Renal Insulin-like Growth Factor Research Articles

Genetic low nephron number hypertension is associated with dysregulation of the hepatic and renal insulin-like growth factor system during nephrogenesis

Low nephron number may represent a major determinant of human primary hypertension in adult life. This hypothesis is supported by a genetic rat model, namely the Munich-Wistar-Frömter (MWF) rat, which demonstrates an inherited deficit in nephron number and the development of spontaneous hypertension. Insulin-like growth factor (IGF) I and II exert endocrine and paracrine effects that are required for normal growth and nephron development. We tested the hypothesis that low nephron number is already present during fetal development, and the expression pattern of important molecules of the IGF system is altered in MWF rat during the critical period of kidney development. We compared MWF and normal Wistar rats during nephrogenesis at day 19 of fetal development (E19) and adult rats at postnatal day 100 (D100). Histomorphometric analysis was performed by stereological methods. Quantitative messenger RNA and protein expression was determined by real-time polymerase chain reaction and enzyme-linked immunosorbent assay. At E19, glomerular density (-32%) and hepatic mRNA (-48%) and protein (-18%) expression of IGF-I were decreased (P < 0.05, respectively), whereas renal mRNA expression of IGF-II receptor (+52%) and IGF binding protein 3 (+113%) were increased in MWF compared with Wistar rats (P < 0.05, respectively). Systolic blood pressure, urinary albumin excretion, and mean glomerular area were significantly elevated in MWF compared with Wistar rats at D100 (P < 0.05, respectively). The fetal expression of IGF system molecules in the MWF rat model points towards a link between the decreased availability of active IGF-I and IGF-II and the fetal development of low nephron number, with manifestation of genetic hypertension in adult life.

Early hyperplastic renal growth after uninephrectomy in adult female rats.

The early, accelerated remnant kidney growth following uninephrectomy (UNX) occurs through alternate mechanisms in juvenile and adult male rats, which may govern the type of renal growth that occurs after UNX. Early compensatory renal growth (CRG) in the adult male rat is GH dependent, but independent of changes in the renal insulin-like growth factor I (IGF-I) system. In contrast, CRG is GH independent in the juvenile male rat, but is associated with significant increases in the renal IGF-I system, and hyperplastic kidney growth. The few studies that examined early CRG in female animals suggest that remnant kidney growth is less than that observed in males, and there is a hyperplastic component, indicating potential gender differences. Whether these differences result from alternate growth mechanisms is unknown. The purpose ofthe present study was to determine the rate, type, and potential mechanism of early remnant kidney growth in adult female rats after UNX. GH levels were determined in conscious, sham-operated, and UNX adult female Wistar rats 24 h postsurgery. Unlike previous findings in adult male UNX rats, pulsatile GH levels were not elevated in UNX female rats. When GH release was suppressed using an antagonist to GH-releasing factor, remnant kidney growth was not different from that in saline/UNX remnant kidneys (25.7+/-4.8% vs. 27.7+/-2.1%, respectively, at 48 h post-UNX). This GH-independent CRG was associated with significant hyperplastic growth in both adult andjuvenile female remnant kidneys, as determined by bromodeoxyuridine incorporation and increases in total DNA. Also associated with the mitogenic growth in the adult female were significant 2- to 4-fold increases in remnant kidney IGF-I receptor gene expression, which occurred in the presence and absence of pulsatile GH secretion. Lastly, the growth rate of adult female remnant kidneys was not different from that observed in male remnant kidneys at these early time points (0.21+/-0.02 vs. 0.20+/-0.02 g at 24 h, and 0.26+/-0.02 vs. 0.30+/-0.03 g at 48 h in female and male remnant kidneys, respectively; P = NS). Thus, in female rats, the initial phase of CRG is GH independent, but is associated with significant increases in remnant kidney IGF-I receptor gene expression and hyperplastic renal growth. This, in addition to previous findings, indicates that there are sex differences in early CRG after UNX. Moreover, the findings confirm that the mechanism governing the initial phase of CRG appears to be a critical determinant for significant hyperplastic remnant kidney growth.

Aminoguanidine ameliorates changes in the IGF system in experimental diabetic nephropathy.

