A number of malignancies are promoted by the activation of Rho GTPases. Regulators of RhoA activity, including the RhoGEF, SmgGDS, can also promote cancer development. Both RhoA and SmgGDS have been shown to enhance NF-ĸB activation in a number of tumors. Utilizing mass spectrometry, we identified the tumor suppressive small GTPase DiRas1 as a novel binding partner for SmgGDS. DiRas1 is a tumor suppressor in malignant gliomas and esophageal squamous cell carcinomas. Examining DiRas gene expression using the ONCOMINE mRNA microarray database (www.oncomine.org) revealed that DIRAS1 and DIRAS2 are markedly decreased in central nervous system tumors, including anaplastic astrocytomas and GBMs, when compared to normal tissue. The role of DiRas1 in other cancers is largely undetermined, and the ways in which DiRas1 may antagonize pro-oncogenic small GTPase signaling is largely unknown. To define how DiRas1 expression may affect SmgGDS functions, we tested the association of DiRas family proteins with SmgGDS. SmgGDS robustly bound wildtype DiRas1, and its closely-related family member DiRas2, but only weakly bound DiRas3 in vitro and in cell lines. In addition, DiRas1, DiRas2, or DiRas3 protein expression decreased proliferation of HEK293T cells. DiRas1 expression could promote less activated RhoA, as a low level of DiRas1 expression caused markedly decreased RhoA-SmgGDS binding in a number of cell lines. DiRas1 expression also abrogated SmgGDS- and RhoA-mediated NF-ĸB activation in a concentration-dependent manner. SmgGDS may exhibit GEF activity toward DiRas1, as a dominant negative DiRas1 (S21N) binds to SmgGDS more than the wildtype protein. However, DiRas1 is predominantly GTP-bound in cells, likely due to a high rate of guanine nucleotide dissociation and low intrinsic GTPase activity. Thus, DiRas1 expression may in part promote tumor suppression by non-productively binding SmgGDS, resulting in less RhoA activity. Taken together, these results suggest that the tumor suppressive small GTPase DiRas1 can antagonize RhoA signaling by competing for binding of the RhoGEF SmgGDS. Further characterizing these actions may lead to novel therapeutic targeting of RhoA activation in cancer malignancies.

Citation Format: Andrew D. Hauser, Kristen M. Barr, Anne C. Frei, Patrick Gonyo, Ellen L. Lorimer, Carol L. Williams, Carmen Bergom. The tumor suppressive small GTPase DiRas1 binds the RhoGEF SmgGDS and antagonizes RhoA activation. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4443. doi:10.1158/1538-7445.AM2014-4443