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Extracellular and Intracellular Signaling (книга).pdf
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Chapter 4

increases the NO production from eNOS, which might contribute to its protective e ects, whereas during cardiac pathology adiponectin inhibits iNOS activation and thus reduces the NO overproduction, which can contribute to increased cardiac injury.

Another potential mechanism underlying the cardioprotective e ect of adiponectin may involve a stimulation of prostaglandin (PG) synthesis via the inducible cyclooxygenase-2 (COX-2) dependent pathway. In this regard, it was shown that adiponectin stimulates PGE2 synthesis and increases COX-2 expression in neonatal rat ventricular myocytes whereas a COX-2 inhibitor abrogated the infarct size sparing e ect of adiponectin in mice subjected to 30 minutes coronary artery occlusion followed by 48 hours of reperfusion.73 While the mechanism underlying the ability of adiponectin to upregulate COX-2 is not fully understood recent evidence suggests that this occurs subsequent to sphingosine kinase-1/sphingosine-1-phosphate receptor-dependent pathway.76

A recent study has also implicated PPARg activation as mediating the anti-hypertrophic e ects of adiponectin in a model of cardiac hypertrophy and remodeling produced by a high-fat diet,77 although a PPARa-dependent e ect has also been implicated at least with respect to angiotensin-2 induced cardiac fibrosis.78 Lastly, it has recently been reported that the anti-hypertrophic e ect of adiponectin in cardiomyocytes was associated with reduced heparin-binding epidermal growth factor (EGF) signaling, which included the downregulation of the EGF receptor.79

4.8 Resistin

A relatively new adipokine, resistin (for ‘‘resistance to insulin’’), was first identified in 2001 as a gene target of the insulin-sensitizing drugs thiazolidinediones (TDZs). Resistin is an adipose-derived secreted factor, produced almost exclusively in white adipose tissue. Resistin is a 12-kDa protein that circulates as either a trimer (monomeric form of the peptide hormone) or a hexamer (dimeric form of resistin). However, controversy remains as to which form, monomeric or dimeric, is responsible for the physiologic properties of the peptide.1 The monomeric form was shown to impair hepatic insulin action more potently than the dimerized form.80 In contrast, the dimerized form of resistin was shown to be more e ective in antagonizing insulin-stimulated glucose uptake in adult murine cardiomyocytes.81 Interestingly, the notion of di erential regulation of hormone signaling based on oligerimization state is shared by another adipokine, adiponectin.82,83 Resistin levels are elevated in patients with heart failure and may represent independent risk factors for heart failure development.84,85 Indeed, a recent study reported that resistin is expressed in the cardiac cell86 and its expression is increased by mechanical stretch, which occurs via a TNFa-dependent pathway involving MAPK and NF-kB activation.87 Resistin is a prohypertrophic factor acting via MAPK activation as well as increased phosphorylation of the insulin receptor

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