Adipose tissue, obesity, and Covid-19
Released mainly by macrophages, albeit also by adipocytes, TNF-a (Tumour Necrosis Factor-alpha) is one of the most significant pro-inflammatory cytokines in our body. According to Kern et al., 16 levels of TNF-a in the blood plasma and AT positively correlate with BMI. Higher levels in the blood plasma are due to a higher propo rtion of M1 ‘classically activated’ macrophages in the A T. 17 It has also been found that a reduction in BMI directly causes a reduction in the levels of TNF-a in the blood plasma. 18 TNF-a has been seen to have a prevalent role in the onset of insulin resistance as a well as a major role in the development of atherosclerosis. 19 Much like TNF-a, IL-6 (Interleukin-6) is yet again prevalent in the immune system. Only one third of IL-6 originates frommacrophages or monocytes within AT. 20 IL-6 is a pro-inflammatory cytokine, which is regularly released by various white blood cells upon infection. The levels of IL-6 are elevated in people with obesity and are also higher in people with type 2 diabetes. Hence it is not surprising that IL-6 is involved in the development of insulin resistance as well as type 2 diabetes. 21 It has also been found that increased secretion of IL-6 and TNF-a lead to a decrease of adiponectin (mentioned below) mRNA in vitro. 22 The best-known anti-inflammatory adipokine,
adiponectin, is almost exclusively produced by adipocytes in the AT. It is produced by bone marrow AT 23 and signals itself through 2 different receptors: AdipoR1 and AdipoR2. 24 Adiponectin has been seen to inhibit inflammation by reducing the secretion of pro- inflammatory cytokines frommacrophages such as IL-6 and TNF-a and directly attenuates vascular inflammation. 25 Its anti-inflammatory properties are evident because it stimulates the production of IL-10, an anti-inflammatory cytokine, from
Picture showing effect of obesity on secretion of adipokines https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4970637/)
macrophages. This adipokine has a preventive effect against developing cardiovascular diseases, notably by reducing the risk of atherosclerosis 26 and also increases insulin sensitivity. The plasma levels of adiponectin negatively correlate with BMI for obese people 27 and negatively correlate with levels of
16 Kern et al., 2001 at https://journals.physiology.org/doi/full/10.1152/ajpendo.2001.280.5.E745?rfr_dat=cr_pub++0pubmed&url_ver=Z39 .88-2003&rfr_id=ori%3Arid%3Acrossref.org. 17 Weisberg et al., 2003 at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC296995/. 18 Ouchi et al. (n.6). 19 Choy et al., 2001 at https://www.jmmc-online.com/article/S0022-2828(01)91419-1/pdf. 20 Mohamed Ali et al, 1997 at https://academic.oup.com/jcem/article/82/12/4196/2866069. 21 Pradhan et al, 2001 at https://jamanetwork.com/journals/jama/fullarticle/1150732. 22 Bruun et al, 2003 at https://journals.physiology.org/doi/full/10.1152/ajpendo.00110.2003?rfr_dat=cr_pub++0pubmed&url_ver=Z39.88- 2003&rfr_id=ori%3Arid%3Acrossref.org. 23 Mancuso et al. (n.3). 24 Yamauchi et al, 2013 at https://www.cell.com/cell-metabolism/fulltext/S1550-4131(13)00006- 5?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS1550413113000065%3Fshowall% 3Dtrue. 25 Mancuso et al. (n.3). 26 Smitka et al. (n.1). 27 Weyer et al, 2001 available at https://academic.oup.com/jcem/article/86/5/1930/2848024.
66
Made with FlippingBook Digital Publishing Software