Adipocyte, Adipose Tissue, and Infectious Disease

The adipocyte and its secretory products have been implicated in a vast array of physiological processes. Work in many laboratories is focusing on the specific contributions of the adipocyte in the areas of metabolism, inflammation, and cancer in order to gain a comprehensive understanding of the systemic influence of the adipocyte under normal and pathophysiological conditions. Until recently, adipose tissue has been considered to be a mere storage compartment of triglycerides. It is now clear that adipocytes are highly active endocrine cells that play a central role in overall energy homeostasis and are important contributors to some aspects of the immune system. They do so not only by influencing systemic lipid homeostasis but also through the production and release of a host of adipocyte-specific and adipocyte-enriched hormonal factors, cytokines, and extracellular matrix components (commonly referred to as “adipokines”). Little attention has been given to the role of adipose tissue in infectious disease. However, the strong proinflammatory potential of adipose tissue suggests an important role in the systemic innate immune response. Furthermore, the adipocyte proper serves as an important target for the intracellular parasite Trypanosoma cruzi, the cause of Chagas’ disease. In chronic Chagas’ disease, adipocytes may represent an important long-term reservoir for parasites from which relapse of infection can occur. In other cases, such as with certain subtypes of adenoviruses, infection with viral particles leads to long-term hyperplasia and hyperproliferation of adipocytes, associating these adenoviral infections with a high propensity for subsequent obesity. Here, we discuss briefly the systemic contributions of adipose tissue to the inflammatory response to infection and provide a brief overview of infectious agents that have a specific impact on adipose tissue. Adipose tissue is composed of many different cell types, but adipocytes are among the most predominant cells. Since different adipose tissue depots show distinct gene expression patterns and vary widely in size and proximity to neighboring organs, individual depots could be viewed as “mini-organs.” Despite differences between the different pads, the depots share similarity with respect to their ability to store lipids and secrete adipose tissue-derived hormones. Adipose tissue stores lipid in the form of triglycerides and cholesterol esters within the lipid droplets that represent specialized organelles inside the adipocyte. Since the lipid droplet is such a large component of the adipocyte (95% of the mass of the adipocyte), changes in the amount of lipid stored within the adipocyte affect fat cell size (ranging from 25 to 250 m). Due to the enormous size of the lipid droplet within a normal adipocyte, these cells were viewed originally merely as lipid storage cells. However, it is now known that adipose tissue acts as an endocrine organ regulating a variety of physiological functions, placing the study of adipokines, or adipose tissue-derived secretory products, at the forefront of research in the field of adipose tissue biology.