IGF-I transcript levels in whole-liver tissue, in freshly isolated hepatocytes, and in cultured hepatocytes from lean and obese Zucker rats.
BACKGROUND
The mechanisms underlying the maintenance of normal to high rates of linear growth and plasma insulin-like growth factor I (IGF-I) levels in spite of a low growth hormone secretion in obese children remain unknown. Among the animal models of early-onset obesity, obese Zucker (FA/FA) rats (which are homozygous for an inactivating missense mutation in the leptin receptor) are particularly appropriate, because their linear growth shows this growth hormone independence.


METHODS
To study the regulation of IGF-I synthesis in this model, we have established primary cultures of hepatocytes derived from 12-week-old Zucker male obese and lean rats. The rat IGF-I gene contains six exons, and alternative splicing generates different mRNAs, one of which (called IGF-1B) has been shown to be decreased by fasting. We report steady state mRNA levels for IGF-I (all transcripts) and for IGF-IB in hepatocytes after 3 days in culture, in freshly isolated hepatocytes, and in whole-liver tissue. RT-PCRs using primers specific for IGF-I or IGF-IB were performed with two different internal competitors for quantification.


RESULTS
In primary cultures of hepatocytes, the IGF-IB mRNA was increased by >50-fold (p = 0.01) in cells derived from obese animals as compared with cells from lean animals. However, these transcript levels were not significantly different when measured in freshly isolated hepatocytes or in whole-liver tissue.


CONCLUSIONS
Increased IGF-IB transcription could be an intrinsic characteristic of cultured hepatocytes harbouring leptin receptors that bear the FA mutation. However, the modulation of this characteristic by cell-cell interactions and by in vivo hormone and metabolic status remains to be studied.
Tenoutasse
S
S
Research Unit on the Biology of Reproduction and Development, Research Center, Ste-Justine Hospital, Montreal, Que., Canada.
Van Vliet
G
G
Ledru
E
E
Deal
C
C
D000818:Animals; D001835:Body Weight / Q000502:physiology; D002469:Cell Separation; D002478:Cells, Cultured; D018076:DNA, Complementary / Q000096:biosynthesis / Q000235:genetics; D022781:Hepatocytes / Q000276:immunology / Q000378:metabolism*; D066298:In Vitro Techniques; D007334:Insulin-Like Growth Factor I / Q000096:biosynthesis*; D008099:Liver / Q000276:immunology / Q000378:metabolism*; D008297:Male; D009765:Obesity / Q000276:immunology / Q000378:metabolism*; D012333:RNA, Messenger / Q000096:biosynthesis; D051381:Rats; D011924:Rats, Zucker; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014158:Transcription, Genetic
D018076:DNA, Complementary; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I