AMP-activated protein kinase fortifies epithelial tight junctions during energetic stress via its effector GIV/Girdin

Abstract

Loss of epithelial polarity impacts organ development and function; it is also oncogenic. AMPK, a key sensor of metabolic stress stabilizes cell-cell junctions and maintains epithelial polarity; its activation by Metformin protects the epithelial barrier against stress and suppresses tumorigenesis. How AMPK protects the epithelium remains unknown. Here, we identify GIV/Girdin as a novel effector of AMPK, whose phosphorylation at a single site is both necessary and sufficient for strengthening mammalian epithelial tight junctions and preserving cell polarity and barrier function in the face of energetic stress. Expression of an oncogenic mutant of GIV (cataloged in TCGA) that cannot be phosphorylated by AMPK increased anchorage-independent growth of tumor cells and helped these cells to evade the tumor-suppressive action of Metformin. This work defines a fundamental homeostatic mechanism by which the AMPK-GIV axis reinforces cell junctions against stress-induced collapse and also provides mechanistic insight into the tumor-suppressive action of Metformin.DOI: http://dx.doi.org/10.7554/eLife.20795.001

The stress polarity signaling (SPS) pathway serves as a marker and a target in the leaky gut barrier: implications in aging and cancer

“…Three studies (Zhang et al, 2006;Lee et al, 2007;Zheng & Cantley, 2007) reported a role of AMP-activated protein kinase (AMPK) in the maintenance of epithelial cell polarity and barrier functions in the context of stress; who or what was its effector at the cell-cell junctions remained unknown. A decade later, Aznar et al (2016) demonstrated that GIV (G-α interacting vesicle-associated protein, also known as Girdin), a multimodular polarity scaffold protein is a substrate of AMPK and defined the molecular mechanisms by which the AMPK-GIV signaling axis protects the epithelium by stabilizing tight junctions (TJs) and preserving cell polarity when challenged with energetic stress. Using MDCK cells as a model of polarized mammalian cells, Aznar et al (2016) showed that energetic stress triggers localized activation of AMPK at the tricellular TJs which mark the most vulnerable cell-cell contacts in sheets of polarized cells.…”

“…A decade later, Aznar et al (2016) demonstrated that GIV (G-α interacting vesicle-associated protein, also known as Girdin), a multimodular polarity scaffold protein is a substrate of AMPK and defined the molecular mechanisms by which the AMPK-GIV signaling axis protects the epithelium by stabilizing tight junctions (TJs) and preserving cell polarity when challenged with energetic stress. Using MDCK cells as a model of polarized mammalian cells, Aznar et al (2016) showed that energetic stress triggers localized activation of AMPK at the tricellular TJs which mark the most vulnerable cell-cell contacts in sheets of polarized cells. A significant part of the junction-stabilizing effects of AMPK agonists such as 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR) and metformin during energetic stress (Zhang et al, 2006;Zheng & Cantley, 2007) appeared to be mediated by AMPK via its downstream effector, pS245-GIV (Aznar et al, 2016).…”

The stress polarity pathway: AMPK 'GIV'-es protection against metabolic insults

“…Schematic showing the pertinent findings reported in by Aznar et al [34]. Top (from left to right) : In normal physiologic states, sheets of polarized epithelial cells maintain barrier integrity by assembling tight junctions TJs; stained here with the TJ-marker and integral membrane protein, Occludin in green.…”

“…Because GIV is ubiquitously expressed junctional scaffold, in both epithelial [36] and endothelial cells [39], it is possible that the stress-triggered mechanisms outlined by Aznar et al, [34] enable the barrier-protective role of AMPK at TJs observed in a diverse organs and tissues, both epithelial and endothelial linings, when challenged with chemical, bacterial and metabolic stressors (Figure 1). …”

GIV/Girdin and Exo70 Constitute the Core of the Mammalian Polarized Exocytic Machinery

“…Here we reveal that the multi-modular polarity scaffold Gα-interacting vesicle associated protein (GIV, a.k.a, Girdin) (Aznar et al, 2016b;Ohara et al, 2012;Sasaki et al, 2015) fulfils all criteria to serve as the mammalian functional homolog of the yeast polarity determinant, Bem1p. The scaffolding function of GIV has been shown to mediate the purposeful dynamic 'glue'-ing of diverse signaling pathways: GIV is a guanine nucleotide exchange factor (GEF) for trimeric GTPases, Gi (Garcia-Marcos et al, 2009;Kalogriopoulos et al, 2019) and Gs (Gupta et al, 2016), a polarity scaffold that binds and modulates via G protein intermediates the aPKC/Par complexes (Ohara et al, 2012;Sasaki et al, 2015); it is a substrate of and major signaling conduit for multiple receptor and non-receptor tyrosine kinases (Lin et al, 2011;Midde et al, 2018;Mittal et al, 2011), serves as a signaling adaptor for growth factor (Lin et al, 2014), integrins (Leyme et al, 2015;Lopez-Sanchez et al, 2015a) and other classes of receptors [reviewed in (Ghosh et al, 2017)], is an actin remodeler (Enomoto et al, 2005) and a scaffold for polymerized microtubules (Aznar et al, 2016b;Nechipurenko et al, 2016). Most of the work on GIV thus far have revealed its ability to serve as an integrator of multiple signaling pathways that enhances multiple fundamental cellular processes, and how its dysregulation (overexpression and hyperactivation) favors cancer invasion and metastasis [reviewed in (Aznar et al, 2016a)].…”