Hypertonic induction of the cell adhesion molecule β₁-integrin in MDCK cells

Cells of many organisms adapt to osmotic stress by accumulating compatible organic osmolytes. In Madin-Darby canine kidney (MDCK) cells, a renal epithelial cell line widely used as a culture model for the study of osmotic regulation in mammals, extracellular hypertonicity induces genes responsible for the accumulation of organic osmolytes. We have recently cloned from these cells a partial cDNA corresponding to a member of the transmembrane 4 superfamily (tetraspan) family, CD9 antigen, and demonstrated its induction by hypertonicity [D. Sheikh-Hamad, J. D. Ferraris, J. Dragolovich, H. G. Preuss, M. B. Burg, and A. Garcia-Perez, Am. J. Physiol. 270 (Cell Physiol. 39): C253-C258, 1996]. An association between CD9 and potential regulatory proteins, such as the heparin-binding epidermal growth factor-like protein and the cell adhesion molecule β₁-integrin, has been reported. Here, using β₁-integrin-specific monoclonal antibodies and an antisense oligonucleotide probe, we demonstrate expression and hypertonic induction of β₁-integrin in MDCK cells. Induction of the mRNA and protein occurs in 2 h and is maximal at 6 h, consistent with a regulatory role in the adaptation to osmotic stress. In addition, we show that accumulation of organic osmolytes markedly attenuates the hypertonic induction of the mRNA, a feature shared with genes involved in hyperosmotic stress response. Finally, we demonstrate that CD9 and β₁-integrin are expressed in association at the cell membrane. Our findings suggest the existence of a cluster of integral membrane proteins that includes but may not be limited to CD9 and the adhesion molecule β₁-integrin, which may play a role in the adaptation of kidney cells to osmotic stress, possibly at the regulatory level.