DOWN-REGULATION OF LAMININ-BINDING INTEGRINS BY 1-ALPHA,25-DIHYDROXYVITAMIN D-3 IN HUMAN-MELANOMA CELLS IN-VITRO

In the present investigation the effect of 1 alpha,25(OH)(2)D-3 on the expression of the integrin laminin receptor on the melanoma cell line SK-MEL-28 has been examined. The SK-MEL-28 cells were shown to contai n high-affinity receptors for 1 alpha,25(OH)(2)D-3 and cell proliferat ion was found to be inhibited in a dose-dependent manner in response t o the hormone, Using monoclonal antibodies against the alpha(6)-sub-un it of the integrin laminin receptor, immunocytochemistry demonstrated that exposure of cells to 1 alpha,25(OH)(2)D-3 for 5 days caused a red uced staining intensity. This observation was further confirmed by dot blot analysis, where a dose-dependent decline of alpha(6)-expression was obtained after treatment of the cells with 1 alpha,25(OH)(2)D-3 fo r 6 days. FAGS-analysis was performed in order to quantify this declin e, and it was found that the level of alpha(6)-subunits on the cell su rface was reduced by more than 40%. Additional investigations includin g Northern blot analyses of poly(A)(+)RNA extracts also showed a dose- dependent reduction of alpha(6) mRNA. Interestingly, the decrease of a lpha(6) expression on the surface of SK-MEL-28 melanoma cells was acco mpanied by a reduced ability of the cells to adhere to an artificial b asement membrane. In conclusion, the present investigation shows that besides having an antiproliferative effect on the SK-MEL-28 melanoma c ells, 1 alpha,25(OH)(2)D-3 is also able to inhibit the surface express ion of the alpha(6)-subunit of the integrin laminin receptor. Moreover , the results strongly indicate that 1 alpha,25(OH)(2)D-3 exerts its r egulatory effect on the alpha(6)-subunit at the transcriptional level rather than at the protein level.