Scientific Abstract
Proposal No. IBD-0012R
Principal Investigator: Lee A. Denson, M.D.
Current Applicant Organization: Cincinnati Children's Hospital Medical Center (Ohio, U.S.A.)
Original Applicant Organization: Yale University (New Haven, Connecticut, U.S.A.)
Project Title: Mechanisms of growth hormone resistance in experimental colitis
Period of Award: October 1, 2002 – March 31, 2006
Chronic inflammation and undernutrition combine to create a state of acquired growth hormone (GH) resistance in patients with inflammatory bowel disease (IBD). In these patients, GH secretion is normal, but production of anabolic mediators including IGF-I is reduced. Consequences may include linear growth failure, wasting, and impaired mucosal healing. Consistent with being GH-resistant, children with Crohn’s disease do not improve linear growth with GH therapy. However, transgenic mice that over express GH do exhibit enhanced mucosal repair in DSS induced colitis, and GH administration has recently been shown to improve disease activity in adults with Crohn’s disease. Therefore, higher doses of exogenous GH can exert beneficial effects. An alternate strategy, which is the focus of the proposed studies, is to identify biological therapies that will specifically restore endogenous GH signaling in liver and intestine. We have recently observed that height velocity has improved in children treated with Remicade, a monoclonal anti-tumor necrosis factor (TNF) antibody. This has indicated that a better understanding of mechanisms of inflammatory GH resistance may lead to additional useful biological therapies. Restoration of endogenous GH signaling in IBD may then enhance both overall growth and anabolic metabolism and mucosal repair.
Mice with experimental colitis due to IL-10 deficiency or TNBS administration develop chronic intestinal inflammation and progressive growth failure. However, the molecular mechanisms by which gut-derived cytokines may cause systemic and local GH resistance are not known. Recent studies from our group and others have demonstrated that mice with colitis are GH resistant, in terms of impaired up regulation of STAT5 and IGF-I. We have determined that this is associated with down regulation of the GH receptor (GHR), combined with increased abundance of the GH signaling inhibitor, CIS. While GH stimulation of intestinal epithelial cell (IEC) proliferation also involves tyrosine kinase activation, the role which downstream transcriptional regulators including STAT5 or AP-1 play in the intestine is much less well defined than in liver. Moreover, whether alterations in the GHR or post-receptor inhibitors including CIS or SOCS-3 also modulate IEC GH signaling in colitis is not known. Thus, the principal objective of this proposal will be to define the cellular responses that regulate hepatic and intestinal GH signaling in chronic colitis. Our working hypothesis is that inflammatory suppression of the GHR and induction of CIS/SOCS-3 combine to impair GH signaling in IBD. We will test this hypothesis in the following aims: Aim 1: Characterize cytokine regulation of hepatic and intestinal GH signaling in chronic colitis. Down regulation of the GHR or induction of CIS/SOCS-3 may impair GH signaling in chronic colitis. Abundance of these proteins relative to GH activation of STAT5, AP-1 and IGF-I will be examined in control, pair-fed, and colitic mice. Complementary studies in liver and intestinal cell lines will directly test mechanisms identified in vivo. Aim 2: Determine whether anti-cytokine agents restore GH signaling in chronic colitis. We have determined that IL-6 mediates acute, LPS induced hepatic GH resistance via CIS/SOCS-3 up regulation, while TNFa down regulates GHR expression. We will determine the ability of anti-cytokine antibodies to alter GHR and CIS/SOCS-3 expression and restore GH signaling in experimental colitis. We will then compare the relative effect of cytokine blockade versus chronic GH or IGF-I administration upon growth, lean body mass, and mucosal repair.
The proposed project meets The Broad Foundation’s goal of funding the early stages of novel research programs. The aims of most IBD research are to determine how mucosal inflammation is initiated and perpetuated, or to identify causative genes. An area that has received less attention is the manner in which chronic intestinal inflammation impairs systemic and local metabolic processes, including GH signaling. The fundamental new direction that will be tested is that mechanisms of GH resistance can be identified which will be amenable to targeted biological therapies. While we and others have characterized hepatic mechanisms of inflammatory GH resistance, whether similar mechanisms are also operative in the intestine is not known. Thus, these types of studies in the intestine are at the earliest stages of exploration. It is anticipated that, by focusing on ways to restore endogenous GH action in colitis, the resulting biological therapies will be optimized to improve linear growth and mucosal healing.