Circulating RANKL is required for glucose homeostasis during pregnancy

Technical Summary: Circulating RANKL is Required for Glucose Homeostasis During Pregnancy

Problem Statement
Gestational diabetes mellitus (GDM) is a prevalent complication affecting 10–15% of pregnancies globally, posing significant risks to maternal and fetal health. The current gold standard for diagnosis, the 75g oral glucose tolerance test (OGTT), is time-intensive and uncomfortable, prompting a search for stable, predictive biomarkers. While various metabolites, miRNAs, and proteins have been investigated, results are often inconsistent, and many lack validation through intervention experiments. Furthermore, the pathogenesis of GDM remains enigmatic, and the specific role of Receptor Activator of NF-κB Ligand (RANKL) in pregnancy-induced glucose intolerance requires clarification, particularly given conflicting data regarding RANKL levels in diabetic versus non-diabetic populations.

Methodology
This study employed a dual approach combining a cross-sectional human population study and animal/cellular experiments:

• Human Population Study: A cross-sectional study was conducted involving 399 pregnant women at 24–28 weeks of gestation (101 diagnosed with GDM, 298 non-GDM controls) and 262 women in early pregnancy. Participants underwent 75g OGTT screening based on IADPSG criteria. Serum RANKL levels were measured using R&D Systems kits. Statistical analyses included logistic regression (crude and adjusted for age, BMI, ethnicity, etc.), restricted cubic spline regression to identify threshold effects, and ROC curve analysis for diagnostic value.
• Animal Model: A GDM mouse model was generated using high-fat diet (HFD)-fed pregnant ICR mice. Pregnant mice were divided into groups: HFD-fed mice injected with recombinant mouse RANKL (1 µg/kg) or saline, and chow-fed mice injected with saline. Glucose tolerance (IPGTT) and insulin tolerance (IPITT) were assessed on gestational days 16 and 17, respectively. Serum insulin and glucose levels were measured at gestational day 18.
• Cell Culture Mechanism: JEG-3 and HTR-8 trophoblast cell lines were treated with varying concentrations of RANKL (0, 25, 50, 100 ng/mL) for 24 hours. RNA extraction and real-time PCR were performed to analyze the mRNA expression of glucose transporter proteins (GLUT1, GLUT2, GLUT3, GLUT4).

Key Contributions and Results

1. Inverse Correlation in Human Subjects:

   • Serum RANKL levels were significantly lower in women with GDM compared to non-GDM controls during mid-pregnancy (192.6 ± 8.53 pg/mL vs. 399.8 ± 33.95 pg/mL; p < 0.0001).
   • A significant negative correlation was observed between serum RANKL levels and glucose metrics, including HbA1c, fasting blood glucose (FBG), 1h/2h OGTT, and the area under the curve (AUC).
   • Logistic regression confirmed that higher serum RANKL levels are associated with a reduced risk of GDM (Adjusted OR = 0.994; 95% CI: 0.99–0.997).
   • Restricted cubic spline analysis identified a threshold range of approximately 800–900 pg/mL where the protective effect on glucose homeostasis plateaus.
   • ROC analysis demonstrated that serum RANKL has high diagnostic value for GDM (AUC = 0.7024), with sensitivity of 0.842 and specificity of 0.574.

2. Therapeutic Efficacy in Animal Models:

   • HFD-fed pregnant mice exhibited impaired glucose and insulin tolerance compared to chow-fed controls.
   • Administration of recombinant RANKL to HFD-fed pregnant mice significantly improved glucose tolerance, insulin tolerance, and serum insulin concentrations compared to saline-treated HFD controls.
   • Notably, RANKL treatment did not alter maternal body weight or weight gain, suggesting the metabolic improvements were independent of weight changes.

3. Mechanistic Insight in Trophoblasts:

   • In vitro experiments revealed that RANKL treatment directly upregulated the mRNA expression of glucose transporter proteins (GLUTs) in trophoblasts.
   • Specifically, RANKL increased the expression of GLUT1, GLUT2, GLUT3, and GLUT4 in JEG-3 cells. In HTR-8 cells, RANKL promoted the expression of GLUT1, GLUT3, and GLUT4, though GLUT2 expression remained unchanged.

Significance and Claims
The authors claim that this study is the first to demonstrate circulating RANKL as both a diagnostic biomarker and a potential therapeutic target for GDM. The study posits that:

• Diagnostic Utility: Low levels of circulating RANKL in the second trimester are indicative of GDM risk, offering a potential alternative or adjunct to the OGTT.
• Therapeutic Potential: RANKL plays a crucial role in maintaining glucose homeostasis during pregnancy. The study suggests that RANKL improves insulin sensitivity and insulin secretion, potentially via the upregulation of glucose transporters in trophoblasts.
• Novelty: Unlike previous studies in non-pregnant populations where RANKL inhibitors were sometimes explored for insulin sensitivity, this research highlights a unique, beneficial role for RANKL specifically within the metabolic context of pregnancy.

The authors acknowledge limitations, including the use of an HFD model which may not fully replicate all systemic metabolic changes of human GDM, the lack of data on lifestyle factors, and the difficulty in determining the specific tissue source of circulating RANKL. They conclude that while these findings are promising, large prospective cohorts and intervention trials are necessary to validate RANKL's clinical application.