Tumor metastasis suppressor nm23H1 regulates Rac1 GTPase by interaction with Tiam1

The putative tumor metastasis suppressor nm23H1 was originally identified i n murine melanomas by subtraction cloning. It displays nucleoside diphospha te kinase activity and regulates cellular events, including growth and deve lopment. Recently nm23H1 has been reported to also act as a GTPase-activati ng protein of the Ras-related GTPase Rad. We attempted to determine whether nm23H1 also regulates Rho-family GTPases. Although we were unable to detec t a direct association between nm23H1 and Rho-family GTPases, nm23H1 was sh own to be associated with a Rad-specific nucleotide exchange factor, Tiam1, by interaction with its amino-terminal region in extracts from the cells e xpressing exogenous Tiam1 and from native tissue. Overexpression of nm23H1 inhibited the Tiam1-induced production of GTP-bound Rad and activation of c -Jun kinase. On the other hand, forced overexpression of the wild type, but not the kinase-inactivated mutant of nm23H1, converted the CDP bound farms of Rad, Cdc42, and RhoA to their GTP-bound forms in vitro by its nucleosid e diphosphate kinase activity, but nm23H1 alone apparently did not produce the GTP-bound form of these GTPases in vivo. These results suggest that nm2 3H1 negatively regulates Tiam1 and inhibits Rad activation in vivo. Moreove r, adhesion-stimulated membrane ruffles of Rat1 fibroblasts were reduced by overexpression of nm23H1. Based on these observations, we concluded that w e had identified a function of nm23H1 as a regulator of Rac1 and that it ma y be related to the effect of nm23H1 as a tumor metastasis suppressor.