Direct stimulation of NADP + synthesis through Akt-mediated phosphorylation of NAD kinase

Abstract

Nicotinamide adenine dinucleotide phosphate (NADP + ) is essential for producing NADPH, the primary cofactor for reductive metabolism. We find that growth factor signaling through the phosphoinositide 3-kinase (PI3K)–Akt pathway induces acute synthesis of NADP + and NADPH. Akt phosphorylates NAD kinase (NADK), the sole cytosolic enzyme that catalyzes the synthesis of NADP + from NAD + (the oxidized form of NADH), on three serine residues (Ser 44 , Ser 46 , and Ser 48 ) within an amino-terminal domain. This phosphorylation stimulates NADK activity both in cells and directly in vitro, thereby increasing NADP + production. A rare isoform of NADK (isoform 3) lacking this regulatory region exhibits constitutively increased activity. These data indicate that Akt-mediated phosphorylation of NADK stimulates its activity to increase NADP + production through relief of an autoinhibitory function inherent to its amino terminus.