Regulation of semicarbazide-sensitive amine oxidase expression by tumor necrosis factor-alpha in adipocytes: functional consequences on glucose transport.

Membrane-associated semicarbazide-sensitive amine oxidase (SSAO) is mainly present in the media of aorta and in adipose tissue. Recent works have reported that SSAO activation can stimulate glucose transport of fat cells and promote adipose conversion. In this study, the murine 3T3-L1 preadipose cell line was used to investigate SSAO regulation by tumor necrosis factor-alpha (TNF-alpha), a cytokine that is synthesized in fat cells and known to be involved in obesity-linked insulin resistance. SSAO mRNA and protein levels, and enzyme activity were decreased by TNF-alpha in a dose- and time-dependent manner, without any change of SSAO affinity for substrates or inhibitors. SSAO inhibition caused by TNF-alpha was spontaneously reversed along the time after TNF-alpha removal. The decrease in SSAO expression also occurred in white adipose tissue of C57BL/6 mice treated with mTNF-alpha. Overall, we demonstrated that reduction in SSAO expression induced by the cytokine had marked repercussions on amine-stimulated glucose transport, in a dose- and time-dependent manner. This effect was more pronounced than the inhibiting effect of TNF-alpha on insulin-stimulated glucose transport. Moreover, the peroxisome proliferator-activated receptor gamma agonists thiazolidinediones did not reverse either TNF-alpha effect on amine-sensitive glucose transport or the inhibition of SSAO activity, whereas they antagonized TNF-alpha effects on insulin-sensitive glucose transport. These results demonstrate that TNF-alpha can strongly down-regulate SSAO expression and activity, and through this mechanism can dramatically reduce amine-stimulated glucose transport. This suggests a potential role of this regulatory process in the pathogenesis of glucose homeostasis dysregulations observed during diseases accompanied by TNF-alpha overproduction, such as cachexia or obesity.