Tönshoff B, Powell D R, Zhao D, Durham S K, Coleman M E, Domené H M, Blum W F, Baxter R C, Moore L C, Kaskel F J
Department of Pediatrics and Physiology, State University of New York at Stony Brook 11794, USA.
Endocrinology. 1997 Mar;138(3):938-46. doi: 10.1210/endo.138.3.4977.
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.
正常胰岛素样生长因子-I（IGF-I）与显著升高的IGF结合蛋白（IGFBP）血浆水平之间的失衡，在慢性肾功能衰竭儿童的生长发育迟缓及分解代谢中起致病作用。为研究这些改变的机制，我们在大鼠尿毒症实验模型（5/6肾切除）以及配对喂养和自由进食的假手术对照组中进行了实验。使用特异性溶液杂交/RNase保护分析法，我们观察到尿毒症动物肝脏中IGF-I信使核糖核酸（mRNA）丰度在稳态时显著降低（为对照组的37±5%），与配对喂养组（65±10%）和自由进食对照组（100±11%）相比均有显著差异（P<0.001）。IGF-I基因表达降低具有明显的器官特异性；在肝脏中最为显著（与配对喂养对照组相比有显著差异），在肺和肌肉组织中也较为明显（与自由进食对照组相比有显著差异）；在肾脏和心脏组织中未观察到变化。为确定尿毒症时肝脏IGF-I基因表达降低的潜在机制，通过特异性溶液杂交/RNase保护分析法分析了同一实验动物肝脏生长激素（GH）受体基因表达。与对照组相比，尿毒症动物肝脏GH受体mRNA丰度降低了20% - 30%。尿毒症时肝脏GH结合蛋白（GHBP）表达也相应降低。尽管肝脏IGF-I mRNA丰度降低，但尿毒症患者血浆IGF-I水平与自由进食对照组并无差异。这种差异可由尿毒症血浆中IGFBPs浓度升高来解释。通过放射免疫分析法（RIA），尿毒症患者血浆IGFBP-1水平升高了4倍；通过Western免疫印迹法，血浆IGFBP-2水平升高了7倍，血浆IGFBP-4水平升高了2倍，与两个对照组相比均有显著差异。完整的IGFBP-3（分子量约48 kDa）和低分子IGFBP-3片段在三组之间无显著差异。通过Northern印迹分析，尿毒症时肝脏IGFBP-1 mRNA水平比对照组高2倍。肝脏组织中IGFBP-2 mRNA丰度升高了4倍，而在肾脏中IGFBP-2 mRNA显著降低（为对照组的30%）。肾脏中IGFBP-4 mRNA升高了50%，但肝脏中未升高。各组间血浆胰岛素和皮质酮水平无差异。我们的研究表明，尿毒症时肝脏IGF-I基因表达特异性降低，部分原因是肝脏GH受体基因表达降低。尿毒症时IGFBPs水平升高的机制之一是肝脏IGFBP-1和IGFBP-2基因表达增加。肝脏IGF-I生成减少与肝脏IGFBP-1和2生成增加之间的失衡，可能在慢性肾功能衰竭的分解代谢和生长障碍中起致病作用。
