How Adipose Tissue Impacts the Mitochondria: Energy Expenditure

Adipocytes have extraordinary capacities and functions that promote homeostasis. Different adipocytes, such as white adipose tissue (WAT) and brown adipose tissue (BAT), possess multiple functions that enable metabolic mechanisms, provide thermogenic protection, and boost insulin sensitivity. However, BAT’s complex composition, lipidic content, high mitochondrial concentration, and low endoplasmic reticulum mass significantly impact metabolic processes.

Adipose tissue:

Adipose content comprises 20% to 28% of most healthy individuals; this amount can vary depending on the patient’s gender. For instance, women’s average body fat range is between 20% and 32%, while men’s healthy fat mass percentage ranges from 8% to 20%. Nevertheless, these numbers vary with the patient’s activity, age, and even race. Therefore, these factors should be considered when assessing body fat mass.

Also, the distribution of this body fat contributes to functionality. For example, subcutaneous adipose tissue comprises the highest portion of the body’s fatty content, is located under the skin, and is characterized by a white color. On the other hand, visceral adipose tissue serves as an organ protectant and surrounds living organs, such as kidneys, gonads, the intestines, and the heart.

Adipose Tissue and its Function:

Adipose tissue is critical for structure, mainly due to its function as connective tissue. 

Indeed, visceral fat enables constant support within the small intestine’s convolutions, allowing a somewhat sturdy position. Another essential adipose function is energetic storage and a metabolism and inflammation modulator. Furthermore, the metabolic function of this tissue extends to several systems, as it modulates immune factors and reproductive and steroid metabolism.

Adipocyte’s cellular composition and structure enable the different functions. Indeed, the different cell types found in adipose tissue are adipocytes, preadipocytes, fibroblasts, macrophages, monocytes, vascular, stromal cells, and innervation cells. Furthermore, depending on the tissue’s cellular composition, it can be characterized under these types:

• Brown adipose tissue
• White adipose tissue

White adipose tissue:

White adipose tissue, or WAT, is characterized by low vascularization and innervation. It is also uniocular, which means it has only one vacuole that stores all the lipid content, mostly triglycerides (99%).

WAT’s role is focused on storing energy, but it also has the potential to modulate inflammatory processes. Indeed, WAT generates adipokines that serve like hormones carrying messages throughout the body.

• Leptin, secreted by adipocytes, is an anorexigenic adipokine that modulates satiety, lipid oxidation, thermogenesis, and insulin sensitivity.
• Adiponectin is an anti-inflammatory adipokine that can potentially suppress hepatic gluconeogenesis and monocyte adhesion.
• Resistin: As the name suggests, resistin is involved in insulin resistance and promotes fatty acid synthesis in the liver.
• Retinol-binding protein four is responsible for retinol transport and inhibits the function of GLUT4, thus contributing to insulin resistance.

Brown adipose tissue:

Brown adipose tissue owes its coloration to the high concentration of mitochondrial cytochromes and vascularization. Furthermore, BAT has multiple lipid vacuoles; their progenitor cell is a skeletal muscle cell instead of an adipocyte, like WAT.

The primary function of BAT is to create thermal energy to regulate body temperature, using uncoupling proteins (UCP) to produce heat instead of ATP. In addition, BAT is located in specific body sites, such as the interscapular, axillary, and cervical regions, which are considered superficial. Meanwhile, the deep sites are the perirenal, periaortic, inguinal, and pericardial regions.

Energy expenditure, WAT, and BAT

BAT promotes heat production through UCPs, which, in turn, upregulates energy expenditure. On the other hand, a fasting state activates hunger signals from the gabaminergic neurons, inhibiting the sympathetic system and, thus, thermogenesis. This mechanism, mediated by the lack of food, reduces energy expenditure.

Structure and composition are essential factors contributing to the different functions of fat mass. Specifically, in BAT, the quantity and concentration of mitochondria are crucial for its thermogenic function. Furthermore, energy homeostasis is vital for properly developing and maintaining these tissues. Studies report that obese mice fed with high fat, high-energy diets have a larger quantity of WAT due to the conversion of BAT into WAT. This, in turn, affects thermoregulation and satiety/hunger neuronal signals, increases tissue inflammation, and affects mitochondrial function. – Ana Paola Rodríguez Arciniega, MS

References:

Frigolet, Mara E, and Ruth Gutiérrez-Aguilar. The colors of adipose tissue.Los colores del tejido adiposo. Gaceta medica de Mexico vol. 156,2 (2020): 142-149. doi:10.24875/GMM.M20000356

Bartelt, Alexander et al. “Brown adipose tissue thermogenic adaptation require Nrf1-mediated proteasomal activity.” Nature medicine vol. 24,3 (2018): 292-303. doi:10.1038/nm.4481

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