1. Introduction
1.2 Aging
1.2.4 IL-6/JAK2/STAT3 Signaling
The interleukin-6 (IL-6)-Janus Kinase 2 (JAK2)-Signal transducer and activator of transcription 3 (STAT3) pathway mediates signaling from the plasma membrane to the nucleus. IL-6 binds to target cells via the low affinity membrane bound IL-6 receptor (IL-6R), which induces homodimerization of glycoprotein 130 (gp130) receptors. The gp130 dimer induces phosphorylation and thus activation of JAK2 which is associated with the intracellular domain of the receptor 104. Activated JAK2 phosphorylates the gp130 receptor, thereby creating binding sites for cytosolic STAT3 via its SH2 domain. Recruited STAT3 is then phosphorylated on tyrosine 705 by JAK2 and this induces homodimerization of two STAT3 molecules, again via their SH2 domains. Homodimeric STAT3 dissociates from the receptor and translocates to the nucleus where it acts as a transcription factor for several genes. Phosphorylation of serine 727 was shown to promote STAT3 transcriptional activity, but is not necessary for its function. Endothelial cells do not express IL-6R and are therefore unresponsive to IL-6. A soluble form of IL-6R (sIL-6R) can be generated by shedding from the membrane bound receptor or by alternative splicing 105,106. sIL-6R can be found in biological fluids where it forms a complex with IL-6 that can bind to gp130 receptors, thus widening the spectrum of IL-6 to non-responsive cells 107. STAT3 was shown to promote p21 expression via transcriptional activation of FOXP3 and by direct binding in the p21 promoter region 108,109. STAT3 was also shown to upregulate the expression of ICAM-1 in human hepatocellular carcinoma cells and in endothelial cells 110,111. It furthermore regulated IL-6 expression by direct binding to the IL-6 promoter region in a PKCε-dependent manner 112,113.
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Figure 8: IL-6/JAK2/STAT3 signaling in endothelial cells. Interleukin-6 (IL-6) forms a complex with soluble IL-6 receptor (sIL-6R) in blood and binds to EC membrane bound gp130 receptors. Upon ligand binding, gp130 receptors dimerize and phosphorylate the associated phosphokinase JAK2. Phosphorylated JAK2 phosphorylates the gp130 receptor, creating binding sites for STAT3 protein. Upon binding to the gp130 receptor, STAT3 is phosphorylated at TYR705 and this induces dimerization and translocation to the nucleus. In the nucleus STAT3 acts as a transcription factor for several genes, e. g.
ICAM-1, p21, and IL-6. Modified from https://www.cellsignal.com/contents/science-pathway-research-immunology-and-inflammation/jak-stat-signaling-pathway/pathways-il6
Circulating biomarkers of inflammatory processes, especially C-reactive protein (CRP) and fibrinogen, were shown to be associated with the development of coronary heart disease over many years 114–
117. Although CRP and fibrinogen are regulated by many cytokines, IL-6 plays a central role 118. Furthermore, increased plasma concentrations of IL-6 predicted both total and cardiovascular mortality over a 5-year period independent of the traditional risk factors for atherosclerosis 119. A SNP in the IL-6 promoter was previously shown to be associated with longevity 120 and serum IL-6 concentration correlated with age in two independent cohorts 121. These observations highlight a
16 central role of IL-6 in cardiovascular disease. Approximately 30% of all circulating IL-6 is produced in subcutaneous adipose tissue, linking obesity with risk for coronary artery disease 122. IL-6 has a variety of functions, including stimulation of hepatic synthesis of acute-phase reactants, activation of endothelial cells, increased coagulation and promotion of lymphocyte proliferation and differentiation 35. Inflammatory activation of endothelial cells induces IL-6 secretion together with inflammatory adhesion molecule presentation on the cell surface 123. As mentioned before, IL-6 plays a central role in propagating the inflammatory response in atherosclerosis development 35. Consequently, exogenous administered IL-6 significantly enhanced the development of fatty lesions in mice, increasing the size of the fatty streak by 1.9 to 5.1-fold over control animals in C57/Bl6 and ApoE-deficient mice upon high fat diet 124.
Global STAT3 knockout is embryonically lethal in mice 125. Induced STAT3 deletion in cardiomyocytes of young mice results in no phenotype under baseline conditions but showed enhanced susceptibility to myocardial ischemia/reperfusion injury and infarction with increased apoptosis, increased infarct size and reduced cardiac function and survival 126. In aged mice, cardiomyocyte-specific STAT3 knockout resulted in reduced myocardial capillary density and increased interstitial fibrosis 126. Vice versa, cardiomyocyte-specific overexpression of STAT3 resulted in increased myocardial capillary density and increased expression of proangiogenic factors VEGF and VE-cadherin 127. STAT3 inhibition was shown to promote satellite cell expansion and tissue repair in aged mice 128. These studies highlight the controversial role of STAT3 in the process of aging. Conditioned medium from STAT3-deficient cardiomyocytes inhibited endothelial cell proliferation and increased fibroblast proliferation, suggesting the presence of paracrine factors attenuating angiogenesis and promoting fibrosis in vitro 126. Proangiogenic STAT3 signaling seems to involve paracrine and autocrine mechanisms in various cells, such as expression and regulation of vascular endothelial growth factor (VEGF) 126,127,129. VEGF was shown to stimulate tyrosine phosphorylation of STAT3, STAT1 and STAT6 and nuclear translocation of the latter two, but not STAT3 129. Taken together, these studies highlight the role of IL-6 in atherosclerosis initiation and progression and also in overall fitness and longevity.
STAT3 was shown to play a role during aging and in some studies STAT3 inhibition had positive effects during aging, e. g. during tissue regeneration, in some studies the opposite was the case, for example in regards to angiogenesis.
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