Decreased levels of hepatic epidermal growth factor receptors in obese hyperglycemic rodents

Stimulation of epidermal growth factor (EGF) receptor autophosphorylation by EGF and phosphorylation of a Mr 52,000 protein endogenous to the membrane extracts were decreased 6-12-fold in liver membrane extracts from mice homozygous for either the ob/ob or db/db mutation when compared to controls. Liver membranes from the mutant mice bound 4-5-fold less 125I-EGF/unit of protein than did their normal littermates, but exhibited normal EGF binding affinity. Similar decreases in EGF binding were noted in liver membranes from homozygous fa/fa Zucker rats, another obese, hyperinsulinemic animal model, when compared to values from control animals. We also immunoprecipitated hepatic EGF receptors from mice injected with [35S]methionine, and found that livers from db/db mice contained approximately 35% of the labeled EGF receptors found in control animals. Both ob/ob and db/db mice had serum immunoreactive EGF levels similar to or lower than those found in unaffected littermates, suggesting that ligand-mediated down-regulation of receptors was not the cause of the decreased EGF binding. In one mutant, db/db, the decreased binding was associated with a 6-fold decrease in the levels of liver EGF receptor mRNA transcripts; in the ob/ob mice, at most a 2-fold decrease in the level of liver EGF receptor transcripts was observed. EGF binding to cultured peritoneal fibroblasts derived from db/db mice was normal, suggesting that the abnormality in the mutant mice might result from altered environmental or tissue-specific factors rather than an abnormal receptor gene. This was supported by Southern blot analysis of DNA from these animals, which showed identical restriction fragment patterns for the EGF receptor gene in both control and mutant animals. These data indicate that three distinct strains of obese hyperglycemic rodents have decreased levels of hepatic EGF receptors, and suggest that this decrease may result from altered environmental stimuli or tissue-specific factors rather than a primary defect in the EGF receptor gene.