UC San Diego

The three Rho subfamily members, RhoA, RhoB, and RhoC, share 85% sequence identity, yet display opposite phenotypes. Whereas RhoA and RhoC proteins promote cellular proliferation, invasion, metastasis, and human cancers, RhoB hinders oncogenesis. How such similar proteins can have such opposing functions is not clearly defined. HMG-CoA reductase inhibitors (statins), typically prescribed for their cholesterol-lowering properties, were additionally found to lower rates of various cancers. Rho proteins were hypothesized to contribute to this anti- tumorigenic effect given that one of the pleiotropic effects of statins is inhibition of Rho prenylation. In our experiments, we found that RhoA and RhoC were more sensitive to the statin-induced decline in prenylation than RhoB, likely due to the ability of RhoB to be additionally farnesylated. In addition, we found that RhoA, RhoB, and RhoC activation and expression were greatly increased in lovastatin-treated cells. An increase in activated, GTP-bound Rho occurred even when RhoA and RhoC were completely unprenylated. This increase in Rho activation was at least partly explained by the decreased ability of unprenylated Rho to associate with its Rho- dissociation inhibitor, RhoGDI\[alpha\]. An increase in activated, yet unprenylated, Rho in response to statins could lead to a potential gain-of-function or a loss-of- function for Rho, and we hypothesized that this might explain how statins provide their anti-oncogenic properties. However, many statin-induced effects on proliferation, transcription, apoptosis, and polyploidy could not be explained by a change in Rho function and we found no evidence that the lovastatin-induced increase in RhoB contributed significantly to the anti-proliferative and pro-apoptotic effects of lovastatin in human leukemia cells. We used a proteomics approach to search for Rho interacting factors with differential binding affinity for RhoA/C and RhoB. Through the use of affinity chromatography and mass spectrometry, we identified a novel RhoA and RhoC isoform-specific interacting protein, IQGAP1. The binding to IQGAP1 was found to be dependent on the activation and prenylation state of Rho. Like RhoA and RhoC, IQGAP1 has many pro-proliferative, pro-invasion, and pro-oncogenesis properties. The lack of RhoB association with IQGAP1 may explain in part how this Rho isoform differs so dramatically in its function from RhoA and C.