Succinate dehydrogenase inhibition leading to increased insulin resistance through reduction in circulating thyroxine

Many environmental contaminants are known or suspected to generate adverse effects through multiple mechanisms, within multiple cell types, and potentially acting across multiple biological systems, thus rendering elucidation of cause and effect highly challenging. The purpose of creating this AOP is to collate evidence for a plausible pathway by which inhibition of a mitochondrial enzyme, succinate dehydrogenase, could, via disruption of the hypothalamus-pituitary-thydroid axis, contribute to the development of a clinically significant metabolic disruption, namely an increase in insulin resistance (IR).

Key to identifying a metabolic effect mediated by thyroid disruption from a direct xenobiotic impact on metabolism is the existence of evidence that qualitatively or quantitatively differentiates between potential impacts via the two routes. Clinical data support the hypothesis that exposure to DEHP - as determined by renal excretion of its major metabolites - has components of both thyroid-mediated (via reduction in circulating free thyroxine concentration) and direct impacts on increase in IR. In vitro data support the hypotheses that (i) inhibition of succinate dehydrogenase can lead to oxidative stress, and (ii) increasing oxidative stress in the thyroid follicular cells can lead to reduction in thyroid hormone secretion. Thus, a plausible link can be established between thyroidal succinate dehydrogenase inhibition and increase in IR.

It must be borne in mind that additional molecular initiating events (MIEs) and alternative pathways to increase in IR are plausible for many xenobiotics.

This AOP was created in order to collate information, and attempt to drive understanding, around several key issues concerning xenobiotic driven disruption of energy metabolism, and the subsequent development of related pathologies. Specifically, the focus is on the nature and extent of the component of this disruption that can potentially be mediated via alteration of thyroid gland function, and consequent changes in circulating thyroid hormone concentrations (Huang et al, 2021).

Amongst classes of xenobiotics, fragmentary evidence indicates that this is one possible route - amongst many - by which toxicity of phthalate esters could manifest itself. This evidence includes the observations that :

1. Phthalate esters can increase reactive oxygen species (ROS) concentrations in multiple in vitro systems and in vivo.
2. Phthalate esters can decrease concentrations of thyroid hormones in vivo.
3. Phthalate exposure has been correlated to pathologies of energy metabolism, such as insulin resistance.

Given the role of thyroid hormones in regulating energy metabolism, including glucose and lipids, the possibility exists for phthalate-induced disruptions in energy metabolism and contribution to subsequent pathologies to be mediated at least, in part, through their impacts on thyroid function.

Identification of molecular initiating event

A significant body of data indicates that:

1. Many xenobiotics generate ROS in vitro and/or in vivo.
2. ROS generation resulting in oxidative stress can be a contributing factor - potentially the most important factor for some xenobiotics - in the induction of toxic pathological outcomes.
3. Multiple mechanisms of excess ROS generation are known, including mechanisms specific to particular cell types, and mechanisms specific to particular xenobiotics or xenobiotic classes.
4. The main source of ROS generation under normal physiological conditions in most cell types is the electron transport chain of the mitochondria.

Consequently, of the multiple potential routes of ROS generation, a mechanism involving increased mitochondrial generation is a strong candidate for a potential MIA.

Identification of adverse outcome

Xenobiotic-induced metabolic disfunction can manifest itself in multiple biochemical and physiolological outcomes. These can be of clinical significance in themselves, of significance when taken in conjunction with other measures, and/or indicators of potential progression to further endpoints that may be of clinical significance. Insulin resistance (IR), a measure of the extent to which cells do not respond normally to insulin, is a component of metabolic syndrome, which, in turn, predisposes to type 2 diabetes and cardiovascular disease. Given this central position in the development of metabolic pathologies, and that it can be determined by simple blood measurements, it represents a quantifiable endpoint of clinical relevance, suitable for identification as a significant adverse outcome in and of itself.

The first iteration of the AOP has been developed in order to outline a plausible route by which stressors that induce oxidative stress could contribute to the global burden of metabolic pathologies acting, specifically, via reduction in thyroid hormone synthesis, and corresponding reduction in circulating thyroxine levels. It was also considered important to identify a particular mechanism that can induce oxidative stress. For phthalate esters, particularly DEHP, there is documented evidence for the chain of events leading from succinate dehydrogenase inhibition, through an increase in thyroid gland oxidative stress and reduction in circulating thyroxine levels, to an increase in insulin resistance (Huang et al, 2021).

Huang, H-B et al (2021) "Mediation effects of thyroid function in the associations between phthalate exposure and glucose metabolism in adults", Environmental Pollution 278, 116799