Uterine Artery Dysfunction in Hypoxic Pregnancy: A Mitochondrial Perspective

Introduction

Pregnancy is characterized by an increase in demand for uteroplacental blood flow to the developing fetus. This increase in blood flow is supplied by the mother to the fetus mainly by the uterine artery. In healthy pregnancy, the uterine artery undergoes several important functional alterations to meet the increase in uteroplacental blood flow demand, including decreased uterine constrictor and myogenic responses, increased vasodilator reactivity, and outward hypertrophic growth (Wang et al., 2024).

Complicated pregnancies and other clinical indications, such as advanced maternal age, gestational diabetes, maternal obesity, and pre-eclampsia, increase the risk for maternal and fetal complications, such as chronic hypoxia, which have serious immediate and long-term effects (Giussani, 2021). These effects contribute to the dysfunctional remodeling of the uterine artery, which is modulated by excessive levels of mitochondrial reactive oxygen species (mtROS). Elevated levels of mtROS disrupt proper regulation of uterine arteries (Wang et al., 2024).

Disruption of the proper regulation of uterine arteries by increased levels of mtROS has been demonstrated in preclinical rodent and sheep models whereby hypoxic pregnancy has a negative impact on uterine artery remodeling and reactivity, mediated in part by increased levels of mtROS (Botting et al., 2020). As such, targeting excess mtROS to improve uterine artery function and blood flow regulation in hypoxic pregnancy represents a promising therapeutic approach.

MitoQ is a mitochondrial-targeted antioxidant which reduces excess mtROS. It is a compound that directly targets the mitochondria as a result of the lipophilic triphenylphosphonium cation attached to an ubiquinol antioxidant. This cation enables the compound to accumulate in the mitochondria at much higher concentrations than other antioxidant compounds (Wang et al., 2024).

As such, Wang et al., (2024) aimed to determine if chronic treatment with MitoQ could prevent uterine artery dysfunction in the setting of a chronic hypoxic pregnancy in rats.

Methods

Wang et al. (2024) used female Wistar rats. They were allocated to one of four groups at Day 6 of gestation (n=10–12/group): (1) Normoxia; (2) Hypoxia (13–14% O₂); (3) Normoxia+MitoQ; and (4) Hypoxia+MitoQ. MitoQ (500uM) was administered through drinking water ad libitum. The rats were sacrificed after two weeks on day 20 of gestation.

Levels of MitoQ uptake by maternal and fetal tissues were determined using mass spectrometry. Wire myography was utilized to assess vasoconstrictor and vasodilator responses in isolated uterine arteries ex vivo in response to common vasoconstrictive (norepinephrine and angiotensin II) and vasodilatory (sodium nitroprusside, SNP; acetylcholine, ACh) agonists. Mechanistic approaches were also employed to determine the role of protein kinase C (phorbol 12,13-dibutyrate; PDBu) and nitric oxide (endothelial NO synthase inhibitor; L-NAME) and Ca²⁺ activated K⁺ channel BKCa (NS1619) in these responses, respectively. Stereology and histology were used to assess morphology (wall thickness and lumen area) of uterine arteries.

Results

Following chronic supplementation, MitoQ levels were increased in the maternal liver and placenta and in the fetal liver and brain.

As compared to normoxic pregnancy, uterine arteries isolated after hypoxic pregnancy constricted to a greater degree in response to norepinephrine and angiotensin II. Increased protein kinase C activation (PDBu) was found to contribute, in part, to this response. Treatment with MitoQ during hypoxic pregnancy prevented the development of this hyperactive vasoconstrictive response.

Uterine artery vasodilation was enhanced in the hypoxic pregnancy group in response to SNP and ACh compared to normoxic pregnancy. This was caused, in part, by increased activation of the Ca²⁺ activated K⁺ channel BKCa (NS1619). MitoQ treatment prevented the increase in the vasodilatory response observed in the hypoxic pregnancy group. Further analysis revealed increased vasodilation in the hypoxic group was a result of exacerbated, and possibly compensatory, NO-dependent vasodilation, which was normalized with MitoQ treatment.

Lastly, relative to normoxic pregnancy, uterine arteries isolated from hypoxic pregnancy had greater wall thickness and an increased ratio of wall to lumen area. Treatment with MitoQ prevented the adverse remodeling of the uterine artery in response to hypoxic pregnancy.

