Structural and Mechanistic Insights into the Regulation of the Fundamental Rho Regulator RhoGDIα by Lysine Acetylation.

Rho proteins are small GTP/GDP-binding proteins primarily involved in cytoskeleton regulation. Their GTP/GDP cycle is often tightly connected to a membrane/cytosol cycle regulated by the Rho guanine nucleotide dissociation inhibitor α (RhoGDIα). RhoGDIα has been regarded as a housekeeping regulator essential to control homeostasis of Rho proteins. Recent proteomic screens showed that RhoGDIα is extensively lysine-acetylated. Here, we present the first comprehensive structural and mechanistic study to show how RhoGDIα function is regulated by lysine acetylation. We discover that lysine acetylation impairs Rho protein binding and increases guanine nucleotide exchange factor-catalyzed nucleotide exchange on RhoA, these two functions being prerequisites to constitute a bona fide GDI displacement factor. RhoGDIα acetylation interferes with Rho signaling, resulting in alteration of cellular filamentous actin. Finally, we discover that RhoGDIα is endogenously acetylated in mammalian cells, and we identify CBP, p300, and pCAF as RhoGDIα-acetyltransferases and Sirt2 and HDAC6 as specific deacetylases, showing the biological significance of this post-translational modification.

Loss of ARHGDIA expression is associated with poor prognosis in HCC and promotes invasion and metastasis of HCC cells.

Rho GTPases control a wide range of cellular processes and contribute to tumor invasion and metastasis. As a regulator of Rho activity, ARHGDIA is aberrantly expressed in several types of tumors and plays different roles in the tumor process. To elucidate the role of ARHGDIA in HCC, we investigated the patterns of its expression, prognosis and clinical profiles in HCC. Functional assays were performed to investigate whether the alteration of ARHGDIA has an effect on cell growth, migration and invasion, as well as the status of Rho GTPases. We found that ARHGDIA was frequently downregulated in HCC and associated with tumor invasion and metastasis. Moreover, ARHGDIA was significantly associated with OS and TTR of HCC patients. Low level of ARHGDIA exhibited a decreased postoperative OS and a shorter TTR compared those with high levels. Functional assays showed that loss of ARHGDIA promoted HCC cell migration and invasion in vitro and lung metastasis formation in vivo. We found that loss of ARHGDIA significantly induced Rac1 and RhoA activation which may contribute to invasion and metastasis of HCC. In conclusion, the present study has identified loss of ARHGDIA contributed to the processes of hepatic tumorigenesis, in particular invasion and metastasis which may provide a potential therapeutic target for HCC.

Loss of RhoGDI is a novel independent prognostic factor in hepatocellular carcinoma.

RhoGDI (Rho GDP-dissociation inhibitor alpha or RhoGDIα) has been identified as a regulator of Rho GTPases, which are essential for tumor progression, but its role in cancer remains controversial and little is known in hepatocellular carcinoma (HCC). Using immunohistochemistry, we analyzed RhoGDI expression in 147 clinicopathologically characterized HCC cases. RhoGDI expression was detected in cytoplasm of HCC tissues. Statistical analysis showed that there was no relationship between RhoGDI expression and clinicopathological features. Importantly, a significant trend was identified between loss of RhoGDI expression in HCC and worsening clinical prognosis. Multivariate survival analysis showed that negative RhoGDI expression was recognized as an independent prognostic factor of patient's survival. Our results suggest that RhoGDI protein is a valuable marker of prognosis for patients with HCC.