Insulin-induced proteolysis of the insulin receptor alpha-subunit from rat liver does not occur in vivo but is prevented in vitro by blood serum proteinase inhibitors.
Insulin binding to rat liver plasma membranes promotes the action of a plasma membrane proteinase that degrades the 135-kDa alpha-subunit of the insulin receptor to a 120kDa product. It has been proposed that this proteolysis may be the initial step leading to down-regulation of the hepatic insulin receptor [Lipson, K. E., Kolhatkar, A. A. & Donner, D. B. (1988) J. Biol. Chem. 263, 10495-10501]. Our results confirm that liver plasma membrane fractions from intact or perfused rat liver and from isolated rat hepatocytes do have a proteinase that degrades the alpha-subunit of the insulin receptor, whose activity increases significantly in the presence of insulin. In contrast to these in vitro results, plasma membranes and Golgi fractions isolated from liver at different times after a single intravenous injection of insulin to rats only contained intact alpha-subunits of the insulin receptor. Insulin administration was associated with a rapid and reversible translocation of insulin receptors from the plasma membranes to Golgi fractions but did not affect their total numbers or the receptor half-life. Incubation of rat liver plasma membranes with human, bovine or rat blood sera totally blocked alpha-subunit proteolysis both in the absence or presence of insulin. The three major serum proteinase inhibitors, alpha 1-antitrypsin, alpha 2-macroglobulin and antithrombin III, blocked alpha-subunit proteolysis of the insulin receptor to a varying extent. alpha 1-antitrypsin exhibited the highest potency, decreasing the amount of 120-kDa product by 70%. When both alpha 1-antitrypsin and antithrombin III were present, inhibition increased to 85%. Thus, the absence of proteolysis of the hepatic insulin receptor in vivo could be mostly accounted for by the antiproteolytic activity of blood serum. These findings suggest that insulin receptor alpha-subunit proteolysis does not represent a mechanism involved in the down-regulation of the insulin receptor in liver.
Sánchez-Casas
P
P
Departamento de Bioquímica y Biología Molecular, Facultad de Medicina, Universidad Complutense, Madrid, Spain.
Yusta
B
B
Blázquez
E
E
D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D015536:Down-Regulation; D006056:Golgi Apparatus / Q000378:metabolism; D006207:Half-Life; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism* / Q000494:pharmacology*; D008099:Liver / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D010477:Perfusion; D011480:Protease Inhibitors / Q000097:blood / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D011972:Receptor, Insulin / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D000515:alpha 1-Antitrypsin / Q000494:pharmacology
D007328:Insulin; D011480:Protease Inhibitors; D000515:alpha 1-Antitrypsin; D011972:Receptor, Insulin