The complexity of multi-cellular organisms demands a dynamic reciprocity between neighboring cells in any given tissue. The disruption in keratinocytes and fibroblasts cross-talk in skin has been linked to an imbalance in extracellular matrix (ECM) expression leading to the onset of fibrosis such as hypertrophic scarring. Our group has recently identified 14-3-3σ or stratifin (SFN) as a potent MMP-1 stimulatory factor in fibroblasts. In this doctoral research project, we hypothesized that SFN can modulate other ECM components and execute its transmembrane signaling through interaction with a receptor on the surface of fibroblasts.
Three specific objectives were accomplished in this project. Under objective 1, ECM gene expression profile of fibroblasts treated with SFN or co-cultured with keratinocytes was characterized by an ECM-pathway specific gene array and revealed that SFN upregulates a wider range of MMPs such as MMP-3, 8, 10, and 24 other than MMP-1. As SFN was not responsible for the keratinocyte-mediated decrease in collagen expression, under objective 2 attempts were made to characterize the nature of a collagen inhibitory factor in keratinocyte-conditioned medium (KCM). Analysis of keratinocyte/fibroblast co-culture and KCM revealed a 30-50 kDa keratinocyte-derived collagen inhibitory factor with stable activity at high temperature (56 ºC) and acidic environment (pH=2, 30 min). Under objective 3, SFN’s transmembrane signaling mechanism was investigated by utilizing a combination of receptor ectodomain biotin labeling, serial affinity purification, and MS/MS to identify aminopeptidase N or CD13 (APN) as a potential SFN receptor in fibroblasts. APN/SFN binding was further confirmed by immunoprecipitation, cross-linking, and co-distribution studies. Expression of APN and SFN increased after wound closure in a rabbit ear fibrotic model as well as a longitudinal study in rats. The transient knockdown of APN blocked SFN-mediated p38 MAPK activation and MMP-1 expression.
Collectively, the findings presented in this thesis provide further support for the importance of keratinocyte-releasable factors in the regulation of ECM and MMP expression in fibroblasts. We also identify APN as a novel cell surface receptor for SFN. Therefore, our findings may provide additional therapeutic tools for the regulation of MMP expression in dermal fibrosis and chronic wound healing disorders.Medicine, Faculty ofMedicine, Department ofExperimental Medicine, Division ofGraduat
