This study examined whether four factors (adiponectin, TNFα, leptin and hsCRP) with associations to insulin sensitivity were altered in pregnancy and whether they could be related to the changes of insulin resistance or insulin secretion of pregnancy, particularly in women with GDM.
Adipocytokine and CRP levels in pregnancy and postpartum
In the pregnant state, TNFα levels were elevated, in accordance with previous literature 3, 4, 21. TNFα was further elevated in pregnant GDM compared to their BMI-matched pregnant control subjects. This is also consistent with previous findings 3, 4, although, to our knowledge, this is the first study to document higher TNFα levels in GDM pregnancies compared to a closely BMI-matched group of healthy control women.
No significant change in adiponectin values from the third trimester to four months postpartum emerged, consistent with another recent study 22. Our GDM subjects did not have significantly different adiponectin levels to the controls, although there was a slight trend for the healthy control women to have higher levels postpartum.
Our finding of elevated leptin in pregnancy may be due to increased secretion from adipocytes in the presence of elevated oestrogen 23 and placental production 24, and is consistent with previous reports 15, 25. We observed reduced leptin levels in GDM compared to that in control subjects. Reports of leptin levels in GDM pregnancy are conflicting, with investigators reporting similar 14, reduced 15 and elevated 13 leptin levels in GDM women compared to healthy pregnancy controls.
HsCRP is an acute phase reactant, elevated levels of which are associated with features of the metabolic syndrome 26, and have also been documented to be elevated in pregnancy 27. Previous reports have shown that where BMI and adiposity are taken into account, hsCRP is not significantly associated with GDM 28, 29. Our finding in closely BMI-matched subjects confirms that hsCRP is linked to adiposity, but not glucose tolerance, in GDM women. There was no association with SI suggesting that the elevation of hsCRP observed is not an important cause or consequence of the reduction of SI in pregnancy.
Adipocytokines and CRP: relationship to insulin sensitivity
In the postpartum state, all of the adipocytokines studied demonstrated their expected association with SI, which confirms the robust nature of our methodology with pregnant and postpartum samples for each subject measured in the same assay.
In pregnancy, only leptin retained a relationship with SI, an association that has been noted previously 3, although it remains unknown if leptin has a direct effect on SI in pregnancy.
In contrast to previous investigators, we did not observe an association between TNFα and SI during pregnancy in either GDM or control women, though this relationship was apparent postpartum. Kirwan et al. noted a correlation between TNFα and SI in a group of 15 women in the third trimester, divided equally into lean normal glucose tolerant (NGT), obese NGT and GDM groups 3. The explanation for the difference in our findings and those of Kirwan et al. may lie in the BMI range of subjects tested. Our GDM and control subjects with an average postpartum BMI of 28 kg/m2 correspond to the five GDM subjects (BMI 30.8 kg/m2) and five obese NGT subjects (BMI 27.3 kg/m2) respectively in the study by Kirwan et al. where the serum TNFα was not significantly different (2.84 ± 0.17 vs 2.80 ± 0.72 pg/mL), despite a difference in SI (4.9 ± 0.8 vs 9.5 ± 1.510−2 mg/kg FFM/min/µU/mL). It appears that the association between TNFα and SI in pregnancy is less evident in this overweight group of subjects. A cross-sectional study by Winkler et al4, who found an association between fasting C-peptide and TNFα also tested a leaner group of women, and this relationship was not apparent on multivariate analysis once BMI was accounted for, which is in keeping with our findings.
In our subjects there, was no association between adiponectin and SI in pregnancy, which is consistent with Ranheim et al. who reported no correlation between Adiponectin and fasting insulin during pregnancy in 51 pregnant women 6. In contrast, Cseh et al7 found reduced adiponectin levels in the second and third trimester compared to the first trimester in a cross-sectional study of healthy pregnant non-diabetic subjects. They postulated that reduced adiponectin has a role in the insulin resistance of pregnancy and especially of GDM where they found further reductions. Our prospective study does not suggest such a role for adiponectin.
