Placental alkaline phosphatase de-phosphorylates insulin-like growth factor (IGF)-binding protein-1

BACKGROUND

Insulin-like growth factors (IGF) regulate fetal growth through their effects on placenta. Their actions are influenced by IGF binding protein-1. Phosphorylated IGFBP-1 (pIGFBP-1) has high affinity for IGF-I and usually inhibits IGF-I activity but during pregnancy, it is de-phosphorylated to generate lower affinity isoforms and consequently, increased IGF bioavailability. Here we investigate the role of placenta in this process.

RESULTS

Our data show that term human placental explants, but not their conditioned medium, can de-phosphorylate IGFBP-1 through the action of placental alkaline phosphatase (PLAP).

DISCUSSION

PLAP-mediated de-phosphorylation of IGFBP-1 may provide a mechanism for controlling IGF-I bioavailability and action at the maternal/fetal interface.

Insulin-like growth factor binding protein-2 (IGFBP-2) is a marker for the metabolic syndrome

AIMS/HYPOTHESIS

IGFs and their binding proteins are increasingly recognised as important in understanding the pathogenesis of cardiovascular disease. Low IGFBP-1, particularly coupled with low IGF-I, is associated with increased cardiovascular risk. In relation to structural and regulatory parallels between IGFBP-1 and - 2 we have now examined the hypothesis that IGFBP-2 may be a marker for cardiovascular risk.

METHODS

Fasting IGFBP-2, IGFBP-1, IGFBP-3, IGF-I, IGF-II, insulin, C-peptide, glucose, lipids, NEFAs, and HbA1c were measured in a cohort of 163 patients with type 2 diabetes. Individuals were categorised according to the presence or absence of the metabolic syndrome.

RESULTS

Patients with the metabolic syndrome had a lower IGFBP-2 concentration. Low circulating IGFBP-2 was associated with elevated fasting glucose (rho = - 0.23, p = 0.003). IGFBP-2 correlated negatively with triglycerides (rho = - 0.19, p = 0.01) and LDL-cholesterol (rho = - 0.20, p = 0.01), and positively with insulin sensitivity (HOMA-S) (rho = 0.26, p = 0.02). Multivariate logistic regression demonstrated that low IGFBP-2 was independently associated with an increased risk of the metabolic syndrome (OR 0.31 [95 % CI 0.11 - 0.90]; p = 0.03). IGFBP-3 did not differ according to the presence or absence of metabolic syndrome.

CONCLUSION/INTERPRETATION

Low IGFBP-2 is associated with multiple cardiovascular risk factors similarly to IGFBP-1. Such associations were not apparent for IGFBP-3. Lack of marked prandial regulation of IGFBP-2, in contradistinction to IGFBP-1, may make IGFBP-2 a more robust biomarker for identification of insulin-resistant individuals at high cardiovascular risk in epidemiological studies.

IGF-binding protein-1 inhibits IGF effects on adipocyte function: implications for insulin-like actions at the adipocyte

IGF action in vivo is acutely regulated by IGF-binding protein-1 (IGFBP-1) and its phosphorylation state is implicated in modulating these effects. Since IGFs have an important regulatory role in adipocyte function, we investigated the effects of phosphorylated IGFBP-1 (pIGFBP-1) and non-phosphorylated IGFBP-1 (npIGF BP-1) on 3T3-L1 preadipocyte proliferation and adipocyte metabolism. IGFs stimulated clonal expansion of 3T3-L1 cells (IGF-I more potently than IGF-II (EC(50): 30 nM and 50 nM)). npIGFBP-1 inhibited IGF-I (50 nM) clonal expansion at a 5:1 molar ratio (P<0.01), whereas pIGFBP-1 (purified from HepG2 cell medium) abolished clonal expansion at a 1:1 molar ratio (P<0.005). In contrast, IGF-II-induced clonal expansion was inhibited 100% at a 1:1 molar ratio of npIGFBP-1. In mature adipocytes, IGF-I was equipotent with insulin in stimulating glucose uptake (EC(50): 10 nM) and inhibiting isoproterenol-induced lipolysis (EC(50): 15 nM). npIGFBP-1 completely reversed IGF-I effects at a 1:1 molar ratio (P<0.01). In summary, IGFs rather than insulin are potent regulators of clonal expansion in 3T3-L1 preadipocytes. Importantly, IGFs are equipotent with insulin in regulating adipocyte metabolism. IGFBP-1 inhibits IGF effects on preadipocyte proliferation and adipocyte metabolism, with pIGFBP-1 being more potent than npIGFBP-1 at inhibiting mitogenic actions. Since IGFBP-1 is acutely regulated by insulin, this could have important consequences in hyperinsulinaemic and insulin-resistant states.

