Age-related changes in insulin receptor mRNA and protein expression in genetically obese Zucker rats.
OBJECTIVE
The mechanisms underlying the age-related decrease in insulin-receptor (IR) binding in genetically obese Zucker rats are not well understood. For this reason, the present study analyzed the expression of IR mRNA and protein in selected tissues from 1- to 4-month-old obese (fa/fa) Zucker rats and lean (Fa/-) age-matched controls.


METHODS
The following parameters were evaluated: (1) IR mRNA level, and proportion of isotypes A (exon 11-) and B (exon 11+) of IR mRNA in liver, brain and kidney; (2) level, molecular size and tyrosine phosphorylation of IR-beta subunit in liver subcellular fractions; and (3) stability of liver IR based on sensitivity in vivo of insulin-binding activity and IR-beta levels in response to tunicamycin, a glycosylation inhibitor.


RESULTS
At one month, IR mRNA level was increased in liver and brain, but decreased in kidneys and, at four months, both mRNA level and isotype B proportion were decreased in liver. From age two months, the following changes in liver IR protein expression were observed: (1) decreased IR-beta level in whole homogenates, but increased IR-beta levels in endosomal fractions; (2) increased IR-beta tyrosine phosphorylation; and (3) at four months, increased levels of both intact IR-beta (95 kDa) and IR-beta fragments (72 and 52 kDa) in lysosomal fractions, along with decreased stability in vivo of the IR.


CONCLUSIONS
These data show that obese Zucker rats display age-related alterations of IR gene expression at both pre- and post-translational stages and, in particular, increased endocytosis and degradation of IR protein.
Amessou
M
M
Inserm, U674, Paris, France.
Tahiri
K
K
Chauvet
G
G
Desbuquois
B
B
D000367:Age Factors; D000375:Aging / Q000235:genetics / Q000378:metabolism*; D000704:Analysis of Variance; D000818:Animals; D015153:Blotting, Western; D015870:Gene Expression / Q000187:drug effects; D020869:Gene Expression Profiling / Q000379:methods; D008297:Male; D009765:Obesity / Q000235:genetics / Q000378:metabolism*; D009928:Organ Specificity; D010766:Phosphorylation; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D055550:Protein Stability; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D055647:Rats, Transgenic; D011924:Rats, Zucker; D011972:Receptor, Insulin / Q000096:biosynthesis* / Q000235:genetics; D054411:Receptors, Leptin / Q000235:genetics; D014415:Tunicamycin / Q000494:pharmacology
D020033:Protein Isoforms; D012333:RNA, Messenger; D054411:Receptors, Leptin; D014415:Tunicamycin; D011972:Receptor, Insulin