Phosphorylated insulin-like growth factor binding protein 1 is increased in pregnant diabetic subjects

During pregnancy, IGFs and their binding proteins (IGFBPs) are important for the growth of fetal and maternal tissues. IGFBP-1 normally circulates as a single, highly phosphorylated species (hpIGFBP-1). However, in pregnancy there are lesser phosphorylated isoforms (lpIGFBP-1) with decreased affinity for IGF-I, allowing for increased IGF bioavailability. Because regulation of IGFBP-1 is abnormal in type 1 diabetes, we examined the impact of this on IGFBP-1 and its phosphorylation status in diabetic pregnancy. We assessed IGFBP-1 in relation to birth weight, maternal weight gain, duration of diabetes, glycemic control, and the presence or absence of retinopathy in 44 diabetic and 11 nondiabetic subjects. We found that in type 1 diabetic patients there was a significant negative relationship between hpIGFBP-1 and birth weight (r = -0.42, P < 0.01) and between the ratio of hpIGFBP-1 to lpIGFBP-1 and birth weight (r = -0.38, P = 0.02) by week 18 of gestation. Multiple regression analysis confirmed that hpIGFBP-1 was the best single predictor of birth weight (R2 = 0.3, P = 0.001) in diabetic subjects using models including other parameters known to influence fetal size. In contrast to hpIGFBP-1 levels, lpIGFBP-1 levels were not associated with birth weight, but were significantly related to initial maternal BMI and maternal weight throughout gestation in diabetic subjects (r = -0.57, P < 0.001). hpIGFBP-1 levels were positively related to duration of diabetes (r = 0.38, P < 0.01). Diabetic subjects had significantly higher hpIGFBP-1 and lpIGFBP-1 levels than nondiabetic subjects (hpIGFBP-1: 215 +/- 21 vs. 108 +/- 13 microg/l, P = 0.01; lpIGFBP-1: 139 +/- 12 vs. 66 +/- 5 microg/l, P < 0.001), but the ratio of hpIGFBP-1 to lpIGFBP-1 was similar in both groups (2.1 +/- 0.3 [diabetic] vs. 1.7 +/- 0.2 [nondiabetic], NS). In summary, maternal IGFBP-1 levels were higher in diabetic than in normal pregnancies. Diabetic subjects with prolonged duration of diabetes and retinopathy had higher total IGFBP-1 levels than those with shorter disease duration. Thus hpIGFBP-1 in diabetic pregnancy is positively related to the duration of diabetes and inversely related to fetal growth, with lpIGFBP-1 being related to maternal weight and BMI. The ratio of hpIGFBP-1 to lpIGFBP-1 may be a more robust indicator of fetal outcome, since it was consistent between diabetic and nondiabetic subjects. Measurement of the different phosphorylated isoforms of IGFBP-1 may increase the usefulness of IGFBP-1 as a predictor of fetal growth in both normal and diabetic pregnancy.