Abstract P4-06-20: The Role of Aktl in RhoC GTPase-Mediated Inflammatory Breast Cancer Invasion

Abstract

Abstract Inflammatory breast cancer (IBC) is a highly aggressive form of locally advanced breast cancer, distinguished from other types of breast cancer by clinical, pathologic, and molecular features. It is clinically distinguished by rapid onset of primary skin changes, including redness of the skin, nipple retraction, and peau d’ aurange. Inflammatory breast cancer accounts for approximately 6-9% of new breast cancers in the United States annually and although the number of women affected by IBC is relatively modest, the overall the 5- and 10-year disease-free survival rates are less than 45% and 20%, respectively, making IBC the most lethal form of breast cancer. Evidence has suggested a role for the phosphoinositide 3-kinase/Akt (Protein Kinase B) signaling pathway in cellular motility, invasion, and metastasis. Recent evidence has shown activation of Akt2 (PKBβ) in breast cancer and its involvement in metastasis. However, the opposite appears to be true in inflammatory breast cancer (IBC), where Akt1 is involved in cellular motility and invasion. We have demonstrated a role for RhoC in conferring a highly invasive phenotype to IBC cells and are suggesting here that Akt1 phosphorylation of RhoC leads to cellular invasion. Farnesyltransferase inhibitors (FTIs) interfere with GTPase farnesylation and activity and we have shown that FTI treatment of SUM149 IBC cells leads to inhibition of the RhoC GTPase-induced invasive phenotype of IBC. RhoC is not a direct target of FTI action, however, its relative, RhoB, is a putative FTI target. It has been shown that FTI inhibition of farnesylated RhoB (fRhoB) leads to an accumulation of geranylgeranylated RhoB (ggRhoB) in the cell, which can interfere with GTPase signals and mimics the effects of FTI in IBC cells. RhoB GTPase, particularly ggRhoB, is an intracellular trafficking protein that regulates cellular localization of Akt1, thus affecting its activation. In this present study our primary objective was to examine the role of Akt1 and RhoC phosphorylation in IBC cell invasion as well as the localization of these various molecules within IBC cells upon FTI treatment. The SUM149 inflammatory breast cancer cell line overexpresses RhoC, lacks the PTEN gene, and consequently expresses a constitutively active Akt. Using the SUM149 cell line, we demonstrate that RhoC is a reputed substrate for Akt phosphorylation. We are able to decrease phospho-RhoC levels and inhibit SUM149 cellular invasion through inhibiting Akt1 activation with commercially available Akt inhibitors and siAkt1. Through immunofluorescence and protein localization experiments we are able to see a shift of phosphorylated Akt1 from the cell cytoplasm to the nucleus upon FTI treatment. We also observe accumulation of ggRhoB over time with localization of Akt1 away from the plasma membrane. These results suggest that Akt, specifically Akt1, phosphorylation of RhoC in inflammatory breast cancer cells is required for cellular invasion. We are also able to suggest that FTI treatment of IBC cells leads to increased ggRhoB levels, which may lead to changes in localization of proteins that will affect RhoC GTPase and the metastatic IBC phenotype. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P4-06-20.