NHE1 is Involved in the Cytoskeletal Remodeling and ECM Deposition of Lung Fibrosis

Abstract

Idiopathic Pulmonary Fibrosis (IPF) is a progressive lung disease characterized by the deregulation of the wound healing process leaving an accumulation of fibroblasts and scarring tissue. Fibroblasts, under normal conditions, support growth and health of connective tissues and maintain the extracellular matrix (ECM). However, as the disease progresses, increases in profibrotic agonists lead to activation of fibroblasts causing stiffening of the cells, increasing in secretion of ECM proteins in the bronchial. The chronic nature of the disease leads fibroblast differentiating into myofibroblast cells making the tissue less compliant. The affected patient’s alveolar architecture becomes damaged which then leads to lung inflexibility, disruption of gas exchange, respiratory failure, and death. Sodium Hydrogen Exchanger Isoform 1 (NHE1) is an ion transporter protein found in the membrane that mediates cellular pH, cell migration, and cytoskeletal anchoring. NHE1 has also been found to impact the pathophysiological development of cancer and other diseases. Interestingly, a number of signaling pathways which contribute to the activation of tumor cells are also involved in profibrotic behavior in fibroblasts and myofibroblast, the purpose of this study was to investigate if NHE1 worsened IPF and if inhibiting NHE1 would decrease the profibrotic activity of fibrosis. We measured two of the indicators of progressing fibrosis: the remodeling of skeletal proteins through formation of actin stress fibers, and the production and secretion of ECM proteins such as collagen and fibronectin. Actin stress fibers have an important role in cell migration and contractility. To stimulate this formation, cells were treated with three agonists that have all been found to increase fibrotic activity: Transforming Growth Factor‐β (TGF‐β), Serotonin (5HT), and Lysophosphatidic Acid (LPA) in the presence and absence of Ethyl‐Isopropyl Amiloride (EIPA), a NHE1 inhibitor. LPA induced stress fibers 43.94 +/‐ 4.4 % over the control, and both TGF‐β and 5HT also stimulated the formation of stress fibers by 32.08 +/‐ 2.1 and 39.1 +/‐ 4.2 %, respectively, over the control non‐treated cells. In each case, the addition of EIPA blocked stress fibers indicating that all three profibrotic factors required NHE1 activity for cytoskeletal remodeling. Additionally, when any combination of agonists were added in combination or a mixture of all three, it was observed that these combinations seemed to block stress fiber formation in comparison to the control. An initial interpretation hints at a competitive or feed‐back loop to controlling fibrosis. The role of NHE1 in TGF‐ β induced fibroblast‐myofibroblast differentiation is also determined and is identified with expression of alpha smooth muscle actin (α‐SMA) with and without EIPA. Finally, the role of NHE1 in production and secretion of ECM proteins, such as collagen and fibronectin, were investigated to detect changes in these proteins in the presence and absence of agonists and EIPA. This work highlights that NHE1 has a role in supporting profibrotic behavior and may be a novel target to fight IPF.