Stem cell factor is expressed by and may affect vascular SMC’s through an autocrine pathway

Abstract

Introduction: Stem cell factor (SCF) is a 45 kDa membrane bound growth factor that is expressed by numerous cell types. SCF is cleaved and released from cell membranes in its active form by proteases such as matrix metalloproteinases-9 (MMP-9). Soluble SCF induces cellular proliferation by binding the c-kit receptor, a tyrosine kinase closely related to the platelet derived growth factor (PDGF) receptor (which is an important mediator of SMC proliferation). Because of the similarities between these receptors we hypothesized that SCF may also play a role in SMC function and thus the development of intimal hyperplasia. Methods: SMC were explanted from human saphenous veins. Expression of SCF and c-kit was measured by immunohistochemistry and/or Western blotting. Results: In order to determine whether human SMC might express SCF, we performed immunohistochemistry on cultured primary cells. We found that SCF was widely expressed on cell membranes (Figure 1A). To determine whether SCF bound to SMC membranes can be cleaved and released by matrix metalloproteinases, we incubated SMC for 6 hours at 37 degrees with 1 μM of MMP-9. Following MMP-9 treatment, SMC bound SCF (cell lysate) decreased by 50% (Figure 1B). Furthermore, MMP-9 treatment resulted in a dramatic increase in soluble SCF (both the 31 and 45 kDa forms) (Figure 1B). To establish if SCF may have an autocrine effect on SMC we searched for expression of its receptor (c-kit protein) in vascular SMC. Using Western blotting we found that the c-kit receptor, measuring 125 kDa, was prominently expressed in primary cultured SMC. Conclusion: Human vascular SMC express membrane bound SCF ligand. SMC exposed to proteases such as MMP-9 release SCF which through an autocrine pathway may affect SMC function via the c-kit receptor.