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PDGF – A KEY MITOGEN FOR INTESTINAL SMOOTH MUSCLE CELLS DURING INFLAMMATION?
RDP Stanzel, AJ Schell, MG Blennerhassett
GIDRU, Department of Biology, Queen’s University, Kingston, Ontario
BACKGROUND: Intestinal inflammation causes structural and functional changes to the smooth muscle. In both humans and animal models of intestinal inflammation, thickening of the smooth muscle layer occurs and is in part due to hyperplasia. The factors involved in this have not been identified. Elsewhere, platelet-derived growth factor (PDGF) is responsible for the proliferation of vascular smooth muscle cells (SMC) to facilitate wound healing, but the role of PDGF in the intestine during inflammation is unknown. Hypothesis: Since PDGF is responsible for SMC proliferation elsewhere during damage, we hypothesized that PDGF is a mitogen for intestinal smooth muscle cells (ISMC) during intestinal inflammation.
Methods and RESULTS: The effects of PDGF addition were studied in vitro using rat ISMC cultures at passages 3-6. PDGF was a potent ISMC mitogen as indicated by an 11-fold increase in thymidine incorporation over control. Western blotting revealed that PDGF stimulated receptor (PDGF-R) phosphorylation within 5 min. Both PDGF-induced thymidine incorporation and receptor phosphorylation were blocked by a PDGF receptor antagonist. PDGF stimulation also activated both ERK and Akt pathways maximally at 5 min post application, with the further consequence of PDGF activation was complete loss of receptor expression within 6 hr. In vivo, western blotting showed PDGF-R in the control smooth muscle layers of the rat colon. However, TNBS-induced colitis reduced PDGF-R expression by 70% at 6 hr of inflammation, suggesting PDGF-R activation based on the findings in vitro. To further assess PDGF activity in vivo, the mitogenic potential of inflamed smooth muscle homogenates was evaluated. Preliminary data show that inflamed homogenates induce a 7-fold increase in thymidine incorporation, which was reduced by 40% by a PDGF-R antagonist.
CONCLUSIONS: In vitro, PDGF is a potent ISMC mitogen and activates ERK and Akt. In vivo, PDGF signaling is activated during inflammation, potentially contributing to ISMC hyperplasia. Therefore, antagonism of PDGF signaling may prevent or reduce smooth muscle layer thickening that occurs during intestinal inflammation.
Supported by the CCFC and a Canadian Digestive Health Foundation/CIHR INMD Doctoral Research Award (RDPS)