VPAC1 Signaling Blocks Ikaros Degradation by a PKA Dependent Mechanism in Activated HuT‐78 Cells

Abstract

Over 50,000 people in the U.S. die annually from Leukemia, and over half of these people have a mutation or splicing deregulation of the tumor suppressor Ikaros (IK) protein. The phosphorylation pattern of IK controls its DNA binding activity; through the antagonistic control of casein kinase II and protein phosphatase 1. IK also contains consensus sequences for other kinases such as PKA, PI3K, and cdk1. T cell activation shifts the equilibrium to favor hyperphosphorylation of IK thereby decreasing DNA binding and cell cycle entry. Vasoactive intestinal peptide receptor 1 (VPAC1) signaling opposes the activation of the T cell receptor by blocking T cell activation through a cAMP/PKA signaling cascade. We hypothesized that VIP/VPAC1 signaling regulates the phosphorylation of IK and impedes cell cycle entry. PMA/ionomycin activation of HuT 78 cells resulted in a decrease in IK protein, but was blocked with the addition of the VPAC1 ligand VIP (10−8M). When cells were treated with either an agonist (VIP10–28) or a VIP+PKA inhibitor (H89) IK protein levels were reduced. When treated with forskolin to induce cAMP, no increase in IK was observed. These data suggest that IK is being regulated through a G protein coupled receptor by PKA in a cAMP independent mechanism. Research supported by NIH‐K01 1K01DK064828 and COBRE 2P20RR05566.