Experts Uncover How Cancer Survives, and Propose a Method to Undermine it
In an interview with Oncology Learning Network, Zhenkun Lou, PhD, Department of Oncology; Haidong Dong, MD, PhD, Department of Oncology, Pharmacology and Experimental Therapeutics; and Robert W. Mutter, MD, Department of Urology (pictured L-R), discussed the clinical significance of their research decoding how certain cancer cells survive when faced with chemotherapy, radiotherapy, and other cancer treatments (Mol Cell. 2019;74:1215-1226).
What existing data led you to evaluate the role of intracellular PD-L1 in cancer?
In reviewing the expression of PD-L1 in human cancer tissue samples, we frequently observed the presence of PD-L1 inside of cancer cells, not just on the cell surface. This was surprising because PD-L1 was originally defined as a protein expressed only at the cell surface (cytoplasm membrane) of cancer cells using flow cytometry.
Therefore, we initially assumed that this evidence of PD-L1 within the cytoplasm and nucleus of cancer cells was an artifact caused by background immunohistochemistry staining. However, when we looked at the expression of PD-L1 in tumor cells after treatment with chemotherapy, we unexpectedly found an increased accumulation of PD-L1 into small vesicles along the cell nucleus.
Also, we observed that knocking out PD-L1 in cancer cells made them more sensitive to chemotherapy and radiotherapy, in vitro.
While the cell surface expression of PD-L1 explained the previously established immune-regulatory function of PD-L1 during the interaction of cancer cells (expressing PD-L1) and immune cells (expressing PD-1, a receptor of PD-L1), the new evidence of PD-L1 within cancer cells and its impact on sensitivity to therapies that cause DNA damage suggested that PD-L1 may have some previously unknown function intracellularly.
Please briefly describe your study and its findings. Were any of the outcomes particularly surprising?
Radiotherapy and most chemotherapies work by damaging the DNA of cancer cells. Unfortunately, some cancer cells may be inherently resistant to these treatments, leading to disease recurrence.
In our studies, we discovered a new way by which cancer cells can be resistant to radiotherapy and chemotherapy. We found that cancer cells use PD-L1 to promote their repairing machinery’s ability to fix the DNA damage caused by either chemotherapy or radiation therapy. Importantly, we identified a new antibody to PD-L1, named H1A, which decreases the levels of PD-L1 and makes cancer cells more sensitive to chemotherapy and radiation therapy.
This new antibody can do so because it promotes PD-L1 degradation by blocking PD-L1’s binding to its stabilizer (CMTM6) in cancer cells.
PD-L1 was originally discovered by Lieping Chen, MD, PhD, and Dr. Dong at Mayo Clinic in 1998 as an immune checkpoint molecule (Nat Med. 1999;5:1365-1369). The external function of PD-L1 expressed by tumor cells is to help tumor cells escape immune attack by shutting down immune cells’ anti-tumor function (Nat Med. 2002;8:793-800).
Blockade of extracellular PD-L1 or PD-1, a receptor of PD-L1, on immune cells has become a standard of care of several human cancers since 2014, based on this discovery. However, the internal function of PD-L1 inside tumor cells had not been clearly defined.
In our new study, we learned that PD-L1 functions as an RNA-binding protein that can prevent RNA from being broken apart by RNA degradation machinery called exosomes. Some of the RNAs that associate with PD-L1 code proteins that help cancer cells to repair the DNA damage caused by radiation or chemotherapy.
In summary, intracellular PD-L1 appears to act as a bodyguard of certain RNAs in cancer cells that promote resistance to therapy.
The FDA approved PD-L1 antibodies that we assessed only blocked the external function of PD-L1. As a result, they did not appear to be helpful in preventing PD-L1 from carrying out its internal function within tumor cells and PD-L1-high cancer cells treated with these antibodies remained resistant to traditional DNA damaging cancer therapies.
In contrast, the H1A antibody blocked the internal function of PD-L1 by causing PD-L1 degradation, sensitizing cancer cells to chemotherapy and radiotherapy.
What are the possible real-world applications of these findings in clinical practice? How will they affect the treatment landscape for patients with cancer?
Our findings suggest that intracellular PD-L1 is a new target to sensitize cancer to DNA damaging therapies. In the future, the H1A antibody that we produced or other strategies that prevent PD-L1 from carrying out its intracellular function may be used to improve the efficacy of traditional cancer therapies.
Such combination therapy strategies targeting intracellular PD-L1 may be particularly attractive in cancer cells that express PD-L1 and exhibit resistance to chemotherapy or radiotherapy.
Further studies are needed, but it could be important for pathologists to pay attention to intracellular PD-L1 expression when they examine tumor specimens as intracellular PD-L1 expression may help clinicians identify cancers most likely to benefit most from this approach.
Do you and your co-investigators intend to expand upon this research?
There is more to learn about the impact of intracellular PD-L1 expression on prognosis of cancer patients and in prediction of therapeutic response.
We are testing our new H1A antibody (that causes PD-L1 degradation) in combination with various drugs, radiotherapy and chemotherapy in Patient-Derived Xenograft (PDX) models in order to identify which combinations are most promising to move forward into clinical trials.
These studies will ultimately determine whether targeting intracellular PD-L1 can improve treatment outcomes of patients with breast cancer and other malignancies.