Activation of integrins by urea in perfused rat liver

Abstract

High concentrations of urea were shown to induce a paradoxical regulatory volume decrease response with K+ channel opening and subsequent hepatocyte shrinkage (Hallbrucker, C., vom Dahl, S., Ritter, M., Lang, F., and Häussinger, D. (1994) Pflügers Arch. 428, 552-560), although the hepatocyte plasma membrane is thought to be freely permeable to urea. The underlying mechanisms remained unclear. As shown in the present study, urea (100 mmol/liter) induced within 1 min an activation of β1 integrins followed by an activation of focal adhesion kinase, c-Src, p38MAPK, extracellular signal-regulated kinases, and c-Jun N-terminal kinase. Because α5β1 integrin is known to act as a volume/osmosensor in hepatocytes, which becomes activated in response to hepatocyte swelling, the findings suggest that urea at high concentrations induces a nonosmotic activating perturbation of this osmosensor, thereby triggering a volume regulatory K+ efflux. In line with this, similar to hypo-osmotic hepatocyte swelling, urea induced an inhibition of hepatic proteolysis, which was sensitive to p38MAPK inhibition. Molecular dynamics simulations of a three-dimensional model of the ectodomain of α5β1 integrin in water, urea, or thiourea solutions revealed significant conformational changes of α 5β1 integrin in urea and thiourea solutions, in contrast to the simulation of α5β1 in water. These changes lead to an unbending of the integrin structure around the genu, which may suggest activation, whereas the structures of single domains remained essentially unchanged. It is concluded that urea at high concentrations affects hepatic metabolism through direct activation of the α5β 1 integrin system. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.