Combinatorial therapeutic targets 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

Colon cancer is the most common cause of cancer-related mortality in non-smoking men and in non-smoking women and it is overall, the second leading cause of cancer deaths in the United States (1). Our pioneering studies analyzing the LOH frequencies of colon cancer showed that SMAD4 is the major target tumor suppressor gene localized to the minimally lost region on chromosome 18q (2). Subsequent studies by others have validated these observations. Interestingly, with the help of model colon cancer cell lines, we found that SMAD4 deficiency and retention of intact TGFβRII, could synergistically increase the levels of VEGF and Glut1 and enhance migration of colon cancer cells concomitant with an increase in MMP9 activity (3). Furthermore, Smad4 depleted cells were more resistant to 5’-fluoruracil (5FU) mediated apoptosis compared to isogenic controls. Additionally, Smad4 specifically interacts with HIF1α under hypoxic conditions providing a molecular basis for the differential regulation of target genes to suppress a malignant phenotype.

Since SMAD4 inactivation confers resistance to 5FU (a common therapeutic agent for colon cancer), we evaluated sensitivity of SMAD4 defective colon cancers to other chemotherapeutic agents and decided to decipher molecular targets for enhancing sensitivity to chemotherapy. We found 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. Our studies found for the first time that SMAD4 interacts with RICTOR to suppress colon cancer progression. 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 (4). Overall, our studies suggest that 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.

REFERENCES:

1. Siegel RL, Miller KD, Jemal A. Cancer statistics, 2021. CA Cancer J Clin. 2021 71(1):7-33.
2. Thiagalingam S, Lengauer 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. 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.