The Roles of RAC1 and RAC1B in Colorectal Cancer and Their Potential Contribution to Cetuximab Resistance.

Simple Summary: Cetuximab is a treatment widely used to treat advanced, metastatic colorectal cancer (CRC), and it works by blocking epidermal growth factor receptor signaling. Unfortunately, patients inevitably develop resistance to cetuximab. The most common resistance mechanisms are well established, but there are still patients that become resistant to cetuximab due to unknown mechanisms. The small guanosine triphosphatases (GTPases) RAC1 and RAC1B have been shown to contribute to CRC progression, but their role in cetuximab resistance is unclear. This review highlights the known cetuximab resistance mechanisms and summarizes how RAC1 and RAC1B could contribute to these resistance mechanisms to propose RAC1 and RAC1B as additional therapeutic targets that could increase the efficacy of cetuximab. Colorectal cancer (CRC) is one of the most diagnosed cancers and a leading contributor to cancer-related deaths in the United States. Clinically, standard treatment regimens include surgery, radiation, and chemotherapy; however, there has been increasing development and clinical use of targeted therapies for CRC. Unfortunately, many patients develop resistance to these treatments. Cetuximab, the first targeted therapy approved to treat advanced CRC, is a monoclonal antibody that targets the epidermal growth factor receptor and inhibits downstream pathway activation to restrict tumor cell growth and proliferation. CRC resistance to cetuximab has been well studied, and common resistance mechanisms include constitutive signal transduction through downstream protein mutations and promotion of the epithelial-to-mesenchymal transition. While the most common resistance mechanisms are known, a proportion of patients develop resistance through unknown mechanisms. One protein predicted to contribute to therapy resistance is RAC1, a small GTPase that is involved in cytoskeleton rearrangement, cell migration, motility, and proliferation. RAC1 has also been shown to be overexpressed in CRC. Despite evidence that RAC1 and its alternative splice isoform RAC1B play important roles in CRC and the pathways known to contribute to cetuximab resistance, there is a need to directly study the relationship between RAC1 and RAC1B and cetuximab resistance. This review highlights the recent studies investigating RAC1 and RAC1B in the context of CRC and suggests that these proteins could play a role in resistance to cetuximab. [ABSTRACT FROM AUTHOR]