Activation of Nuclear Factor-κB by High Molecular Weight and Globular Adiponectin

Abstract

Adipose tissue secretes a wide range of hormones named adipokines, and these may play a role in obesity-related inflammation. Adiponectin is an exceptional adipokine because low plasma concentrations are associated with obesity, type 2 diabetes, and cardiovascular diseases. It has been observed that plasma adiponectin concentrations are elevated during inflammatory conditions like preeclampsia and arthritis. Nuclear factor-κB (NF-κB) is an essential transcription factor for expression of inflammation-related proteins. We have used U937 cells stably transfected to express luciferase under the control of NF-κB to examine if adiponectin may modulate NF-κB activity. Physiological concentrations of native adiponectin induced NF-κB activity. This effect was relatively strong compared with proinflammatory adipokines like leptin, resistin, and IL-6. The enhanced NF-κB activity was attributed to the high molecular weight adiponectin isoforms. NF-κB was not activated by mutated adiponectin that is unable to form high molecular weight complexes. Furthermore, the C-terminal fragment, globular adiponectin, markedly increased NF-κB reporter activity, cytokine release, and mRNA expression of inflammation marker genes, at higher levels than stimulation with TNF-α and lipopolysaccharide. NF-κB activation by globular adiponectin was not affected by antibody inhibition of toll-like receptor 4 or TNF receptors 1 and 2 but was attenuated by inhibitors of p38 MAPK, phosphatidylinositol 3-kinase, and protein kinase C. Analyses of the p65 subunit of NF-κB in different leukocyte cell lines showed activation of two monocytic cell lines (U937 and THP-1) by native and globular adiponectin. Our results indicate that adiponectin has proinflammatory properties in monocytic cells.