Activation of integrins by urea in perfused rat liver.
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 β(1) integrins followed by an activation of focal adhesion kinase, c-Src, p38(MAPK), extracellular signal-regulated kinases, and c-Jun N-terminal kinase. Because α(5)β(1) 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 p38(MAPK) inhibition. Molecular dynamics simulations of a three-dimensional model of the ectodomain of α(5)β(1) integrin in water, urea, or thiourea solutions revealed significant conformational changes of α(5)β(1) integrin in urea and thiourea solutions, in contrast to the simulation of α(5)β(1) 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 α(5)β(1) integrin system.
Reinehr
Roland
R
Clinic for Gastroenterology, Hepatology and Infectious Diseases, Heinrich-Heine-University Düsseldorf, D-40225 Düsseldorf, Germany.
Gohlke
Holger
H
Sommerfeld
Annika
A
Vom Dahl
Stephan
S
Häussinger
Dieter
D
D000818:Animals; D015153:Blotting, Western; D004789:Enzyme Activation / Q000187:drug effects; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D051416:Focal Adhesion Protein-Tyrosine Kinases / Q000378:metabolism; D022781:Hepatocytes / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D039081:Integrin alpha5beta1 / Q000235:genetics / Q000378:metabolism*; D048031:JNK Mitogen-Activated Protein Kinases / Q000378:metabolism; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D010477:Perfusion; D055503:Protein Multimerization; D017433:Protein Structure, Secondary; D051381:Rats; D017208:Rats, Wistar; D013890:Thiourea / Q000494:pharmacology; D014508:Urea / Q000494:pharmacology*; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism
D039081:Integrin alpha5beta1; D014508:Urea; D051416:Focal Adhesion Protein-Tyrosine Kinases; D048049:Extracellular Signal-Regulated MAP Kinases; D048031:JNK Mitogen-Activated Protein Kinases; D048051:p38 Mitogen-Activated Protein Kinases; D013890:Thiourea