Flavin-containing monooxygenases (FMO) have been implicated in the regulation of glucose and lipid homeostasis and in the development of atherosclerosis. However, we have shown that increased FMO expression is associated with improved insulin action in livers of protein kinase C (PKC)δ- and PKCε-deficient mice.

Here we examined the interaction between lipid oversupply and FMO3 expression in hepatocytes. Palmitate treatment of HepG2, HuH7 or IHH hepatocytes resulted in downregulation of FMO3 mRNA. Reporter assays indicated that PKC activation in combination with fatty acid treatment reduced FMO3 promoter activity, consistent with increased FMO3 levels in PKC knockout mice.

We then investigated whether FMO3 expression affected insulin signalling, glucose metabolism and ER stress. Palmitate treatment reduced insulin action at the level of Akt phosphorylation and glycogen synthesis, but these were not significantly improved by FMO3 overexpression. However, the fatty acid also increased the levels of several endoplasmic reticulum (ER) stress markers and activation of caspase 3, which were counteracted by FMO3 overexpression and exacerbated by FMO3 knockdown. FMO activity can improve ER redox balance and protein disulphide bond formation in yeast, and we examined whether a similar function protected hepatocytes against ER stress induction. Using a fluorescent indicator of ER protein thiol redox poise, we showed that in fact FMO3 promoted a decrease in ER redox and did not prevent the decrease caused by palmitate. Both the fatty acid and FMO3 also increased reactive oxygen species levels in hepatocytes. These data indicate that the enzyme protects against ER stress independently of redox balance.

FMO3 expression also prevented upregulation of the gluconeogenic enzyme PEPCK by pharmacological ER stress inducers and by palmitate. ER stress and PEPCK levels were also reduced in livers of fat-fed PKCδ-deficient mice. Our data indicate that FMO3 can contribute to the regulation of glucose metabolism in the liver by reducing lipid-induced ER stress and the expression of PEPCK, independently of insulin signal transduction. This is an important consideration, because FMO3 has recently been highlighted as a therapeutic target to prevent the development of diabetes-induced atherosclerosis.