Adipose Tissue Quality in Aging: How Structural and Functional Aspects of Adipose Tissue Impact Skeletal Muscle Quality

Myokines and adipokines in sarcopenia: understanding cross-talk between skeletal muscle and adipose tissue and the role of exercise

Detrimental age-associated changes in skeletal muscle and adipose tissue increase the risk of sarcopenia. Age-related changes in myokines, such as myostatin and irisin, as well as adipokines, such as leptin and adiponectin, contribute to cross-talk between muscle and adipose tissue. These age-related changes in myokines and adipokines have important implications for sarcopenia, however, recent literature highlights discrepancies in these relationships. Exercise may alter serum profiles and muscle receptor expression of these factors, but future work is needed to determine whether these changes in myokines and adipokines relate to improvements in muscle mass and function. Here, we describe myokine-mediated and adipokine-mediated interactions between muscle and adipose tissue, and discuss the fundamental importance of these cytokines to understanding the development of sarcopenia.

Suppressing adipocyte inflammation promotes insulin resistance in mice

OBJECTIVE

Obesity-induced insulin resistance is closely associated with chronic subclinical inflammation in white adipose tissue. However, the mechanistic involvement of adipocyte-derived inflammation under these disease conditions remains unclear. Our aim was to investigate the relative inflammation-related contributions of adipocytes and macrophages to insulin sensitivity.

METHODS

RIDα/β is an adenoviral protein complex that inhibits several inflammatory pathways, including TLR4, TNFα, and IL1β signaling. We generated novel mouse models with adipocyte-specific and macrophage-specific doxycycline (dox)-inducible RIDα/β-transgenic mice (RID^ad and RID^mac mice, respectively).

RESULTS

RIDα/β induction significantly reduced LPS-stimulated inflammatory markers, such as Tnf, Il1b, and Saa3 in adipose tissues. Surprisingly, RID^ad mice had elevated levels of postprandial glucose and insulin and exhibited glucose intolerance and insulin resistance, even under chow-fed conditions. Moreover, the RID^ad mice displayed further insulin resistance under obesogenic (high-fat diet, HFD) conditions despite reduced weight gain. In addition, under pre-existing obese and inflamed conditions on an HFD, subsequent induction of RIDα/β in RID^ad mice reduced body weight gain, further exacerbating glucose tolerance, enhancing insulin resistance and fatty liver, and reducing adiponectin levels. This occurred despite effective suppression of the inflammatory pathways (including TNFα and IL1β). In contrast, RID^mac mice, upon HFD feeding, displayed similar weight gain, comparable adiponectin levels, and insulin sensitivity, suggesting that the inflammatory properties of macrophages did not exert a negative impact on metabolic readouts. RIDα/β expression and the ensuing suppression of inflammation in adipocytes enhanced adipose tissue fibrosis and reduced vascularization.

CONCLUSION

Our novel findings further corroborate our previous observations suggesting that suppressing adipocyte inflammation impairs adipose tissue function and promotes insulin resistance, despite beneficial effects on weight gain.