Platelet-Derived Growth Factor-BB is the Dominant Mitogen for Intestinal Smooth Muscle Cells in the Trinitrobenzenesulfonic Acid Model of Rat Colitis
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In normal adult physiology, intestinal smooth muscle cells (ISMC) are characterized as contractile and non-proliferative. Inflammation induces permanent changes to the intestine including hypertrophy of the smooth muscle layer largely due to smooth muscle cell (SMC) proliferation. While the consequences of this hyperplasia are largely unknown, increased muscularis mass may present permanent challenges to organ motility. Similar SMC hyperplasia is observed in other inflammatory pathologies including atherosclerosis and pulmonary arterial hypertension (PAH) where SMC de-differentiate into a ‘synthetic’ phenotype and the mitogens responsible for hyperplasia have been well studied. However, there are limited investigations of SMC mitogens in intestinal inflammation. The identification of these factors may be of critical importance in the case of intestinal strictures, whereby recurring inflammation can lead to bowel obstruction requiring surgical intervention. A novel, primary rat ISMC model was developed to identify the factors responsible for ISMC proliferation in vitro. Primary ISMC cultures are likely more representative of SMC in vivo than the commonly used late-passage cultures. As such, this primary ISMC model is valuable in the evaluation of mitogens involved in the onset of proliferation. This primary ISMC model was used to conduct a comprehensive evaluation of potential mitogens including basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), insulin-like growth factor-1 (IGF-1) and platelet-derived growth factor-BB (PDGF-BB. This work identified IGF-1 and PDGF-BB as ISMC mitogens. However, multiple lines of evidence indicated that PDGF-BB was a more potent mitogen and the involvement of PDGF-BB was subsequently examined in vivo using the trinitrobenzenesulfonic acid (TNBS) model of rat intestinal inflammation. While control ISMC lacked expression of the PDGF-BB receptor (PDGF-Rβ), robust expression was observed within only 6 hr following the induction of TNBS inflammation. By Day 2, when ISMC proliferation in vivo is maximal, freshly isolated ISMC showed on-going PDGF-Rβ activation that was further increased by exogenous PDGF-BB. Taken together, the conclusions from this work in vitro identify PDGF-BB as a potent ISMC mitogen in vivo. Further, this work establishes PDGF-BB and its receptor as potential targets in the medical treatment of intestinal stricture formation.