Our observation of reduced SI in former GDM women postpartum is in keeping with the findings reported by previous investigators 9, 30, and, given the association of adiponectin with SI, one would expect that, in a larger sample, GDM women would have statistically significantly reduced adiponectin levels. Indeed, a recent study of 180 women found reduced adiponectin levels in GDM 12. However, we would speculate that this was due to their underlying lower insulin sensitivity in the non-pregnant state rather than changes in pregnancy, as suggested by Winzer et al's study of 89 former GDM subjects 31. Our paired pregnant and postpartum data highlights that the association of adiponectin with SI in pregnancy is weaker than that observed postpartum.
Adiponectin and TNFα produce opposite effects on insulin signalling—with TNFα inhibiting 32 and adiponectin increasing 33 tyrosine phosphorylation of the insulin receptor. TNFα may inhibit the synthesis of adiponectin 5. The ratio of these cytokines may therefore be an important determinant of insulin sensitivity. In our subjects, postpartum adiponectin/TNFα correlated with SI and its related parameters of TG, HDL, % fat and BMI, and had a higher correlation with SI than either adiponectin or TNFα alone.
Lactation and the oral contraceptive pill use (by a minority of subjects) may have had an impact on adiponectin levels and SI in our subjects. Combs et al. noted an inhibitory effect of prolactin and oestrogen on adiponectin levels in mice 34, which may be important in our study given that 32/40 subjects were lactating at the time of the postpartum study. However, the adiponectin levels were consistent with previous studies for women 35, 36 and we found no evidence of altered adiponectin levels in the non-breastfeeding subjects (non-lactating: mean adiponectin 6.9 ± 1.7 vs lactating 8.4 ± 0.6µg/mL, p = NS).
There are profound alterations in lipid levels during normal pregnancy. Under the influence of elevated oestradiol, TG, HDL and cholesterol synthesis are stimulated and levels increase by 200–310%, 15–40% and 30–65% respectively 37, 38. The normal relationships between plasma TG, HDL and SI are to some extent modulated by these hormonal influences. However, our finding of an association between the changes in HDL and TG with the changes in SI indicates that there is still an influence of SI on HDL and TG in pregnancy. The question then arises as to whether this relationship may exist for the adipocytokines. There was no association, however, between % change TNFα, adiponectin or leptin with change in SI for each individual, which suggests that in our study population neither TNFα nor adiponectin are responding to or driving the changes in SI during pregnancy.
Adipocytokines and CRP: relationship to insulin secretion
In GDM subjects, the physiological decrease in SI in pregnancy may unmask an abnormality in glucose metabolism, which is not apparent in the non-pregnant state. Thus, it is the defect in insulin secretion, which differentiates those women who develop GDM from those who maintain normal glucose tolerance 39. A novel finding of this study was the association between serum TNFα and acute insulin secretion—AIRg. In pregnancy, for all subjects, TNFα correlated with AIRg, although this only remained significant for the GDM group postpartum. Furthermore, for GDM subjects, there was a significant association between TNFα and disposition index in pregnancy. This raises the possibility that in pregnancy TNFα could be having a deleterious impact on insulin secretion in GDM subjects. This concept is supported by in vitro evidence of a TNFα inhibiting beta cell function 40, 41.
We also observed an association between serum leptin levels and AIRg for control subjects in pregnancy and postpartum. This is most likely due to leptin's close association with BMI and SI—which, because of the inverse relationship between AIRg and SI in normal subjects, will produce an association between leptin and AIRg. This was not seen in GDM subjects, where their insulin secretory deficit has uncoupled the association between AIRg and SI.
In conclusion, our findings suggest that adiponectin, TNFα and hsCRP do not appear to contribute greatly to pregnancy-induced insulin resistance in GDM or in women with a healthy pregnancy, while leptin retains its association with SI during pregnancy. TNFα is elevated in GDM pregnancy, and may play a role in impairing insulin secretion in these subjects.