Formation of advanced glycation end-products (AGEs) has been implicated in the development of diabetic complications. As well as causing changes in structural proteins, AGEs may also alter gene expression of growth factors in vitro. The insulin-like growth factor (IGF) system, including IGF-I and modulatory IGF binding proteins (IGFBPs), is dysregulated during the development of diabetic nephropathy. Quantitative in situ hybridization histochemistry and immunohistochemistry were used to determine the effects of aminoguanidine, an inhibitor of AGE formation, on gene expression of IGF-I and IGFBPs in kidneys of long-term (8 months duration) streptozotocin-diabetic rats. Diabetes was associated with increased renal expression of IGFBP-1 mRNA (diabetes 824+/-236 vs control 264+/-76 arbitrary units, P<0.01) and decreased expression of mRNAs for IGF-I (diabetes 39+/-7 vs control 185+/-23 arbitrary units, P<0.001) and IGFBP-4 (diabetes 139+/-25 vs control 383+/-54 arbitrary units, P<0.001). Aminoguanidine treatment inhibited the effects of diabetes on renal expression of mRNA for IGF-I, IGFBP-1 and IGFBP-4. The changes in IGF-I and IGFBP-1 mRNA levels were reflected in altered peptide levels. In diabetic kidneys, IGFBP-5 mRNA levels were slightly decreased to 75% of control levels (P<0.01); aminoguanidine had no effect on IGFBP-5 mRNA levels. These results suggest that amelioration of changes in the renal IGF system by aminoguanidine may contribute to the renoprotective effects of the latter, which have been previously shown to inhibit structural and functional aspects of diabetic nephropathy in the rat.

Octreotide prevents the early increase in renal insulin-like growth factor binding protein 1 in streptozotocin diabetic rats.

The early renal growth in streptozotocin (STZ)-induced diabetic rats is preceded by a transient rise in renal tissue insulin-like growth factor (IGF)-I concentration. Administration of the long-acting somatostatin analog octreotide to STZ diabetic rats inhibits the early increase in kidney IGF-I and the increase in kidney size without affecting metabolic control. We studied the effects of octreotide treatment on the intrarenal IGF axis at 2 and at 7 days after the induction of STZ diabetes. Two days after induction of diabetes, kidney IGF-I was increased from 850 +/- 43 ng/g tissue in controls to 1,648 +/- 165 ng/g tissue (P < 0.001) in diabetic animals. The diabetes-associated increase in renal IGF-I 48 h after STZ injection was totally prevented by octreotide (IGF = 780 +/- 57 ng/g tissue). However, 7 days after the induction of diabetes, kidney IGF-I was similar to that of control and was not affected by octreotide. No difference in serum IGF-I was observed between controls and diabetic rats after 2 days of diabetes; however, octreotide treatment resulted in a significant decrease of serum IGF-I after 2 days when compared with control rats (P < 0.05). Renal IGF-I mRNA was significantly decreased to the same extent in both diabetic groups 2 and 7 days after the induction of diabetes, while renal IGF-I receptor (IGF-IR) mRNA was unchanged in rats from either group. Two days after induction of diabetes, renal insulin-like growth factor binding protein (IGFBP)-1 mRNA and 30-kDa IGFBPs (containing IGFBP-1) increased by 186 and 192%, respectively, in untreated diabetic animals compared with controls. Octreotide treatment prevented the diabetes-associated rise in renal IGFBP-1 mRNA and protein. However, 7 days after the induction of diabetes, renal IGFBP-1 mRNA and protein were similarly increased in both octreotide-treated or untreated diabetic rats. Renal IGFBP-3 gene expression and protein and IGFPB-5 mRNA remained unchanged after 2 and 7 days of diabetes when treated or untreated with octreotide. We conclude that the well-known inhibitory effect of octreotide on the early increase in renal IGF-I concentration and renal size in diabetes may be mediated through a direct effect on renal IGFBP-1 levels.

The Insulin-Like Growth Factor–I Axis in Acute Renal Failure

We have examined the response of the renal insulin-like growth factor (IGF-I) axis to acute ischemic injury in the rat Key findings. included a de-crease in IGF-I mRNA and peptide levels, a decrease in GH receptor gene plus protein expression and a decrease in the IGF binding proteins except for IGF binding protein 1. Administration of GH to compensate for the reduced GH receptor binding corrected the IGF-I mRNA levels suggesting a relative GH deficiency. Interestingly, IGF-I receptor mRNA levels were unchanged while plasma membrane IGF-I receptor number increased two fold. This appeared to be due to a redistribution of receptors to a membrane location. IGF-I receptor autophosphorvlation and tyrosine kinase activity were intact despite severe uremia for up to 6 days. We propose that this increase of functional IGF-I receptors following acute tubular necrosis will sensitize the kidney to the administration of exogenous IGF-I.

