative effect.