Discussion and Future Directions

The onset of gestational hypoxia during late pregnancy poses an acute risk to the mother and the fetus. Also, offspring born from hypoxic pregnancies face elevated risk of long-term cardiovascular dysfunction in adulthood, including endothelial dysfunction, hypertension, and adverse cardiac wall remodeling (Giussani, 2021). This association was confirmed in sheep exposed to hypoxic pregnancies, as signs of cardiovascular impairment, such as endothelial dysfunction and hypertension, were observed in adult offspring (Botting et al., 2020). These findings were a result of uterine artery dysfunction caused, in part, by increases in mtROS. Here, Wang et al found that treatment with a mitochondrial targeted antioxidant (MitoQ) prevented hypoxic pregnancy-driven uterine artery dysfunction characterized by exaggerated vasoconstrictor and vasodilator responses and maladaptive vascular remodeling in rats (Wang et al., 2024).

Interestingly, Wang et al. (2024) uncovered a potential dichotomy for reducing mtROS to regulate vasodilation. Treatment with MitoQ during hypoxic pregnancy reduces mtROS to normalize compensatory, and potentially pathological, NO-dependent vasodilation. However, it is generally understood that reducing excess mtROS with MitoQ will increase bioavailability of NO to increase endothelium-dependent vasodilation (Gioscia-Ryan et al., 2014); this is also illustrated with normoxic pregnancy in Wang et al. (2024). Therefore, the mechanisms and the context (i.e., type of artery and physiological circumstance) with which mtROS is reduced should be considered when targeting mtROS to normalize vascular function.

Overall, these are results that require further exploration. However, prior to clinical translation, several experiments need to be performed. We would like to state that this list is by no means exhaustive but represents a start in the direction of translating the findings of Wang et al. (2024) to humans.

One set of experiments could be to assess the level of mtROS in isolated human umbilical or placental arteries with hypoxic pregnancy compared to normoxic pregnancy. These arteries also play a key role in blood and nutrient delivery and gas exchange between the mother and fetus and may exhibit a similar dysfunctional phenotype as the less accessible uterine artery. Hypoxic pregnancy can lead to oxygen deficits in maternal and fetal compartments, which are also supplied with blood by umbilical and placental arteries. Therefore, dysfunction in these arteries may also contribute to developmental issues and cardiovascular dysfunction (Giussani, 2021).

Wang (2024) and Botting (2020) each found increases in MitoQ uptake in the maternal liver and placenta and the fetal liver and brain. Thus far, chronic supplementation of MitoQ has been shown to be safe for (1) mothers in two preclinical mammalian models (i.e., sheep and rats) of hypoxic pregnancy (Botting et al., 2020; Wang et al., 2024) and (2) in the fetus and developing adult in a sheep model of hypoxic pregnancy (Botting et al., 2020). Botting et al. (2020) found no adverse effects on development through adulthood in sheep born to mothers treated with MitoQ during hypoxic pregnancy. The sheep had lower long-term risk for cardiovascular disease. Hence, another important series of experiments could be to establish definitive preclinical evidence for the safety of MitoQ supplementation in offspring development into adulthood across multiple/additional preclinical models prior to clinical translation. Future experiments could include tracking intergenerational effects of MitoQ administration in offspring, marking developmental goals and behavior including the ability to also produce healthy offspring. Experiments could be designed such that offspring born of chronic hypoxic pregnancies are assessed for these changes with and without MitoQ administration.

Lastly, there should be continued exploration of underlying molecular causes for increased long-term cardiovascular disease and developmental delays in offspring born from hypoxic pregnancy, and how MitoQ might mitigate these adverse effects. It is known that increases in mtROS plays a crucial role in uterine artery dysfunction, however, because many of these adverse physiological outcomes occur years later it is possible that long-lasting epigenetic changes may also be observed. Long-lasting epigenetic changes could mediate the long-term adverse cardiovascular and developmental outcomes observed in offspring of hypoxic pregnancies. Wang et al. (2024) mentions the role of TET methylcytosine dioxygenase 2 (TET2), an enzyme that regulates demethylation of DNA (an epigenetic mark) to alter gene expression, in modulating mtROS. Importantly, TET2 is downregulated in hypoxic pregnancy. Knockdown of TET2 in sheep uterine arteries during normoxic pregnancy increases mtROS and myogenic tone, similar to hypoxic exposure. Treatment with MitoQ blocks these acute physiological effects (Hu et al., 2023). In doing so, MitoQ treatment may also disrupt the potential development of a chronic hypoxic pregnancy-mediated maladaptive epigenetic landscape that could, in theory, drive increases in cardiovascular disease and delays in development long-term.

Conclusion

In summary, chronic hypoxic pregnancy results in uterine artery dysfunction characterized by hyperactive vasoconstrictor and vasodilator responses and maladaptive vascular remodeling in rats (Wang et al., 2024). Increased mtROS is a key mediator in the development of uterine artery dysfunction with chronic hypoxic pregnancy and treatment with MitoQ prevents this. These are important and exciting findings; however, future research is still needed to translate these observations to humans. Most importantly, this includes the assessment of acute and long-term safety of the fetus and offspring throughout and following MitoQ treatment during hypoxic pregnancy, respectively.