Insulin-like growth factor binding protein 1 response to acute insulin induced hypoglycaemia in type 1 diabetes

Influence of circulating epinephrine and norepinephrine on insulin-like growth factor binding protein-1 in humans

The aim of the present study was to investigate the influence of circulating epinephrine (Epi) and norepinephrine (Norepi) on serum insulin-like growth factor binding protein-1 (IGFBP-1) concentrations. Healthy men received 0.3 nmol.kg.min Epi iv (n = 6), 0.5 nmol.kg.min Norepi iv (n = 7), or saline (n = 5) during 30 min. Arterial blood samples were obtained before, during, and 120 min after infusion. During the catecholamine infusion arterial Epi and Norepi plasma concentrations reached 6.35 +/- 0.53 and 15.65 +/- 2.71 nmol/L, respectively, which resulted in significant increases in glucose concentrations. When Epi was infused, IGFBP-1 increased from 45 +/- 6 micrograms/L to 76 +/- 10 micrograms/L (P < 0.05) 60 min after the infusion. Epi was also followed by increases in insulin, C-peptide, and glucagon. Norepi resulted in a slight increase in circulating IGFBP-1 (43 +/- 6 to 54 +/- 8 nmol/L, NS). The findings suggest that Epi, at plasma concentrations similar to those reached during physical stress, stimulates the production of IGFBP-1 in humans.

Characterization of insulin-like growth factor binding proteins and regulation of IGFBP3 in human mesangial cells

IGF-I regulates renal growth and development. Insulin-like growth factor binding proteins (IGFBPs) are synthesized by the kidney and may modulate the local autocrine and/or paracrine actions of IGF-I. We have previously demonstrated that mesangial cells (MC) release IGF-I and IGF-binding activity; however, the specific IGFBPs produced by these cells and the factors involved in their regulation are unknown. We examined MC for expression of IGFBP-1 to -6 mRNAs and proteins. RNase protection assays using total RNA demonstrated that MC express all of the IGFBPs. [125I]IGF-I Western ligand blot of conditioned medium demonstrated that MC release IGFBPs of 24, 29, 32 kDa, and a doublet at 46 kDa, consistent with IGFBP-4, -5, -2 and -3, respectively. IGFBP species of 28 and 34 kDa were also detected. Since IGF-I and TGF-beta are implicated in glomerular hypertrophy and matrix expansion, we tested their effect on IGFBPs released by MC. IGF-I (100 ng/ml), TGF-beta (2 ng/ml) and forskolin (10(-5) M) differentially regulated the abundance of IGFBPs released in the conditioned medium in a time-dependent manner. IGF-I and TGF-beta were potent inducers of the release of IGFBP3 protein; however, TGF-beta, but not IGF-I, increased IGFBP3 mRNA levels. Recombinant IGFBP3 was tested for its effect on IGF-I-induced mitogenesis. IGFBP3 inhibited IGF-I-stimulated DNA synthesis in a dose-dependent manner with a peak effect observed at 50 nM IGFBP3. Although TGF-beta is a potent inhibitor of IGF-I-stimulated DNA synthesis, this effect is not mediated via IGFBPs. Expression of IGFBP-1 to -6 by MC suggests that these proteins may modulate IGF-I bioavailability in the glomerulus. IGF-I itself, TGF-beta and cAMP agonists may indirectly modulate the effects of IGF-I via the release of IGFBPs by MC.

Insulin-like growth factor binding proteins as glucoregulators

Circulating insulin-like growth factors (IGFs) represent an important pool of potential hypoglycemic activity, which is largely inhibited by their sequestration in a heterotrimeric complex comprising growth factor, IGF-binding protein-3 (IGFBP-3), and acid-labile subunit (ALS). Less than 1% of total IGFs circulate in the free form, yet even this amount might contribute significantly to circulating insulin-like activity. The ternary binding protein complex appears to inhibit insulin-like activity of bound IGFs by preventing their egress from the circulation. Although the integrity of this complex might be affected by limited proteolysis of IGFBP-3 in pregnancy and catabolic conditions, the evidence that this increases IGF bioavailability, and thus hypoglycemic potential, is as yet unclear. However, in patients with IGF-II-secreting tumors, hypoglycemia may result from a failure of the ternary complex to adequately sequester the IGFs. Improvement in complex formation, by treatment with corticosteroids or growth hormone, alleviates the hypoglycemia, even if (as seen with growth hormone treatment) IGF-II hypersecretion persists. In these patients, blood glucose levels are inversely correlated with IGFBP-2 levels, suggesting that this protein might play a part in transporting IGFs to their target tissues. Conversely, ALS levels correlate positively with blood glucose, emphasizing the importance of the ternary complex in preventing hypoglycemia. Unlike the other IGF-binding proteins, IGFBP-1 is acutely regulated in the circulation, in a manner consistent with its acting as a glucose counterregulator. It might act in this way by inhibiting the activity of free IGFs in the circulation.(ABSTRACT TRUNCATED AT 250 WORDS)