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

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 (Ser⁴⁴, Ser⁴⁶, and Ser⁴⁸) 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.