Inflammatory responses to exercise and the role of orexigenic hormones

Chronic inflammation is a result of a disruption to the inflammatory process, often associated with a low-grade systemic increase in pro-inflammatory cytokines, such as IL-6 or TNF-α. Chronic inflammation has been implicated in the development of serious health conditions such as cardiovascular disease, neurodegenerative diseases and type 2 diabetes.

It is unsurprising that research into methods of attenuating chronic inflammation is of major interest. It is well established that chronic exercise training, or increased physical activity is an effective method of curbing chronic inflammation. This attenuation of the release and circulating levels of inflammatory cytokines (such as IL-6 and TNF-α) and upregulation of anti-inflammatory cytokines (such as IL-10) is referred to as the anti-inflammatory effects of exercise. Although, the mechanisms behind the anti-inflammatory effects of exercise are not completely understood.

Fully elucidating the anti-inflammatory effects of exercise by investigating populations such as highly trained individuals or the acute effects of exercise, provides a unique point of view to solving the problem of chronic inflammation. Ghrelin is an orexigenic hormone established to be sensitive to acute exercise (transiently supresses) and obesity (chronically supresses). Interestingly, some studies have implicated ghrelin treatment in the attenuation of markers of inflammation in in vivo and in vitro models. A key element for ghrelin to have anti-inflammatory effects is its receptor, GHS-R1a, which is expressed on immune cells.

Recently, another orexigenic hormone called asprosin, has been discovered. It is not well understood but initial findings largely implicate it in metabolism. Some reports show asprosin may also be sensitive to exercise, however current evidence is limited. Although its relationship to inflammation is not clear, this hormone is of great interest as a potential target in the treatment of chronic inflammation. Due to its potential sensitivity to exercise it is of even greater relevance in this thesis.

Therefore, this thesis aims to use acute exercise and chronically trained people as models to uncover mechanisms contributing to the anti-inflammatory effects of exercise. Specifically, this thesis aims to presents a series of investigations to ultimately examine how orexigenic hormones may play a role in the anti-inflammatory mechanisms seen in response to exercise training and in physically active individuals.

The first main experimental chapter was a cross-sectional population characterisation of GHS-R1a+ immune cell subsets, plasma ghrelin, asprosin and IL-6 concentrations (n=74). We found that men had a greater number and percent of GHS-R1a+ lymphocytes compared to women. Sex and age were the primary factors to predict the amount of GHS-R1a+ lymphocytes. In addition, we report that women taking oral contraceptives had lower plasma asprosin concentrations compared to women who did not. We also found among women, age and physical actively level were the main predictors of plasma asprosin. It was found that women who scored in a higher physical activity category, and those who were older were predicted to have higher plasma asprosin concentrations. These findings led to two primary research questions (1) What are the effects of an acute bout of exercise and training status on GHS-R1a+ immune cells in young men and (2) what are the effects of oral contraceptive use, physical activity and menstrual cycle phase on plasma asprosin concentrations in pre-menopausal women.

Next, an investigation into the effects of acute exercise on GHS-R1a+ immune cells and their subsets was conducted in trained (V̇O2peak ≥60 mL·kg-1·min-1; n=12) and untrained (≤45 mL·kg-1·min-1; n=6) men. This study used a V̇O2peak cycling test and physical activity questionnaires to validate training status. In the main visits participants cycled for either 1 h at 65% V̇O2peak or rested for 1 h. Whole blood was collected Pre-, Post-, 1 h Post- and 24 h Post-trial. Plasma was collected for ghrelin, asprosin and IL-6 analysis and PBMCs were isolated for flow cytometry GHS-R1a identification and subsequent in vitro ghrelin treatment and LPS stimulation. We saw that among trained men (n=10) at 1 h post-exercise and rest trial, there was an increase in plasma asprosin concentrations, and more research is needed to understand this relationship. In addition, our findings suggest that trained men had more GHS-R1a+ PBMCs at rest compared to untrained men, whereas untrained men had an increase in GHS-R1a+ PBMCs after exercise compared to untrained men. The secretion of IL-6 by PBMCs in response to LPS-stimulation in vitro did not show any attenuation of IL-6 with ghrelin treatment. Although further investigation is needed to confirm, these findings could suggest that GHS-R1a is adaptive to training and could be a potential mechanism contributing to the anti-inflammatory effects of exercise.

This thesis next examined WBC count and fasting plasma asprosin, ghrelin and IL-6 in trained and untrained women (n=32) with regular menstrual cycles (not using oral contraceptives; n=8 untrained, n=6 trained) or using oral contraceptives (n=10 untrained, n=8 trained) during early follicular, late follicular and mid-luteal menstrual phases (or the time-period equivalent for oral contraceptive users). As determined in the cross-sectional study, oral contraceptive users had lower plasma asprosin concentrations compared to non-users. Asprosin concentrations varied more across a cycle in untrained than trained women. It was also found plasma asprosin correlated negatively with lymphocyte counts but only when all data were analysed together. With asprosin being a newly discovered molecule, it is vital that frequently used medication such as oral contraceptives and the cyclic changes in menstrual cycle are considered in the design of future investigations of plasma asprosin concentrations. There may be some relationship between asprosin and markers of inflammation, however the data presented in this chapter are highly preliminary.

In conclusion, this thesis aimed to use exercise and training to understand how orexigenic hormones interact with the anti-inflammatory effects of exercise. The role of asprosin remains to be understood but preliminary data presented in this thesis could provide justification for further investigation of asprosin role in inflammation and exercise. GHS-R1a, the ghrelin receptor, may be involved in training induced anti-inflammatory mechanisms, but further investigations are needed to corroborate these findings. Further elucidation of the GHS-R1a pathway and how that relates to exercise could lead to novel treatments of chronic inflammation to ultimately improve the health and morbidity of populations with chronic inflammation.