Low circulating levels of insulin-like growth factor binding protein-1 (IGFBP-1) are closely associated with the presence of macrovascular disease and hypertension in type 2 diabetes

The IGF system is increasingly implicated in the development of cardiovascular disease. The effects of circulating IGFs on the vasculature are largely modulated by IGFBPs, which control their access to cell-surface IGF receptors. IGFBP-1 has been proposed as the acute regulator of IGF bioavailability because of its metabolic regulation by glucoregulatory hormones. Posttranslational phosphorylation of IGFBP-1 significantly increases its affinity for IGF-I and therefore represents a further mechanism for controlling IGF bioavailability. We have therefore examined the IGF system and IGFBP-1 phosphorylation status, using specifically developed immunoassays, in a cohort of 160 extensively characterized type 2 diabetic subjects on two occasions 12 months apart. Total IGFBP-1 (tIGFBP-1), which is predominantly highly phosphorylated, was significantly lower in subjects with known macrovascular disease (geometric mean [95% CI], 48.7 microg/l [33.7-63.6]) than in patients with no vascular pathology (80.0 microg/l [52.2-107]; F = 5.4, P = 0.01). A similar relationship was found for highly phosphorylated IGFBP-1 (hpIGFBP-1) concentration (known macrovascular disease, 45.1 microg/l [35.1-55.2]; no macrovascular disease, 75.8 microg/l [56.2-95.3]; F = 4.8, P = 0.01). Logistic regression showed that for every decrease of 2.73 microg/l in IGFBP-1 concentration, there was a 43% increase in the odds of a subject having macrovascular disease (odds ratio 0.57 [95% CI 0.40-0.83]; P = 0.001). hpIGFBP-1 correlated negatively with systolic blood pressure (rho = -0.30, P < 0.01), diastolic blood pressure (rho = -0.45, P < 0.001), and mean arterial pressure (MAP) (rho = -0.41, P < 0.001). Linear regression modeling showed that 40% of the variance in tIGFBP-1 was accounted for by MAP, triglycerides, and nonesterified fatty acids. In contrast, levels of nonphosphorylated and lesser-phosphorylated IGFBP-1 (lpIGFBP-1) were unrelated to macrovascular disease or hypertension but did correlate positively with fasting glucose concentration (rho = 0.350, P < 0.01). tIGFBP-1 concentrations were higher in subjects treated with insulin alone (n = 29) than for any other group. This effect persisted after adjustment of tIGFBP-1 levels for BMI, C-peptide, age, and sex (F = 6.5, P < 0.001, rho = - 0.46). Such an effect was not apparent for lpIGFBP-1. We conclude that low circulating levels of hpIGFBP-1 are closely correlated with macrovascular disease and hypertension in type 2 diabetes, whereas lpIGFBP-1 isoforms are associated with glycemic control, suggesting a dual role for IGFBP-1 in the regulation of IGF actions in type 2 diabetes. Our data suggest that high circulating concentrations of highly phosphorylated IGFBP-1 may protect against the development of hypertension and cardiovascular disease by reducing the mitogenic potential of IGFs on the vasculature.