Effect of lipopolysaccharide (LPS) on HAEC cells. Does nicotinamide N-methyltranferase sensitize HAEC cells to LPS?

Treatment of endothelial cells with bacterial lipopolysaccharide (LPS) evokes a number of metabolic and functional consequences which built a multifaceted physiological response of endothelium to bacterial infection. Here effects of LPS on human aortic endothelial cells (HAEC) have been investigated. Among the spectrum of biochemical changes substantially elevated N-nicotinamide methyltransferase (NNMT) protein level was particularly intriguing. This important enzyme may potentially affect cellular metabolism by two means: direct regulation of methylnicotinamide level and availability of nicotinamide, that at least potentially may influence NAD+ synthesis, and regulation of S-adenosylmethionine concentration and therefore controlling methylation of many proteins including chromatin. This may have epigenetic consequences. This paper is focused on NNMT, despite the fact that in the presence of LPS additional effects of this compound mask pure (canonical) consequences of the elevated NNMT protein which are an increased MNA synthesis or reduced NAD+ level. On the other hand, however, it has been shown that silencing of the NNMT-encoding gene prevents several changes which are observed in control HAECs treated with LPS. They include significantly increased calcium response to thapsigargin (store-operated calcium entry), altered energy metabolism which is switched to anaerobic glycolysis and rearrangement of the mitochondrial network. However, a biochemical mechanism behind the protective consequences of the NNMT deficiency in cells treated with LPS remains unexplained.