We observed a significant decrease between the first and second trimesters of gestation in both humanin and MOTSc levels. The decrease in humanin was comparable among pregnant women who later developed GDM and those who did not. However, the decrease in MOTSc was only significant in the group of pregnant women who developed diabetes.
To our knowledge, there are no published studies on the plasma levels of MDPs (Humanin and MOTSc) in the early stage of gestation and their evolution throughout pregnancy. Furthermore, only one previous study has evaluated humanin levels during pregnancy with a single determination between 24–28 weeks [10] and no study has focused on MOTSc levels during pregnancy.
We also explored the relationship between MDPs and insulin resistance during early gestation. We found an association between low levels of humanin and a higher risk of presenting a high HOMA-IR index (≥ p70) in both the first and second trimesters of gestation; however, this association decreased in strength and lost statistical significance after adjusting the analysis by BMI. In fact, the identification of a negative correlation between the levels of humanin and BMI in both trimesters reinforces the hypothesis that BMI is an important confounder when interpreting the relationship between humanin and insulin sensitivity. Regarding MOTSc, surprisingly, the findings were not the same in the first trimester, where we did not identify a significant association with the HOMA-IR index and the second trimester, where we observed a significant relationship between the MOTSc levels and the HOMA-IR index with a higher risk of presenting a HOMA-IR index above p70 in pregnant women with lower MOTSc levels. This association was maintained after adjusting the analysis for both BMI and age, and it was mainly derived from a higher risk among pregnant women with MOTSc levels in the lower tertile. In addition, a more pronounced decline in MOTSc levels between the first and second trimesters resulted in a higher risk of having a HOMA-IR index above p70. Therefore, our results suggest that MOTSc levels, especially the decrease between the first and second trimesters of gestation, are associated with an increased risk of insulin resistance during early gestation. The correlation with the value of HOMA-IR in the second trimester, statistically significant although small, as well as the associations found when dichotomizing insulin resistance based on a cutoff point ≥ P70 of HOMA-IR, would support this hypothesis.
Our findings are consistent with previous studies that have identified a relationship between MDPs and insulin resistance. There is evidence of the insulin-sensitizing properties of MDPs mainly derived from cellular and animal models [11]. MOTSc is detected in the circulation, and its target organs are primarily skeletal muscle and fat. Administration of MOTSc in mice resulted in increased glucose uptake, primarily by skeletal muscle tissue, prevented the development of insulin resistance induced by a high-fat diet and reversed age-associated insulin resistance via activation of AMPK and SIRT1 [12]. Furthermore, MOTSc improves insulin sensitivity and increases beta-oxidation by targeting three metabolic pathways: sphingolipid metabolism, monoacylglycerol metabolism and dicarboxylate metabolism [13]. Humanin has been shown to decrease beta cell apoptosis in vitro and delay the development of diabetes in mouse NOD in vivo [14]. Finally, it has been described how people with type 2 diabetes mellitus have lower levels of humanin and MOTSc than people without diabetes, and their levels correlate with the HbA1c value [15].
Despite the relationship of MDPs with insulin sensitivity during pregnancy, their predictive capacity for the development of GDM was poor according to the AUC values obtained. This probably reflects the complexity of GDM, where insulin resistance is only one of the factors involved in its physiopathology. In fact, the ability to diagnose GDM in the first trimester remains controversial, and all parameters investigated have been poorly predictive of oral glucose tolerance test outcomes in the third trimester [16]. The highest precision achieved by a model defined as the summation of seven binary variables recommended by the National Institutes of Health (NIH) was only 30%, and its AUC for GDM was 0.682 [17].
Our study has several limitations. First, the small sample size could attenuate our ability to identify significant differences between pregnant women with and without GDM. Second, the single-center character of the study, and third the use of HOMA-IR as a marker of sensitivity to insulin. However, we consider that this mathematical model may be suitable to estimate the longitudinal changes in insulin sensitivity in our study population and has been shown to be an independent risk factor for the development of GDM [18]. Furthermore, a good correlation between HOMA-estimated insulin resistance and the euglycemic clamp [19] or minimal model [20] has been described. Further studies with larger sample sizes should extend and corroborate our results. In relation to the strengths of the study, it is worth mentioning the effort to study the associations through different analysis strategies, exploring also the dose-response pattern, and the control of confounding in the design phase through matching and by using multivariate analysis.
In conclusion, we found a significant decline in humanin and MOTSc levels between the first and second trimesters of pregnancy. The decrease in humanin was significant in pregnant women who developed diabetes and those who did not, while that of MOTSc was only significant in pregnant women who developed diabetes. Moreover, a greater decrease in MOTSc levels is associated with a higher risk of presenting a high HOMA-IR in the second trimester, while the relationship between humanin and HOMA-IR is attenuated and becomes nonsignificant after including BMI in the analysis. Thus, our results suggest that MOTSc levels, especially a strong decrease between the first and second trimesters of gestation, may be involved in the progressive increase in insulin resistance starting from early gestation.