Specific internalization of basal membrane domains containing the integrin alpha 6 beta 4 in dispase-detached cultured human keratinocytes.

The integrin alpha 6 beta 4 is polarized towards the basal side of basal keratinocytes and helps anchor these cells to the basement membrane components. We have found that cultured human epidermal keratinocytes, when detached from their culture substratum, as for grafting, using the enzyme dispase, rapidly internalize the basal membrane domains containing the integrin alpha 6 beta 4, while integrins of the very late antigen subtype remain on the cell surface. Detachment and incubation at 4 degrees C prevent this internalization, as well as the contraction of the detached sheet area. Subsequent incubation at 37 degrees C initializes this contraction and allows the basal integrin alpha 6 beta 4 to be internalized. We took advantage of this blockage to label upon detachment using immunogold techniques, the alpha 6 subunit present on the basal cell surface; then we studied its internalization with the electron microscope. This internalization pathway differs from classical receptor-mediated endocytosis, and intermediate filaments might possibly play a role in this process. Interestingly, 1 h after their internalization from the basal membrane, a third of the gold particles labeling the alpha 6 subunit was found between lateral membranes of basal cells, strongly suggesting that the integrin alpha 6 beta 4 can be partly recycled to the cell surface in these conditions.