Author ORCID Identifier

https://orcid.org/0000-0002-4576-3728

Year of Publication

2021

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department/School/Program

Toxicology and Cancer Biology

First Advisor

Dr. Yekaterina Zaytseva

Second Advisor

Dr. Daret St. Clair

Abstract

Colorectal cancer (CRC) remains a leading cause of cancer-related deaths in the world, comprising over 1 million new cases each year and over 500,000 deaths. CRC, when detected at an early stage of disease development, can be effectively treated, with a 5-year survival rate of over 90%. Such standard treatments include surgical resection of the primary tumor in combination with adjuvant chemotherapy. However, even with advancements in surgical procedures and chemotherapeutic targets, when CRC progresses to a more advance stage, the 5-year survival rate decreases significantly to just under 14%. This stark decrease in patient survival rate can be directly contributed to the lack of effective chemotherapeutics as well as relapse, which occurs in 30-50% of patients treated for CRC and results in a far more aggressive disease. With such a large rate of CRC relapse and drastically low survival rates, the need for new therapeutic targets for the disease are particularly needed.

Dysregulation of fatty acid (FA) metabolism has been identified as a hallmark of cancer and a key proponent to CRC development and progression. FAs are used for a multitude of purposes within the cell, such as signaling molecules, components of membrane synthesis, and most importantly, as a direct energy source. Two different pathways contribute to FA utilization in cancer cells. First, FAs can be synthesized de novo. FA Synthase (FASN), a key enzyme of de novo FA synthesis, catalyzes the synthesis of palmitate from acetyl-CoA and malonyl-CoA. Another prominent pathway utilized by cells to obtain FAs is exogenous FA uptake via transmembrane FA transporters including FA Translocase (CD36). CD36 is a multifunctional glycoprotein, is primarily involved in the binding and transportation of low-density lipoproteins and long-chain/ultra-long-chain FAs. My research assessed the role of CD36 in CRC growth and metastasis as well as the relationship between CD36 and FASN, particularly, the aspect of this relationship when de novo lipid synthesis is inhibited using novel FASN inhibitors.

FASN upregulation has been previously shown to contribute to primary CRC growth and progression to metastasis. Chemical inhibition of FASN via a novel FASN inhibitor TVB-3664, has shown significant promise in the treatment of CRC in vitro by reducing CRC proliferation via a decrease is cellular respiration. Interestingly, pre-clinical evaluation of this inhibitor in patient derived xenografts (PDXs) in vivo suggests that high expression of FASN does not determine a positive response to FASN-targeted therapy and only approximately 30% of PDXs exhibit significant tumor reduction. These results suggest that there may be alternative pathways which may contribute to FASN inhibition resistance.

CD36 has been studied in various diseases, including glioblastoma, breast, ovarian, and oral carcinomas, where it has been found to significantly contribute to disease progression and metastasis. However, CD36 has not yet been thoroughly investigated in CRC, and has not been studied in relation to de novo lipid synthesis. My studies show that CD36 is upregulated in CRC primary and metastatic tumors and an increased expression level of CD36 is associated with an increase in FA uptake. Furthermore, CD36 inhibition significantly decreases proliferation of CRC established and primary cell lines in vitro, and knockdown of CD36 reduces subcutaneous xenograft tumor growth in vivo. Most excitingly, inhibition of FASN significantly and specifically upregulates CD36 expression, but not other FA transporters, in human tissues, established and primary cell lines, and genetically modified mice with heterozygous and homozygous deletion of FASN. This upregulation of CD36 in the presence of FASN inhibition resulted in a CD36 specific increase in the uptake of exogenous FA analogues, further supporting the role of CD36 as a potential compensatory mechanism to FASN-targeting therapy. Additionally, inhibition of FASN and CD36 in combination resulted in an additive effect on the reduction of CRC cell proliferation.

Late-stage metastatic CRC has a substantially lower rate of 5-year survival compared to earlier stage disease. As aforementioned, in addition to primary CRC cell survival and proliferation, CD36 was shown to be further upregulated in CRC metastatic tissues. My studies demonstrate that CD36 promotes CRC cell invasion and colony formation in vitro. Additionally, an increase in CD36 is associated with more metastatic CRC cell lines and promotes metastasis of CRC cells in vivo. Interestingly, CD36 expression is associated with matrix-metallopeptidase-28 (MMP28), the newest member of the matrix-metallopeptidase family of proteins. MMP28 is involved in the degradation of the extracellular matrix (ECM), particularly the disassembly of collagen fibers within cell-cell adhesions. MMP28 has been suggested in the metastasis of various diseases including lung and gastric cancer. MMP28 had not yet been investigated in CRC. Knockdown of CD36 decreases MMP28 expression and increases the expression of e-cadherin, a critical protein involved in cell-cell adherent junctions and whose loss is a well-known marker for epithelial to mesenchymal transition (EMT) in various cancers. Furthermore, knockdown of MMP28 is sufficient to significantly upregulate e-cadherin expression suggesting that CD36 potentially regulates metastasis through early EMT via the upregulation of MMP28 and loss of e-cadherin.

New therapeutic strategies in the treatment of CRC are urgently needed to improve the survival rates of those patients with late-stage disease. Altered FA metabolism is a common characteristic of many cancers including CRC. The studies described here investigate the role of CD36 in CRC and upregulation of CD36 as a potential compensation mechanism to FASN-targeted therapy which is currently being tested in several clinical trials. The results of my studies demonstrate that CD36 may present a viable target for treatment of both primary and metastatic CRC. Furthermore, this study provides the rationale for targeting CD36 in combination with FASN inhibitors and demonstrates that inhibition of CD36 increases efficacy of FASN-targeted therapy via a reduction of the free-FA pools available to CRC tumor cells. Further investigation into the mechanistic functions of both CD36 and FASN in CRC as well as the contribution of CD36 to CRC metastasis is needed to provide more efficacious treatment strategies of CRC.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2021.370

Funding Information

This study was supported by the National Institute of Environmental Health and Safety T32 Grant (no. T32ES07266) from 2018-2020.

This study was also supported by the National Institutes of Health - National Cancer Institute P20 Grant (no. GM121327) and the R01 Grant (no. ES07266) from 2017-2019.

This study was also supported by the National Institutes of Health - National Cancer Institute R01 Grant (no. CA249734) in 2021.

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