Influence of triiodothyronine (T3) and its intracellular pathways on local adenosine-mediated signaling in human adipose tissue browning

Abstract

Adipose tissue (TA) is divided into white adipose tissue (TAB) and brown adipose tissue (TAM). TAB has a large lipid accumulation, whereas TAM has less lipid accumulation and a large number of mitochondria, presenting as a marker the mitochondrial uncoupling protein-1 (UCP-1). It is known that TAB can be converted to TAM, a process known as browning. TAB expresses purinergic system receptors, which when activated can modulate lipid metabolism, and can be activated by adenosine triphosphate (ATP) and its derivatives, such as adenosine (ADO). In addition, TAB is modulated by thyroid hormones (HT), especially triiodothyronine (T3), through different cellular pathways, including MAPK/ERK and PI3K. There is an important relationship between TH and purinergic signaling in different physiological and pathological systems, however, this relationship in adipose tissue is poorly elucidated in the literature. It is known that the action of T3, as well as the activation of purinergic receptors, stimulate the TAB browning process. In previous results, it was shown that T3 increased the levels of mRNA coding for UCP1 in parallel with the reduction in the expression of enzymes responsible for the metabolization of adenosine (e.g. ADA, ADK) in human subcutaneous adipocytes. Considering that the reduction of adenosine extracellular levels can determine changes in signaling mediated by its membrane receptors, we now intend to evaluate the action of T3 and associated MAPK/ERK and PI3K signaling pathways on adenosine release and its, eventual, relationship with browning in human subcutaneous adipose tissue. Human preadipocytes (HPAd-802s-05A) will be subjected to a differentiation protocol and subsequently subjected to treatment with T3 (10 nM, for 24h) in the presence or absence of inhibitors of the MAPK/ERK pathways (PD98059, 5 ¼M ) and PI3K (LY294002, 5 µM). Adipogenic differentiation will be evaluated by Oil Red staining; browning will be evaluated by protein expression of UCP-1 by Western Blot and extracellular release/accumulation of ADO will be evaluated by Ultra high-performance liquid chromatography (UHPLC). The understanding of the action of T3 by different cellular pathways on adenosine release and on the induction of adipose tissue browning has never been done before and will allow a better understanding of the functioning of adipose tissue, especially the relationship between the role of thyroid hormones and the pathways of local purinergic signals (autocrine/paracrine) involved in the browning of adipose tissue and, therefore, in the control of obesity.