The effect of metabolic imbalance in the retina and beyond: new insights from the role of antioxidants, thyroid hormones and β3-adrenoceptor

The study of the molecular mechanisms underlying the pathological processes induced by metabolic imbalance could provide important insights for the development of novel therapeutic strategies to counteract the onset of diseases in particularly sensitive tissues. The high energy demand required by retinal neurons renders them highly susceptible to various metabolic stressors, inducing imbalanced metabolism-related pathological processes such as oxidative stress and inflammation, then triggering neurodegenerative phenomena and loss of visual function. Thus, an overall improvement in the retinal components altered by hyperglycemia could be achieved by preventing the onset of oxidative stress induced by metabolic imbalance. In this context, the role of the thyroid hormone (TH) and β-adrenergic systems in response to metabolic imbalances, including changes in oxygen and glucose levels, was investigated in dysfunctional retinas, given the central role of the TH system in metabolic regulation and the predominant expression of β3-adrenoceptor (β3-AR) in the retinal vasculature. In addition, the systemic role of β3-AR was investigated in the potential cross-talk between adipose tissue and the heart, given the close correlation between altered adipose tissue and increased risk of cardiovascular disease onset and the maximal expression of β3-AR in adipose tissue, where it plays a pivotal role in regulating metabolic processes reflected in a modulation of the overall metabolic state. Therefore, this thesis is focused on providing further insights into the molecular mechanisms underlying dysfunctional metabolism-driven pathological processes in retinal diseases, suggesting possible new therapeutic approaches aimed at preventing or counteracting retinal neurovascular alterations and related loss of visual functions. Moreover, the preliminary study on the putative role of β3-AR in the adipose tissue-heart communication provides initial evidence for potential new therapeutic strategies using β3-AR agonism in adipose tissue as an additional cardioprotective effect to counteract the pathological signs of heart failure induced by obesity. In particular, the effects of oral administration of a natural compound with pronounced antioxidant and anti-inflammatory properties, including cyanidin-3-glucoside (C3G), verbascoside and zinc, were first evaluated in a rat model of streptozotocin (STZ)-induced diabetic retinopathy (DR), considering oxidative stress and related inflammation as the metabolic dysfunction-induced upstream processes that promote neurovascular alterations typical of DR. In this model, the compound demonstrated efficacy in counteracting oxidative stress and inflammatory processes, thereby preventing hyperglycemia-induced neurovascular alterations, including BRB dysfunction, retinal cell degeneration and visual loss. Hence, the beneficial effects displayed by the oral administration of the compound in preventing oxidative stress and inflammation suggested a possible new non-invasive treatment for DR. Furthermore, a putative retinal modulation of thyroid hormone (THs) levels in response to hyperglycemic-related stress was evaluated in 16-week-old db/db mice, due to their genetic background-related spontaneous development of T2DM and obesity state. In response to high glucose exposure, a retinal low-T3 state (LT3S) was shown as a local adaptive mechanism against the persistent increase in metabolic rate. Therefore, the LT3S onset suggested an early retinal response to hyperglycemic stress, although it could turn detrimental in the long term exacerbating mitochondrial dysfunction and related retinal energetic failure. Additionally, a putative molecular mechanism involving the activation of the Nrf2/HIF-1 axis in response to hyperglycemia-induced oxidative stress was investigated in the human MIO-M1 cell line. Together, these findings would demonstrate a retinal response against hyperglycemic stress in the early stages of DR, in which oxidative stress-related pathways are crucial for the LT3S onset, thus suggesting the pivotal involvement of the TH system even in the retinal metabolic regulation and providing further insights regarding possible therapeutic strategies for DR. The effect of β3-AR agonism was first investigated using the oxygen-induced retinopathy (OIR) mouse model, given its predominant localization in the retinal vasculature. Being more susceptible to hypoxia-induced pathological angiogenesis and showing higher β3-AR expression than other mouse strains, 129S mice were used in this study to better reproduce the typical hallmarks of preterm infant immature retinas affected by retinopathy of prematurity (ROP). In this respect, β3-AR agonism with BRL 37344 showed an efficacy in counteracting the OIR-induced neurovascular alterations, preventing vascular and astrocyte template regression in the central retina, mid-peripheral engorged tuft formation, retinal ganglion cells (RGCs) degeneration and vision loss. Therefore, pharmacological modulation of β3-AR activity could offer novel insights into therapeutic strategies aimed at counteracting pathological OIR-induced retinal angiogenesis. In order to evaluate the systemic role of β3-AR in a condition of dysregulated metabolism induced by obesity, the effects of β3-AR agonism, delivered by CL 316,243-releasing osmotic mini-pumps, were studied in 12-week-old C57BL/6J mice fed a diet enriched in fat and sugar for 28 weeks to mimic the major pathological signs of cardiac and white adipose tissue (WAT) dysfunction. β3-AR agonism showed efficacy in maintaining glucose and insulin tolerance, as well as preventing obesogenic diet-induced cardiac hypertrophy and interstitial fibrosis. In addition, reduced immune cell proliferation and upregulated metabolic markers typical of brown adipose tissue (BAT) suggested a β3-AR-mediated anti-inflammatory and ‘beiging’ effect in WAT, previously associated with a beneficial effect on overall metabolic status, providing first indications of a potential new therapeutic approach based on the systemic β3-AR-mediated effects in the prevention of the obesity-induced cardiovascular alterations through adipose tissue-heart communication. Taken together, these findings provide new insights into the molecular mechanisms involved in metabolically induced retinal and cardiac diseases, thus suggesting potential new therapeutic approaches aimed at counteracting the onset of pathological processes induced by metabolic dysfunction and the progression of related diseases.