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Decreased hepatic insulin-like growth factor (IGF)-I and increased IGF binding protein-1 and -2 gene expression in
B Tönshoff1, D R Powell, D Zhao
1Department of Pediatrics and Physiology, State University of New York at Stony Brook 11794, USA. Burkhard_Toenshoff@krzmail.krz.uni-heidelberg.de
Insights
Chronic renal failure in children causes growth problems due to an imbalance in insulin-like growth factor-I (IGF-I) and IGF binding proteins (IGFBPs). Uremia reduces IGF-I gene expression and increases IGFBP production, contributing to growth failure.
Area of Science:
- Endocrinology
- Nephrology
- Molecular Biology
Background:
- Chronic renal failure (CRF) in children is associated with growth retardation and catabolism.
- This is linked to an imbalance between insulin-like growth factor-I (IGF-I) and IGF binding proteins (IGFBPs).
- The underlying mechanisms require investigation in an experimental model.
Purpose of the Study:
- To investigate the mechanisms behind altered IGF-I and IGFBP levels in uremia.
- To analyze hepatic IGF-I and growth hormone (GH) receptor gene expression in a rat model of uremia.
- To determine the impact of uremia on the expression of various IGFBPs.
Main Methods:
- Experimental model of uremia in rats (5/6 nephrectomy).
- Pair-fed and ad libitum-fed sham-operated control groups.
- Solution hybridization/RNase protection assay for gene expression analysis (IGF-I, GH receptor, IGFBPs).
- RIA and Western immunoblot for plasma IGFBP quantification.
Main Results:
- Uremic rats showed significantly reduced hepatic IGF-I mRNA abundance compared to controls.
- Hepatic GH receptor mRNA abundance was also reduced in uremic animals.
- Plasma levels of IGFBP-1, IGFBP-2, and IGFBP-4 were significantly increased in uremia, while IGFBP-1 and IGFBP-2 mRNA levels were elevated in the liver.
Conclusions:
- Hepatic IGF-I gene expression is specifically reduced in uremia, partly due to decreased GH receptor expression.
- Increased hepatic gene expression of IGFBP-1 and IGFBP-2 contributes to elevated plasma IGFBP levels in uremia.
- The imbalance between reduced IGF-I production and increased IGFBP production likely drives catabolism and growth failure in CRF.
Abstract:
The imbalance between normal insulin-like growth factor-I (IGF-I) and markedly increased IGF binding protein (IGFBP) plasma levels plays a pathogenic role for growth retardation and catabolism in children with chronic renal failure. To investigate the mechanism of these alterations, experiments were performed in an experimental model of uremia in rats (5/6 nephrectomy) and in pair-fed and ad libitum-fed sham-operated controls Using a specific solution hybridization/RNase protection assay, we observed a marked reduction of hepatic IGF-I messenger RNA (mRNA) abundance at steady state in uremic animals (37 +/- 5% of control) compared both with pair-fed (65 +/- 10%) and ad libitum-fed controls (100 +/- 11%) (P < 0.001). Reduced IGF-I gene expression was clearly organ-specific; it was most pronounced in liver (significant vs., pair-fed controls) and lung and muscle tissue (significant vs., ad libitum-fed controls); no change was observed in kidney and heart tissue. To determine a potential mechanism of reduced hepatic IGF-I gene expression in uremia, the hepatic GH receptor gene expression in the same experimental animals was analyzed by specific solution hybridization/RNase protection assay. Uremic animals had a 20-30% reduction of hepatic GH receptor mRNA abundance compared with controls. Hepatic GHBP expression in uremia was decreased in parallel. Despite the reduction of hepatic IGF-I mRNA abundance, plasma IGF-I levels in uremia were not different from ad libitum-fed controls. This discrepancy is explained by an increased concentration of IGFBPs in uremic plasma. By RIA, plasma IGFBP-1 levels in uremia were increased 4-fold; by Western immunoblot, plasma IGFBP-2 levels were increased 7-fold and plasma IGFBP-4 levels were increased 2-fold compared with both control groups. Intact IGFBP-3 (M(r), approximately 48 kDa) and low molecular IGFBP-3 fragments were not significantly different among the three groups. By Northern blot analysis, hepatic IGFBP-1 mRNA levels in uremia were 2-fold higher than in controls. IGFBP-2 mRNA abundance in liver tissue was increased 4-fold, whereas in kidney there was a significant reduction of IGFBP-2 mRNA (30% of control). IGFBP-4 mRNA was increased by 50% in kidney but not in liver. Plasma insulin and corticosterone levels were not different among the groups. Our study shows that hepatic IGF-I gene expression was specifically reduced in uremia, partially as the consequence of a reduced hepatic GH receptor gene expression. One of the mechanisms contributing to increased IGFBP levels in uremia is increased hepatic gene expression of IGFBP-1 and IGFBP-2. The imbalance between reduced hepatic IGF-I production and increased hepatic IGFBP-1 and 2 production is likely to play a pathogenic role for catabolism and growth failure in CRF.