Renal hypertrophy in hyperglycemic non-obese diabetic mice is associated with persistent renal accumulation of insulin-like growth factor I.

The non-obese diabetic mouse is a model of spontaneous insulin-dependent diabetes as a result of autoimmune destruction of pancreatic beta cells, similar to the disease seen in human Type I diabetes. This mouse strain develops glomerular lesions reminiscent of those seen in human disease. The study presented here investigated the changes in renal insulin-like growth factor (IGF) system in hyperglycemic non-obese diabetic mice. Female non-obese diabetic mice and their age- and sex-matched controls were euthanized 4 days, 2 wk, and 4 wk after the onset of glycosuria. Kidney weight increased in diabetic mice, beginning at 2 wk after the onset of glycosuria. This renal hypertrophy was associated with an increase in renal extractable IGF-I protein. However, a decrease in IGF-I mRNA was observed at the same time. Serum IGF-I levels remained stable after 2 wk of diabetes and decreased at 1 month. No change was detected in renal IGF-I receptor mRNA levels. Renal cortical IGF binding protein (IGFBP)-1 mRNA levels were increased. Ligand blot analysis revealed a significant increase in serum and renal 30-kd IGFBP and a decrease in serum and kidney IGFBP-3 and IGFBP-4 at 30 days of diabetes. Insulin therapy prevented the increases in kidney weight, renal IGF-I, and 30-kd IGFBP, but did not reverse the decreased serum IGF-I levels observed at 1 month of diabetes. In summary, renal hypertrophy in non-obese diabetic mice is associated with a persistent accumulation of renal IGF-I and, IGFBP-1. These changes were partially reversed with insulin therapy, which did not correct the hyperglycemia, suggesting an important role for insulin deficiency in mediating these changes in the IGF system. These findings suggest that the IGF system may play a potential role in the development of diabetic nephropathy.

Increased activity of the insulin-like growth factor system in mesangial cells cultured in high glucose conditions. Relation to glucose-enhanced extracellular matrix production.

Recent evidence suggests that several growth factors participate in diabetic glomerular disease by mediating increased extracellular matrix accumulation and altered cell growth and turnover leading to mesangial expansion. Transforming growth factor (TGF)-beta has been demonstrated to be upregulated both in vivo and in vitro, whereas studies on the activity of the renal insulin-like growth factor (IGF) system in experimental diabetes have provided conflicting results. We investigated the effects of prolonged exposure (4 weeks) of cultured human and rat mesangial cells to high (30 mmol/l) glucose vs iso-osmolar mannitol or normal (5.5 mmol/l) glucose levels on: 1) the autocrine/paracrine activity of the IGF system (as assessed by measuring IGF-I and II, IGF-I and II receptors, and IGF binding proteins); and, in parallel, on 2) TGF-beta 1 gene expression; 3) matrix production; and 4) cell proliferation. High glucose levels progressively increased the medium content of IGF-I and the mRNA levels for IGF-I and IGF-II, increased IGF-I and IGF-II binding and IGF-I receptor gene expression, and reduced IGF binding protein production. TGF-beta 1 transcripts and matrix accumulation and gene expression were increased in parallel, whereas cell proliferation was reduced. Iso-osmolar mannitol did not affect any of the above parameters. These experiments demonstrated that high glucose levels induce enhanced mesangial IGF activity, together with enhanced TGF-beta 1 gene expression, increased matrix production, and reduced cell proliferation. It is possible that IGFs participate in mediating diabetes-induced changes in matrix turnover leading to mesangial expansion, by acting in a paracrine/autocrine fashion within the glomerulus.

Enhanced renal IGF-I expression following partial kidney infarction

Renal insulin-like growth factor (IGF)-I expression is enhanced in tissue that remains following removal of kidney mass. To characterize the expression of renal IGF-I after reduction of kidney mass by partial unilateral infarction, we measured levels of IGF-I extracted from noninfarcted (remnant) renal tissue that remained after one-half unilateral kidney infarction that was performed without (1/2NX) or with (1 1/2NX) contralateral nephrectomy. Levels of IGF-I extracted from remnant renal tissue after 1/2NX increased significantly, peaking on day 3 after renal mass reduction, and then returned toward baseline. Steady-state levels of IGF-I mRNA were also elevated on day 3, suggesting that the increase in IGF-I results from enhanced synthesis. A similar pattern of increased extracted IGF-I and elevated IGF-I mRNA occurred after 1 1/2NX. Levels of IGF-I extracted from remnant renal tissue 3 days after 1 1/2NX were not higher than levels extracted from remnant tissue 3 days after 1/2NX, and both were higher than levels of IGF-I extracted from contralateral kidneys 3 days after unilateral nephrectomy. Therefore, levels of IGF-I did not correlate with the extent of reduction of renal mass per se. We conclude that partial renal infarction provides a stimulus to enhance IGF-I expression. Increased renal IGF-I must be considered in the interpretation of findings originating from use of remnant kidney models of chronic renal failure.

