EGF and TGF-β signaling synergy in β-cell proliferation

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Project Summary and Relevance (Abstract) The overall goal of our research program is to determine the optimal way to induce β-cell proliferation, both in vitro and in vivo, to improve β-cell mass in patients with diabetes mellitus. Over the last several years, we have used various mouse models to study β-cell neogenesis, regeneration, and proliferation. Transforming growth factor beta (TGF-β) signaling, and a key intracellular component of TGF-β signaling, smad7, seem to play an important role in regulating β-cell proliferation. However, apparently inconsistent roles for TGF-β signaling in different models of β-cell proliferation led us to realize that epidermal growth factor (EGF) signaling was also key, and that synergy between EGF signaling and TGF-β signaling was at the heart of inducing optimal β-cell proliferation. We believe that smad7 is a key mediator of this synergy. There is ample evidence in the literature for such synergy between EGF signaling, TGF-β signaling, and smad7. This synergy appears to come through three pathways of interaction. First, EGF receptor signaling can specifically enhance pro-proliferative aspects of TGF-β receptor signaling, but second, it can also at the same time specifically suppress anti-proliferative (cytostatic) actions of TGF-β receptor signaling. Third, TGF-β receptor signaling can in turn enhance EGF receptor signaling. In the literature these studies are generally focused on the proliferation of cancer cell lines, but here in this proposal, based on our preliminary data, we wish to apply these same principles to β-cell proliferation. In particular, one of the most potent known β-cell mitogens, GLP-1, was recently found to work through EGF receptor signaling. This grant proposal will study these potential pathway interactions through two Specific Aims. Specific Aim 1: Determine molecular mechanisms for EGF receptor signaling-induced alterations in TGF-β signaling that lead to enhanced β-cell proliferation. Here, we will pursue the possibility that EGF receptor signaling selectively enhances TGF-β receptor pro-proliferative signals, and specifically suppresses anti-proliferative (cytostatic) TGF-β receptor signals, both mediated by smad7. Specific Aim 2: Determine a role for TGF-β signaling and smad7 in enhancing the β-cell proliferation induced by EGF receptor signaling and by GLP-1 signaling. Here, we will round out the three synergistic pathways that we hypothesized contribute to optimal β-cell proliferation. In addition, we will study a potential synergistic role for TGF-β receptor signaling specifically in augmenting the powerful GLP-1 mitogenic effect on β-cells, since GLP-1 works through the EGF receptor signaling pathway.