RICTOR is a therapeutic target for chemotherapy resistant metastatic colon cancer

Sam Thiagalingam, PhD

Biomedical Genetics, Genetics & Genomics and Pathology & Laboratory Medicine,Boston University School of Medicine, Boston, MA, USA

Colorectal cancer (CRC) is the second leading cause of cancer deaths in the United States and the Western countries, and the majority of the cases are diagnosed at the metastatic stage due to the late onset of symptoms (1). Surgery has limited therapeutic role with metastatic colorectal cancer (mCRC) as only 10-15% of patients have resectable lesions. Therefore, breakthroughs in the discovery of effective therapeutic targets/agents based on the molecular basis of metastatic colorectal cancer (mCRC) are urgently needed for controlling the spread and eradication of the cancer cells. Currently, the use of chemotherapeutic agents, 5-fluorouracil (5-FU), irinotecan (CPT-11) and oxaliplatin (L-OHP), have been the standard care as adjuvant therapy for the mCRC patients who undergo surgical resection as well as in the remaining cases to significantly improve overall survival (OS). However, chemotherapy is not effective in every patient and resistance to chemotherapy limits the success rate.

Several studies have confirmed that loss of heterozygosity (LOH) at the chromosome 18q occurs at an advanced stage of sporadic colon cancer and it is associated with a poor prognosis and resistance to the most common chemotherapeutic agents such as 5-fluorouracil (5-FU). Our pioneering study showed that the high LOH frequencies of chromosome18q in colon cancer targets SMAD4, a central player in the cannonical TGFβ signaling pathway, for inactivation (2), and studies by us and others also found that SMAD4 deficiency was responsible for resistance to 5-FU-mediated apoptosis (3). Furthermore, our in silico Kaplan-Meier analysis found significantly decreased survival in patients with low levels of SMAD4 expression. An association between SMAD4 defect and resistance to chemotherapy may indicate similar pathways affect most mCRCs.

Since deciphering molecular targets to enhance sensitivity to chemotherapy is becoming a priority for effectively treating colon cancers, we evaluated if SMAD4 inactivation also confers resistance to irinotecan and oxaliplatin, other common chemotherapeutics often used along with 5-FU, and identified SMAD4-interacting proteins using co-immunoprecipitation followed by mass spectrometry as candidate precision therapeutic target(s) to improve chemotherapy. We found that SMAD4 inactivation also renders resistance to irinotecan but not to oxaliplatin, and showed for the first time that SMAD4 interacts with RICTOR to suppress colon cancer progression. Expression levels of SMAD4-interacting candidates were also correlated to survival in silico revealing higher levels of RICTOR or AKT, in general irrespective of SMAD4 status, also correlated with poor survival, suggesting them as strong prognostic biomarkers and therapeutic targets. Moreover, overexpression of SMAD4 or depletion of RICTOR was sufficient to suppress AKT signaling and restore sensitivity to irinotecan in SMAD4-deficient colon cancer cells, and experimental pharmacological inhibition of AKT sensitized SMAD4-negative colon cancer cells to irinotecan in vitro and in vivo. Overall, overexpression of RICTOR is not only a prognostic biomarker but also provides a mechanistic rationale for its targeted inactivation as a distinctive combinatorial therapeutic opportunity with chemotherapy for subsets of colon cancers that are either SMAD4-negative or exhibit overexpression of RICTOR/AKT (4).

REFERENCES:

1. Siegel RL, Miller KD, Wagle NS, Jemal A. Cancer statistics, 2023. CA Cancer J Clin. 2023 73(1):17-48.

2. Thiagalingam S, Linguae C, Leach FS, Schutte M, Hahn SA, Overhauser J, Willson JKV, Markowitz S, Hamilton SR, Kern SE, Kinzler KW, and Vogelstein B. 1996. Evaluation of candidate tumour suppressor genes on chromosome 18 in colorectal cancers. Nature Genet. 13(3): 343-346.

3. Papageorgis P, Cheng K, Ozturk S, Gong Y, Lambert AW, Abdolmaleky HM, Zhou JR, Thiagalingam S. 2011. Smad4 Inactivation Promotes Malignancy and Drug Resistance of Colon Cancer. Cancer Res. 71(3): 998-1008.

4. Wong CK, Lambert AW, Ozturk S, Papageorgis P, Lopez D, Shen N, Sen Z, Abdolmaleky HM, Győrffy B, Hui F, and Thiagalingam S. 2020. Targeting RICTOR sensitizes SMAD4-negative colon cancer to Irinotecan. Mol. Cancer Res. 18(3):414-423.