Regulation of Mouse Hepatic α-Amino-β-Carboxymuconate-ϵ-Semialdehyde Decarboxylase, a Key Enzyme in the Tryptophan-Nicotinamide Adenine Dinucleotide Pathway, by Hepatocyte Nuclear Factor 4α and Peroxisome Proliferator-Activated Receptor α

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Abstract
Nicotinamide adenine dinucleotide (NAD) plays a critical role in the maintenance of cellular energy homeostasis. α-Amino-β-carboxymuconate-ϵ-semialdehyde decarboxylase (ACMSD) is the key enzyme regulating de novo synthesis of NAD from l-tryptophan (Trp), designated the Trp-NAD pathway. Acmsd gene expression was found to be under the control of both hepatocyte nuclear factor 4α (HNF4α) and peroxisome proliferator-activated receptor α (PPARα). Constitutive expression of ACMSD mRNA levels were governed by HNF4α and downregulated by activation of PPARα by the ligand Wy-14,643 ([4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio]acetic acid]), as revealed by studies with hepatic HNF4α-null mice and PPARα-null mice, respectively. Transient transfection and electrophoretic mobility shift analyses showed an HNF4α binding site in the Acmsd gene promoter that directed transactivation of reporter gene constructs by HNF4α. The Acmsd promoter was not responsive to PPARα in transactivation assays. Wy-14,643 treatment decreased HNF4α protein levels in wild-type, but not PPARα-null, mouse livers, with no changes in HNF4α mRNA. These results show that Wy-14,643, through PPARα, post-transcriptionally down-regulates HNF4α protein levels, leading to reduced expression of the HNF4α target gene Acmsd.