Gestational diabetes mellitus (GDM), a condition in which the state of pregnancy induces the development of diabetes, is characterized by heightened maternal insulin resistance. The levels of sex steroid hormones generally increase during pregnancy. It is thought that imbalance in the levels of steroids like estradiol (E2) and progesterone (P4) with respect to each other, may increase susceptibility towards GDM. To understand the metabolic effects of these steroids, ovariectomized (OVX) rats were treated with E2 or P4 at dosages mimicking the true hormonal status as in pregnancy. E2 significantly reduced the body weight gain (145.4±1.4% to 108.3±0.8%, p<0.001, n≥12) as well as the cumulative food intake (391.3±14.6 g to 312.5±9.0 g, p<0.001) over the course of the 23 day-treatment period. It also decreased the quantity of accumulated gonadal white adipose tissue (GWAT) in the body (3.3±0.2 g to 1.1±0.1 g, p<0.001) and repressed expression of lpl (1.3±0.2 fold, p<0.05) and other lipogenesis markers. P4, on the other hand, enhanced lpl expression (3.7±0.2 fold, p<0.001), but did not affect the total quantity of GWAT. Further, E2 treatment brought about an increase in the expression of insulin sensitivity markers like insr in the GWAT (4.5±0.6 fold, p<0.001) and soleus skeletal muscle (6.2±0.3 fold, p<0.001), as well as an increase in the protein levels of GLUT4.
GDM susceptibility in pregnant women is most commonly associated with SNPs in the tcf7l2 gene, the product of which is an effector of the canonical Wnt signaling pathway. It has also been reported that certain actions of steroid hormones are mediated by Wnt signaling. Moreover, we found that tcf7l2 and other components of this pathway (β-catenin protein, lrp5) were up-regulated following treatments with E2 (3.8±0.2 fold, p<0.001 in GWAT; 5.3±0.2 fold, p<0.001 in soleus) and P4 (2.1±0.2 fold, p<0.05 in GWAT; 2.9±0.3 fold, p<0.001 in soleus). We therefore hypothesized that the metabolic actions of these steroids may be mediated by Wnt signaling. To test this hypothesis, we conducted experiments in which OVX rats treated with steroids as described above, were additionally treated with niclosamide (NIC), a Wnt pathway inhibitor. NIC in conjunction with E2 increased GWAT accumulation and lipogenesis, thereby reversing the action of E2. NIC treatment in OVX rats did not change these parameters, indicating that this effect is specific to the inhibition of Wnt signaling modulated by E2. Additionally, NIC inhibited the E2-modulated increase in insulin sensitivity in GWAT and soleus. Taken together, the results suggest that the actions of E2 on insulin sensitivity and lipogenesis are mediated by the Wnt signaling pathway. No such observation was made with respect to the effect of P4 on lipogenesis. Understanding the mechanistic actions of these steroids may play an important role in devising methods to prevent conditions like GDM before its onset.