Down-regulation of Laminin-Binding Integrins by 1α,25=Dihydroxyvitamin D₃ in Human Melanoma Cells in Vitro

Abstract

In the present investigation the effect of 1α,25(OH)₂D₃ 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 contain high-affinity receptors for 1α,25(OH)₂D₃ and cell proliferation was found to be inhibited in a dose-dependent manner in response to the hormone. Using monoclonal antibodies against the α₆-sub-unit of the integrin laminin receptor, immunocyto-chemistry demonstrated that exposure of cells to 1α,25(OH)₂D₃ for 5 days caused a reduced staining intensity. This observation was further confirmed by dot blot analysis, where a dose-dependent decline of α₆expression was obtained after treatment of the cells with 1α, 25(OH)₂D₃ for 6 days. FACS-analysis was performed in order to quantify this decline, and it was found that the level of α6-subunits on the cell surface was reduced by more than 40%. Additional investigations including Northern blot analyses of poly(A)⁺RNA extracts also showed a dose-dependent reduction of α₆ mRNA. Interestingly, the decrease of α₆ expression on the surface of SK-MEL-28 melanoma cells was accompanied by a reduced ability of the cells to adhere to an artificial basement membrane. In conclusion, the present investigation shows that besides having an antiproliferative effect on the SK-MEG28 melanoma cells, 1α,25(OH)₂D₃ is also able to inhibit the surface expression of the α₆-Subunit of the integrin laminin receptor. Moreover, the results strongly indicate that 1α,25(OH)₂D₃ exerts its regulatory effect on the α₆-subunit at the transcriptional level rather than at the protein level.

Key Words:

• melanoma cells
• integrins
• laminin receptor
• invasion
• 1α
• 25-dihydroxyvitamin D₃