The cellular effects of insulin-like growth factor I (IGF-I) are modified by a family of binding proteins (IGFBPs) that act as reservoirs in serum for the growth factor and are produced locally by tissues, including the kidney. Because regulation of these proteins may influence renal repair, either directly or by their interactions with IGF-I, we studied gene expression during the recovery from renal failure induced by folic acid and during the compensatory increase in renal function following uninephrectomy (UNX). Expression of IGF-I, the IGF-I receptor (IGF-IR), and all six IGFBPs was detected using an ribonuclease protection assay. IGFBP-5 was the most abundant binding protein mRNA present in kidney, whereas IGFBP-2 and -6 were the least abundant. During regeneration following folic acid-induced acute renal failure, IGF-I, IGFBP-3, and IGFBP-5 mRNAs declined in abundance approximately two- to threefold. On the other hand, IGF-IR, IGFBP-1, and IGFBP-2 were increased (approximately 2-, 6-, and 6-fold, respectively) in the first 24 h. IGFBP-1 mRNA remained elevated for at least 3 days. Despite the known increase in cellular RNA content following UNX, little difference in specific expression of mRNAs was observed. Because IGFBP-1 has been shown to stimulate cell migration and has previously been localized to the distal nephron, the site of greatest injury in the folic acid model, these data are compatible with the notion that this protein may function either directly to affect cellular repair or act as a reservoir for IGF-I under conditions of cellular damage.
This study examines growth-regulating adaptations made by the proximal nephron in response to hypophysectomy (HYPX). Fourteen days after HYPX, circulating insulin-like growth factor I (IGF-I) levels were diminished, averaging 97 +/- 7 compared with 650 +/- 69 ng/ml in controls (n = 5, P < 0.001). Similar data were observed at day 7. Binding of 125I-IGF-I to isolated glomerular membranes and proximal tubule basolateral membranes (BLM) was increased in HYPX rats. Affinity labeling of membranes with 125I-IGF-I followed by electrophoresis on 6% polyacrylamide gels demonstrated two bands, one of approximately 140 kDa and another of > 200 kDa. The lower-molecular-mass protein, which has been identified as the alpha-subunit of the IGF-I receptor, and the higher-molecular-mass species were both upregulated by HYPX. Ligand blotting with IGF-I demonstrated a 31-kDa protein in both membranes, identified by immunostaining as IGF binding protein 5 (IGFBP-5), not IGFBP-1 or IGFBP-2. Affinity labeling documented an upregulation of this protein in both membranes after HYPX. Ligand blotting demonstrated a 31-kDa protein in HYPX cortical but not normal cortical or medullary cytosol that was immunostained with IGFBP-5 antibodies. RNA prepared from normal kidney cortical tissue demonstrated a 6.0-kb IGFBP-5 transcript, which was increased at day 14 after HYPX. Adaptations in the kidney after HYPX include an upregulation of the IGF-I receptor as well as IGFBP-5.(ABSTRACT TRUNCATED AT 250 WORDS)
Insulin-like growth factor (IGF)-binding proteins and the IGF-I receptor in rat kidney were studied to gain perspective on their potential roles in the control of renal cell mass. Thirty days after nephrectomy (UNx), membranous whole-kidney binding of IGF-I averaged 9.5 +/- 1.0 pmol/kidney, whereas binding to sham-operated controls (SNx) averaged 6.3 +/- 0.8 pmol/kidney (N = 6; P < 0.01). Scatchard analysis of IGF-I binding per milligram of protein to glomerular membranes or proximal tubule basolateral membranes (BLM) at Day 30 did not reveal significance differences in Bmax or KD between SNx and UNx; however, protein per glomerulus increased from 361 +/- 33 ng/glomerulus in SNx animals to 826 +/- 123 ng/glomerulus in UNx rats (N = 6; P < 0.002). Histomorphometrics documented an increase in proximal tubule circumference per cell and an increase in the basolateral/basement membrane ratio. IGF-I affinity labeling studies demonstrated three proteins in both glomerular membranes and proximal tubule BLM; molecular weight approximately 140,000 d, the alpha subunit of the IGF-I receptor, a protein > 200,000 d, and a protein approximately 31,000 d that was immunostained with IGF-binding protein-5 antibodies. Differences in expression between SNx and UNx were not observed at Day 30 in either glomeruli or BLM. These studies suggest that cytosolic hypertrophy of proximal nephron structures is accompanied by membrane hypertrophy with a fixed density of IGF-I receptor and IGF-binding protein-5.
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