Transforming growth factor β (TGF-β) is a multifunctional protein with roles in cancer, wound healing, fibrotic disorders, and immune modulation. As such, a primary objective of our laboratory is to understand the pathway(s) downstream of TGF-β receptor binding. We are addressing this question from two distinct approaches. The first employs a chimeric receptor strategy designed to investigate the specific TGF-β receptor complex(es) regulating TGF-β action. Use of this system has allowed us to determine that the diverse cellular phenotypes induced by TGF-β occur through coordinate regulation between the endocytic and signaling activity of defined TGF-β receptor interactions. In order to determine the functional significance of these associations, specific projects include (1) identifying and characterizing the celular machinery and receptor elements mediating TGF-β receptor downregulation, recycling, and polarized epithelial cell trafficking; (2) characterizing the proteins, receptor elements, and modifications involved in association with the clathrin AP-2 complex and their role in receptor signaling; and (3) defining the interrelationship between TGF-βR endocytosis and signaling in normal proliferative control and cancer.

The second approach for investigating TGF-β action employs a genetic screen to directly identify novel downstream intermediates in the TGF-β receptor signaling pathway(s). We have recently reconstituted TGF-β signaling in the genetically amenable organism S. cerevisiae. Use of this system has allowed us to determine that a member of the p21 activated kinase family (PAK2) is regulated by TGF-β in mammalian cells. This represents a previously unknown signaling pathway for TGF-β and may be the mechanism through which TGF-β receptor binding is coupled to cytoskeletal changes and enhanced cell migration/metastasis. Current projects are investigating upstream and downstream regulators of PAK2 and how this Smad-independent pathway integrates TGF-β signaling with proteins in the WASP, arp2/3, and/or ERM families.

We have recently identified the c-Abl nonreceptor tyrosine kinase as a novel Smad-independent target of TGF-β signaling. C-Abl activation occurred within minutes of TGF-β stimulation and is needed for TGF-β mediated morphologic alterations and colony formation in soft agar. While this (in itself) is extremely exciting as it is the first demonstration that serine/threonine receptor kinases can couple to tyrosine kinase signaling pathways, we extended these in vitro cellular phenotypes dependent upon TGF-β to an animal model of lung fibrosis where TGF-β is believed to be causal. The data show that inhibition of c-Abl activity with imatinib mesylate (Gleevec) abolished in vivo lung fibrosis associated with bleomycin treatment. Thus, c-Abl activity is not only necessary for TGF-β action in vitro, but in vivo pathologies dependent upon TGF-β can also be prevented by c-Abl inhibition. These findings provided the preclinical culture/animal work to generate a multi-center Phase I/II clinical trial evaluating imatinib vs. placebo for treatment of idiopathic pulmonary fibrosis (IPF). This investigator-initiated study is centered at Mayo and partially funded by Novartis. Current projects (1) address the spatio-temporal relation of c-Abl and PAK2 activation; (2) investigate the role(s) of c-Abl in TGF-β morphologic alteration; and, in collaborative studies with Drs. Shah, Sarkaria, Storgard, and Hirschberg (UCLA), (3) are addressing imatinib treatment in animal models of cirrhosis, radiation-induced pneumonitis, scleroderma, and renal fibrosis, respectively.