Cellular metabolism-induced epigenetic regulation is essential for the maintenance of cellular homeostasis. Nicotinamide N-methyltransferase (NNMT) is emerging as a key point of intersection between cellular metabolism and epigenetic regulation and has a central role in various physiological and pathological processes. NNMT catalyzes the methylation of nicotinamide (NAM) using the universal methyl donor S-adenosyl methionine (SAM) to yield S-adeno-syl-L-homocysteine (SAH) and N1-methylnicotinamide (MNAM)

directly linking methylation balance with nicotinamide adenosine dinucleotide (NAD

+

) contents. NNMT acts on either the SAM-methylation balance or both NAD

+

metabolism

depending on the tissue involved or pathological settings where metabolic demand is increased. Under physiological conditions

the liver act as an essential metabolic organ with abundant NNMT expression

while NNMT hepatic function is not mediated by its methyltransferase activity due to other major methyltransferases such as glycine N-methyltransferase (GNMT) in the liver. However

hepatic NNMT

as well as its metabolite is improperly regulated and linked to the worse pathological states in liver diseases

including alcoholic liver disease

non-alcoholic fatty liver disease (NAFLD)

liver cirrhosis

and hepatocellular carcinoma (HCC)

suggesting a potential role in the process of liver diseases. In this review

we summarize how NNMT regulates cell methylation balance and NAD metabolism

and extensively outline the current knowledge concerning the functions of NNMT in hepatic metabolism including glucose

lipid and energy

with a specific focus on the contribution of NNMT to the pathophysiology of liver-related diseases. NNMT is involved in the development and progression of liver diseases. Understanding the complex NNMT regulatory network and its effects on pathogenesis could provide new therapeutic strategies in the context of liver diseases.