Strong fences keep trouble at bay: How the AMPK-GIV pathway maintains epithelial barriers.
We probably don’t need a moonshot to understand some human cancers. Occasionally, basic cell research provides amazing insights when paired with keen observational judgment. In a groundbreaking research study soon to be published in eLife, a group of UC-San Diego cell biologists unravel a complex interplay between stress-related signaling, cellular polarity, and development of cancer phenotypes, mediated by the newly described AMPK-GIV axis.
Defining a novel pathway
AMPK, aka AMP-activated protein kinase, is recognized as a major stress and metabolic regulator, situated at a pivotal switch point in hepatic gluconeogenesis, cholesterol synthesis, fatty acid oxidation and glucose uptake pathways. By conformational changes in the three independent subunits, this kinase can sense modulating ratios of cellular ATP, the primary energy source for cellular physiology and the degradative metabolite AMP, and swiftly react to increasing energy needs by stimulating glucose uptake and fatty acid oxidation.
Sitting at an intersection between cancer, energy balance and AMPK pathways is a multipurpose regulatory adaptor protein, GIV/Girdin originally discovered in 2005 by Dr. Marilyn Farquhar. Many adaptor proteins are found to be multifunctional but the singular GIV (G-alpha interacting vesicle associated protein) takes that to the extreme. The N-terminal domain, described in detail in this study, lacks the G-protein binding and effecting domains found in the C-terminal. Much is currently known about the structure and functions of the C-terminal domain but roles and functions for the N-terminal domain has remained mysterious until now.
AMPK is the hammer and GIV the nail
Anyone who has helped their child build with Legos can appreciate how solid these linked structures can be. A similar effect is found in nature. Cells that are solidly “linked” together form stable and unchanging barriers. These highly polarized epithelial cells are the basis for most of our physical integrity. But maintenance of this cementing cellular structure – the tight junction (TJ) – requires significant energy input, since external forces are constantly placing stresses and strains on each connected cell. This is the point where the AMPK-GIV axis steps in. Cells that have reduced AMPK expression or functional loss of AMPK activity lose cell polarity through weakened tight junctions, an effect exacerbated during periods of energetic stress. But the precise mechanism of AMPK’s actions on TJ proteins was unknown.
Other published studies find a binding role for GIV with a number of TJ associated proteins but no one had put AMPK and GIV together before now. Using bioinformatics to determine putative phosphorylation sites in the N-terminal and following up with biochemical and mutational studies, the group lead by Dr. Pradipta Ghosh, MD conclude that GIV is a substrate for activated AMPK and a single phosphorylation event at serine 245 (S245) is enough to trigger TJ stability during energy deprivation. Stability equates to homeostasis, limiting potential damage.
Why basic science maintains relevancy
As a stand-alone discovery, this interesting revelation moves our knowledge of TJ regulation forward, a static field for almost a decade. The major novel findings from this study are not simple cell biology results. When this group mined cancer mutation databases, they uncovered one specific mutation in two colorectal tumors that disrupted the phosphorylation of GIV by AMPK, resulting in weakened TJ, uncoupled cell energetics and unchecked cell proliferation. In brief, they have given us an enormous leap forward in our understanding of many epithelial cell derived cancers, with this basic cell observation.
Recent interest in cell junctions has been lagging behind other fields of cell biology. Gap junctions are well characterized as direct portals between cells, i.e. electrical conductors in specific neuronal synapses, direct ion or protein exchangers and impulse synchronizers between cardiomyocytes. Tight junctions maintain our physical barriers against invaders. But just like a building, structure precedes function, once structural integrity is lost, the building collapses. Yet for the past decade, our interest has turned elsewhere to find critical initiators of cancer. Let’s hope that it doesn’t take another 10 years to take advantage of this critical finding in the fight against colorectal, ovarian and some forms of breast cancer.
Reference:
AMP-activated protein kinase fortifies epithelial tight junctions during energetic stress via its effector GIV/Girdin.
Nicolas Aznar, Arjun Patel, Cristina Rohena, Ying Dunkel, Linda Joosen, Vanessa Taupin, Irina Kufareva, Marilyn Farquhar and Pradipta Ghosh, (2016) eLife.
Story posted ahead of anticipated formal publication.