Understanding the Impact of Extracellular Matrix on Integrin Signalling during Liver Fibrosis

Abstract

Liver fibrosis is characterised as scarring of the liver parenchymal tissue due to excess deposition of extracellular matrix (ECM) proteins in response to injury. Liver fibrosis is a major cause of morbidity and mortality worldwide. Liver fibrosis-linked mortality has increased significantly in the UK during the past 50 years due to increased risk factors, inefficient diagnostics, and, most importantly, a lack of effective treatment options. Currently, the only treatment for end-stage liver fibrosis, known as cirrhosis, is a transplant. There is an urgent need for anti-fibrotic therapies; however, this has proven difficult to researchers in the past. Hepatic stellate cells (HSCs) are the key cells responsible for the deposition of ECM proteins during fibrosis. In healthy tissue HSCs are quiescent and are responsible for maintaining a healthy basement membrane and storage of lipid droplets. Once activated through biochemical or mechanical cues, HSCs undergo transdifferentiation becoming myofibroblast-like. This results in a profibrotic phenotype of increased contractility, migration to the site of injury, and rapid proliferation, as well as increased deposition of ECM components, particularly collagen type I. Integrins are transmembrane receptors that span the intracellular and extracellular space, enabling bidirectional communication between the cell and ECM. Integrins are critical in how the cell responds to external cues and, as such, are an attractive target for therapeutics. Integrin beta 1 (ITGB1), in particular, has been established as a critical mediator of HSC activation and progression of fibrotic disease. However, as ITGB1 is ubiquitously expressed it cannot be targeted therapeutically. To address this issue, we investigated downstream targets of ITGB1 and identified the group I p21-activated kinases (PAKs), comprising PAK1, PAK2, and PAK3. PAK1 has been identified as a regulator of various pathways associated with disease development, such as cell motility, cytoskeletal reorganisation, cell survival, and apoptosis, and as a result has become the focus of recent efforts in the search for treatments of various cancers. We investigated the role of PAK1 using a global PAK1-null mouse and established a functional role for PAK1 in HSCs during liver fibrosis in vitro. Transcriptomic and functional analyses demonstrated the mechanisms regulated by PAK1 during HSC activation. Additionally, we investigated PAK1 in vivo using established models of liver fibrosis. Loss of PAK1 ameliorated the development and severity of fibrotic disease following bile duct ligation-induced liver injury. We sought to further examine the response of PAK1 to mechanical stimuli to elucidate the mechanisms governed by PAK1 in HSCs during liver fibrosis. We determined that PAK1 is a mechanosensitive mediator of HSC activity through various mechanisms, including Hippo signalling and nuclear structure and function. Taken together, these data demonstrate an important and specialised role for PAK1 in liver fibrosis and, therefore, highlight PAK1 as a possible target for the development of inhibiting compounds with which to treat patients.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Mark Travis (Supervisor) & Karen Piper Hanley (Supervisor)