Renal growth hormone receptor gene expression: relationship to renal insulin-like growth factor system

In order to elucidate potential sites of direct GH action on the kidney, we used in situ hybridization to localize GH receptor (GHR) gene expression during the course of development and in the adult rat. In order to illuminate potential interactions between GH and insulin-like growth factor-I (IGF-I) in regulating renal function, we compared the anatomical localization of GHR messenger RNA (mRNA) with that for the IGF-I receptor and for IGF-I in the rat kidney. Low levels of GHR mRNA were present in the kidney from before birth and increased in abundance until postnatal day 40. Hypophysectomy resulted in a decrease and GH treatment resulted in an increase in renal GHR mRNA levels. Renal GHR mRNA was most abundant in the proximal straight tubule, with lesser levels present in the medullary thick ascending limb (MTAL), and it was not detected in the glomerulus or inner medulla. In contrast, IGF-I receptor mRNA was concentrated in the glomerulus, distal nephron and collecting system. The only point of convergence for GHR and IGF-I receptor mRNAs was in the MTAL, where IGF-I mRNA was localized. This segregation of GHR and IGF-I receptor gene expression in the kidney suggests that each hormone has distinct spheres of action along the nephron, with GH acting directly on the proximal straight tubule, whereas IGF-I may act on the glomerulus, distal nephron, and collecting duct. GHR expression in the MTAL, which is the site of renal IGF-I synthesis, supports the view that GH has a direct effect on renal IGF-I synthesis. Finally, it appears that in the kidney, as in other GH-sensitive tissues, GH may regulate its receptor levels.

Anatomical relationships in the patterns of insulin-like growth factor (IGF)-I, IGF binding protein-1, and IGF-I receptor gene expression in the rat kidney.

The rat kidney is both a target of circulating insulin-like growth factor-I (IGF-I) and a site of local IGF-I production. In order to identify which renal structures produce IGF-I and the functionally related IGF binding protein 1 (IGFBP-1), and which structures are potential sites of circulating or endogenous renal IGF action, we have employed in situ hybridization to localize IGF-I, IGFBP-1, and IGF-I receptor messenger RNAs (mRNAs) in the rat kidney. The effects of hypophysectomy (Hx) and GH replacement on renal IGF-I, IGFBP-1, and IGF-I receptor gene expression have also been evaluated. IGF-I and IGFBP-1 mRNAs are both localized in the epithelial cells of medullary thick ascending limbs (TALs) of Henle's loops in the normal rat kidney. IGF-I receptor mRNA is also abundant in TALs, but, in addition, is distributed throughout the distal nephron and collecting duct, and in the glomerulus, with lowest levels found in proximal tubules. Hx and GH treatment had complex effects on patterns of renal IGF-I and IGFBP-1 gene expression. In general, Hx resulted in decreased IGF-I and increased IGFBP-1 mRNA levels, and GH treatment produced the opposite effects, while IGF-I receptor mRNA levels were not significantly effected by either treatment. However, the most dramatic effect produced by the interruption of the pituitary-renal axis was the demonstration of reciprocal changes in IGF-I vs. IGFBP-1 gene expression in individual kidneys and even in individual nephrons, suggesting a local interaction between IGF-I and IGFBP-1 in the regulation of their respective mRNA levels. Functional implications issuing from these anatomical relationships in renal patterns of IGF-I, IGFBP-1, and IGF-I receptor gene expression are that IGF-I, if secreted into the tubular lumen, possibly carried or modulated by IGFBP-1, may act on luminal TAL and downstream receptor sites. The specific physiological role of IGF-I produced in TALs is open to speculation. Glomerular IGF-I receptor sites, based on their localization upstream and distant from local sources of IGF-I production, are predicted to be targets for circulating IGFs.

Transient increase in renal insulin-like growth factor binding proteins during initial kidney hypertrophy in experimental diabetes in rats.

