Our growth regulation research has been concerned with oncogenes and tumor suppressor genes as positive and negative regulators of normal and neoplastic growth. The main current project is concerned with a tumor suppressor gene family, DLC1-3, and the targets that it regulates. DLC1 is inactivated in a variety of tumors, but many aspects of it mechanism of action remains poorly understood. It negatively regulates Rho, via its Rho-GAP activity, but it is likely to encode other activities, as other Rho-GAPs are not known to be inactivated in cancer. We have recently determined that DLC1 interacts with members of the tensin gene family. The interaction occurs via a region of DLC1 for which no function had been previously identified. DLC1 binds to two adjacent regions located at the C-terminus of each tensin protein, the SH2 domain and the PTB domain, with the SH2 binding predominating. We constructed a DLC1 point mutant that does not bind the isolated tensin SH2 domain and is deficient for interaction with full-length tensin. When physiologic levels of this mutant DLC1 protein were expressed in a lung cancer cell line in which endogenous DLC1 was spontaneously inactivated, it was found to be less active than wild type DLC1 in suppressing anchorage-independent growth and cell migration, and to be less closely associated with focal adhesions, although the levels of endogenous Rho-GTP in cells expressing the mutant were as low as those expressing the wild type DLC1. We conclude that tensin is a physiologically relevant target of DLC1. Furthermore, the association between DLC1 and tensin does not appear to be essential for DLC1 to carry out its Rho-GAP function. Instead, we found that the DLC/tensin interaction interfered with the binding of signaling molecules to the SH2 and PTB domains of tensin, which implies that this interference may contribute to the biological activity DLC1. Given the biological importance of the interaction between DLC1 and tensin, we are currently exploring whether tensin might play a role in tumorigenesis. We are also trying to identify other activities of DLC1 to understand its mechanism of action better and to more fully substantiate our hypothesis that DLC1 is frequently inactivated in cancer because it encodes a multifunctional protein.
