Cell adhesion molecule IGPR-1 activates AMPK connecting cell adhesion to energy sensing and autophagy

Abstract

Cellular stress caused by loss of extracellular matrix (ECM)-cell interaction involving integrins induces autophagy and autophagy is considered to protect cells from anoikis (i.e., cell death due to loss of cell adhesion). Immunoglobulin (Ig) and proline-rich receptor-1 (IGPR-1) is a newly identified cell adhesion molecule that mediates cell-cell adhesion and supports cell survival in response to anoikis.However, whether IGPR-1 is activated in response to and mediates autophagy is not known. In this study, we demonstrate that IGPR-1 is activated by autophagy inducing stimuli, such as nutrient deprivation, rapamycin and lipopolysaccharide (LPS). We have identified IκB kinaseβ (IKKβ) as a key serine/threonine kinase activated by autophagy stimuli and mediates phosphorylation of IGPR-1 at Ser220. Activation of IGPR-1, in turn, stimulates phosphorylation of AMP-activated protein kinase (AMPK), which leads to phosphorylation of key pro-autophagy proteins, ULK1 and Beclin-1, increased LC3-II levels and accumulation of LC3 punctum. This study demonstrates that IGPR-1 is activated by and regulates autophagy, connecting cell adhesion to autophagy, a finding that has important significance for autophagy-driven pathologies such cardiovascular diseases and cancer. Introduction:Autophagy (also called macroautophagy), the lysosomal degradation of cytoplasmic organelles or cytosolic components, is an evolutionarily conserved cytoprotective mechanism that is induced in response to cellular stress, such as nutrient withdrawal, loss of cell adhesion, and flow shear stress, or by therapeutic genotoxic agents and others 1, 2 . Cell-extracellular matrix (ECM) adhesion through engagement of cell adhesion molecules (CAMs) (e.g., integrins) is thought to regulate autophagy 3, 4 and more importantly loss of cell adhesion to ECM is linked to induction of autophagy 4, 5 .Upon autophagy, unc-51-like kinase 1 (ULK1 also known as ATG1) associates with autophagy-related protein 13 (ATG13), and focal adhesion kinase family interacting protein of 200 kD (FIP200) to form the ULK1 complex. ULK1 interaction with ATG13 and FIP200 is critical for ULK1 kinase activity and stability 6 .The ULK1 complex translocates to autophagy initiation sites and recruits the class III phosphatidylinositol 3-kinase, vacuolar protein sorting 34 (VPS34) complex consisting of BECLIN-1 (the mammalian orthologue of the yeast autophagy protein Apg6/Vps30 7 and multiple other ATGs leading to the phagophore formation 8 . The serine/threonine protein kinase mTOR (mechanistic target of rapamycin) complex 1 (mTORC1) is a key regulator of autophagy in response to nutrient availability. In the presence of amino acids, mTORC1 is activated and suppresses autophagy through phosphorylation of ULK1 and ATG13. However, upon nutrient deprivation, mTORC1 activity is inhibited leading to the activation of ULK1 that induces the autophagy program 9, 10 . Suppression of mTORC1 activity by AMP-activated protein kinase (AMPK) is central to regulation of autophagy. AMPK inactivates mTORC1 throu...