The insulin-like growth factors, insulin-like growth factor I and insulin-like growth factor II are bound to six distinct classes of insulin-like growth factor binding proteins (IGFBPs) in the circulation and in extracellular fluids. Diabetic renal hypertrophy is preceded by a transient increase in kidney insulin-like growth factor I suggestive of a renotropic function for insulin-like growth factor I. In order to examine a possible involvement of IGFBPs in initial diabetic kidney growth and in kidney insulin-like growth factor I accumulation, we studied rat kidney IGFBPs by ligand blotting during the first 4 days after induction of diabetes. Six distinct bands were identified in kidney and liver tissue with apparent molecular weight values of 38-47 (doublet), 34, 30, 24 and 20 kDa. The 38-47 kDa doublet band probably corresponds to the insulin-like growth factor binding subunit of IGFBP-3, the 24 kDa band to IGFBP-4 and the 30 kDa band to IGFBP-1 and/or IGFBP-2, as these IGFBPs in rats have similar molecular weight. In untreated diabetic rats a transient increase in the kidney 30 kDa band was demonstrable 24 h after induction of diabetes with a maximal rise (two-fold) after 48 h, followed by a decrease to baseline values after 4 days. In untreated diabetic rats the 38-47 kDa doublet band also increased (two-fold) in kidney during the first 2 days after induction of diabetes, followed by a subsequent decrease. Insulin-treatment prevented both the increase in the 30 kDa and in the 38-47 kDa bands.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of insulin-like growth factor I infusion on renal hypertrophy in experimental diabetes niellitus in rats

Initial diabetic renal hypertrophy is preceded by a transient increase in kidney insulin-like growth factor I suggesting that insulin-like growth factor I may be implicated in diabetic kidney growth. The present study was undertaken to examine the effects of exogenous insulin-like growth factor I infusion on diabetic renal hypertrophy at a time when renal insulin-like growth factor I concentration had returned to normal and the initial steep kidney growth rate had diminished to a much slower rate. Groups of rats with diabetes duration of 5 days were infused s.c. for 4 subsequent days with equimolar concentrations of insulin-like growth factor I (36 nmol/day) or insulin (35 nmol/day). Insulin infusion lowered blood glucose to a normal level within 2 days and induced an average body-weight gain of 9.3 +/- 0.6 g/day. Insulin-like growth factor I infused diabetic rats maintained the original diabetic state with blood glucose levels comparable to those of 0.154 mol/l NaCl-infused diabetic rats, but had nevertheless an average body-weight gain of 6.8 +/- 1.0 g/day while untreated diabetic rats had a lower body-weight gain amounting to 3.3 +/- 0.8 g/day (p less than 0.01). The kidney weight at day 9 in untreated diabetic animals was about 25% greater than that of non-diabetic control animals, while in insulin-like growth factor I treated diabetic rats a further increase (p less than 0.05) was seen, amounting to 36% above control level. No increase was seen in the insulin-treated diabetic group.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal insulin-like growth factor I and growth hormone receptor binding in experimental diabetes and after unilateral nephrectomy in the rat.

We have measured specific binding of insulin-like growth factor I and growth hormone to renal plasma membranes from control, streptozotocin-diabetic, insulin-treated diabetic, uninephrectomised and combined diabetic-uninephrectomised male Wistar rats. Control, insulin-treated and uninephrectomised rats had similar body weights after 7 days (243 +/- 2 g), whereas diabetic and diabetic-uninephrectomised animals were significantly lighter (219 +/- 4 and 203 +/- 4 g, p less than 0.05). Blood glucose concentrations were similar in the diabetic and diabetic-uninephrectomised animals (around 26 mmol/l) but significantly lower in the insulin-treated group. Right kidney weight increased by 14% in the control, insulin-treated and sham-nephrectomised animals, by 33% in the diabetic group, 38% in thenephrectomised animals and 60% in the diabetic-nephrectomised group. The renal content of insulin-like growth factor I was similar and stable in the control, insulin-treated and sham-nephrectomised animals (208 +/- 14 ng/g wet weight) but rose to a peak of 669 +/- 35 ng/g in the diabetic group (p less than 0.001), 871 +/- 34 ng/g in the nephrectomised animals (p less than 0.001) and 1012 +/- 43 ng/g in the diabetic-uninephrectomised group (p less than 0.001). Maximum binding of insulin-like growth factor I fell on day 1 in the diabetic group (8.3 +/- 1.4 vs 5.2 +/- 0.71 x 10(-11) mol/l; p less than 0.01) but thereafter was identical to control animals.(ABSTRACT TRUNCATED AT 250 WORDS)