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Scientists Explain Why Ovarian Cancer Becomes Resistant To Chemotherapy

2016-05-26
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    The reason why ovarian cancer becomes resistant to chemotherapy over a period of time has always fascinated and puzzled the researchers. Now, a team of researchers from the University of Michigan has come up with a possible reason.  The tumor microenvironment is made up of an array of cell types, yet effector T cells and fibroblasts constitute the bulk of the tissue. The investigators believe that understanding the interplay between these two cell types holds the key to how ovarian cancer cells develop resistance.

 

    The new study suggests that the fibroblasts surrounding the tumor work to block chemotherapy, which is why nearly every woman with ovarian cancer becomes resistant to treatment. Conversely, the scientists published evidence that T cells in the microenvironment can reverse the resistance phenotype—suggesting a whole different way of thinking about chemotherapy resistance and the potential to harness immunotherapy drugs to treat ovarian cancer.

 

    “Ovarian cancer is often diagnosed at late stages, so chemotherapy is a key part of treatment,” explained co-senior study author J. Rebecca Liu, M.D., associate professor of obstetrics and gynecology at the University of Michigan. “Most patients will respond to it at first, but everybody develops chemoresistance. And that’s when ovarian cancer becomes deadly.”

 

    Dr. Liu continued, stating that “in the past, we’ve thought the resistance was caused by genetic changes in tumor cells. But we found that’s not the whole story.”

 

    The University of Michigan team looked at tissue samples from ovarian cancer patients and separated the cells by type to study the tumor microenvironment in vitro and in mice. More importantly, the scientists linked their findings back to actual patient outcomes.

 

    The results of this study were published recently in Cell through an article entitled “Effector T Cells Abrogate Stroma-Mediated Chemoresistance in Ovarian Cancer.”

 

    Ovarian cancer is typically treated with cisplatin, a platinum-based chemotherapy. The researchers found that fibroblasts blocked platinum. These cells prevented platinum from accumulating in the tumor and protected tumor cells from being killed off by cisplatin.

 

    “We show that fibroblasts diminish the nuclear accumulation of platinum in ovarian cancer cells, resulting in resistance to platinum-based chemotherapy,” the authors wrote. “We demonstrate that glutathione and cysteine released by fibroblasts contribute to this resistance.”

 

    T cells, on the other hand, overruled the protection of the fibroblasts. When researchers added the T cells to the fibroblast population, the tumor cells began to die off.

 

    “CD8+ T cells abolish the resistance by altering glutathione and cystine metabolism in fibroblasts,” the authors explained. “CD8+ T-cell-derived interferon (IFN)γ controls fibroblast glutathione and cysteine through upregulation of gamma-glutamyltransferases and transcriptional repression of system xc cystine and glutamate antiporter via the JAK/STAT1 pathway.”

 

    By boosting the effector T cell numbers, the researchers were able to overcome the chemotherapy resistance in mouse models. Moreover, the team used interferon, an immune cell-secreted cytokine, to manipulate the pathways involved in cisplatin.

 

    “T cells are the soldiers of the immune system,” noted co-senior study author Weiping Zou, M.D., Ph.D., professor of surgery, immunology, and biology at the University of Michigan. “We already know that if you have a lot of T cells in a tumor, you have better outcomes. Now we see that the immune system can also impact chemotherapy resistance.”

 

    The researchers suggest that combining chemotherapy with immunotherapy may be effective against ovarian cancer. Programmed death ligand 1 (PD-L1) and PD-1 pathway blockers are currently FDA-approved treatments for some cancers, although not ovarian cancer.

 

    “We can imagine re-educating the fibroblasts and tumor cells with immune T cells after chemoresistance develops,” Dr. Zou remarked.

 

    “Then we could potentially go back to the same chemotherapy drug that we thought the patient was resistant to. Only now we have reversed that, and it’s effective again,” Dr. Liu concluded.

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