Mechanotransduction: Forcing a change in metabolism

Abstract

Epithelial and endothelial cells experience numerous mechanical cues throughout their lifetimes. Cells resist these forces by fortifying their cytoskeletal networks and adhesions. This reinforcement is energetically costly. Here we describe how these energetic demands are met. We focus on the response of epithelial and endothelial cells to mechanical cues, describe the energetic needs of epithelia and endothelia, and identify the mechanisms these cells employ to increase glycolysis, oxidative phosphorylation, and fatty acid metabolism. We discuss the similarities and differences in the responses of the two cell types.

Figure 1.. Mechanotransduction by cadherins in epithelia and endothelia.

In epithelia A, forces imparted by a variety of stimuli, such as air flow, are sensed by E-cadherin which undergoes conformational changes. These conformational changes allow for the increased intracellular signaling and recruitment of proteins to the cadherin cytoplasmic domain. Key among these are β-catenin and α-catenin which undergo conformational changes which expose binding sites for proteins such as vinculin, and F-actin. Phosphorylation of vinculin at Y822 is critical to downstream activation of the Rho family of GTPases. Active RhoA stimulates Rho kinase (ROCK) which promotes myosin light chain (MLC) phosphorylation by directly adding phosphates groups and inactivating the myosin light chain phosphatase (MLCP). In endothelia B, the mechanosensing complex responds to shear from the flow of blood and other sources and is more intricate. VE-cadherin is part of a mechanosensory complex, including platelet endothelial cell adhesion molecule 1 (PECAM-1), and vascular endothelial growth factor receptor 2 (VEGFR2) that are required for responding to changes in tension. Each member of the mechanosensory complex has a distinct role in transmitting force. PECAM-1 transmits mechanical force in response to shear and its association with vimentin is critical. VE-cadherin bears tension and responds by recruiting binding partners, such as β-catenin and α-catenin. This complex, like the E-cadherin mechanosensing complex, binds vinculin and F-actin and stimulates downstream signaling leading to increased myosin light chain phosphorylation and increased contractility and actin cytoskeletal rearrangements. VEGFR2 activates downstream signaling via PI3K. For both cell types, there is substantial crosstalk between this mechanotransduction machinery and that which is rich in integrins and is present at sites where the cell adheres to the extracellular matrix (ECM).

Figure 2.. Mechanisms epithelia and endothelia employ to adjust glycolytic metabolism in response to force.

A, in epithelia, E-cadherin which is responsible for the adhesion of cells to their neighbors, senses changes in tension and initiates multiple effects. (1) It stimulates recruitment of proteins and activation of downstream signaling. Liver kinase B1 (LKB1) is recruited to the cadherin cytoplasmic domain and activates AMP-activated protein kinase (AMPK). AMPK stimulates recruitment of ankyrin G to E-cadherin which allows for complex formation between E-cadherin and glucose transporter 1 (GLUT1) and the uptake of glucose. (2) Force also stimulates the release and activation of glycolytic enzymes, such as aldolase, from the actin cytoskeleton. (3) Force imparted by plating epithelial cells on stiff substrates, leads to increased levels and activity of the most important regulator enzyme of glycolysis, phosphofructokinase-1 (PFK-1). In addition, force also inhibits PFK-1 degradation by causing F-actin to sequester tripartite motif containing 21 (TRIM21), the E3 ubiquitin ligase, that degrades PFK-1. B, in endothelia, a mechanosensing complex of VE-cadherin, PECAM, and VEGFR2 senses changes in tension and increases in AMPK. Increased AMPK activation triggers PFKFB3 phosphorylation, enhancing glycolysis by increasing fructose-2,6-bisphosphate levels, an allosteric activator of PFK-1. Shear stress also stimulates endothelial nitric oxide synthase (eNOS), which enhances nitric oxide production. The exact mechanism remains unclear, but AMPK mediated phosphorylation may be involved. Nitric oxide stimulates glycolysis by targeting multiple glycolytic components.

Figure 3.. Other metabolic alterations in epithelia and endothelia in response to force.

A, In epithelia, shear stress induces the degradation of lipid droplets (lipophagy) promoting the availability and increased β-oxidation of fatty acids for ATP production. Additionally, force leads to mitochondrial reorganization and metabolic reprogramming. Flow promotes AMPK activation which phosphorylates PGC-1α and increases mitochondrial biogenesis, and changes in the stiffness of epithelial cells alters mitochondrial function by changing solute carrier family 9 member A1 (SLC9A1) ion transport and heat shock factor-1 transcription in a process known as mitohormesis. B, Endothelial cells utilize fatty acids as a fuel source. shear activates the fatty acid transporter CPT1A, thereby increasing fatty acid transport and fatty acid oxidation (FAO). Shear stress affects mitochondrial dynamics in endothelia. Shear increases mitochondrial fusion events through increased production of optic atrophy protein 1 (OPA1) and mitofusion 2 (MTF2) and sequesters the endothelial fission protein dynamin-related protein 1 (DRP1) away from mitochondria. Additionally, shear induced AMPK triggers mitochondrial biogenesis via peroxisome proliferator-activated receptor-gamma coactivator 1α (PGC-1 α) phosphorylation.