REV1 Inhibition Enhances Radioresistance and Autophagy

SIMPLE SUMMARY: Cancer resistance to therapy continues to be the biggest challenge in treating patients. Targeting the mutagenic translesion synthesis (TLS) polymerase REV1 was previously shown to sensitize cancer cells to chemotherapy. In this study, we tested the ability of REV1 inhibitors to radiation therapy and observed a lack of radiosensitization. In addition, we observed REV1 inhibition to trigger an autophagy stress response. Because reduction of REV1 triggered autophagy and failed to radiosensitize cells, we hypothesize REV1 expression dynamics might link cancer cell response to radiation treatment through the potential induction of autophagy. ABSTRACT: Cancer therapy resistance is a persistent clinical challenge. Recently, inhibition of the mutagenic translesion synthesis (TLS) protein REV1 was shown to enhance tumor cell response to chemotherapy by triggering senescence hallmarks. These observations suggest REV1’s important role in determining cancer cell response to chemotherapy. Whether REV1 inhibition would similarly sensitize cancer cells to radiation treatment is unknown. This study reports a lack of radiosensitization in response to REV1 inhibition by small molecule inhibitors in ionizing radiation-exposed cancer cells. Instead, REV1 inhibition unexpectedly triggers autophagy, which is a known biomarker of radioresistance. We report a possible role of the REV1 TLS protein in determining cancer treatment outcomes depending upon the type of DNA damage inflicted. Furthermore, we discover that REV1 inhibition directly triggers autophagy, an uncharacterized REV1 phenotype, with a significant bearing on cancer treatment regimens

The role of REV1 in cancer resistance to radiation therapy

Despite the availability of several treatment modalities, cancer resistance is a prominent clinical challenge. Cancer recurrence is often observed in patients after initial treatment leading to acquired resistance. Amongst the different biological pathways responsible for treatment resistance, translesion synthesis (TLS), a DNA damage tolerance mechanism has recently been implicated in causing cancer resistance to chemotherapy (Yamanaka et al., 2017). The REV1 TLS protein plays a major role in this pathway, where it functions as a key scaffolding protein that helps facilitate DNA damage bypass. Recently, targeting REV1 with small molecule inhibitors was shown to sensitize cancer cells to chemotherapeutic drugs (Wojtaszek et al., 2019). However, it is not known whether REV1 inhibition would similarly sensitize cancer cells to radiation therapy, another common treatment module in cancer patients. We proposed that REV1 is a potential target for sensitizing cancer cells to radiation treatment. We performed cytotoxicity and colony survival assays upon treating cancer cells with REV1 inhibitors and ionizing radiations to determine whether REV1 inhibition similarly sensitizes cancer cells to ionizing radiation (IR) treatment. Remarkably, we found that REV1 inhibition fails to sensitize cells to radiotherapy. Instead, REV1 inhibition induces autophagy which might be enhancing radioresistance. Immunofluorescence assays were used in the detection of autophagy. Ascertaining the mechanisms of REV1-inhibition dependent cancer resistance is a key component in addressing the clinical challenge of cancer therapy resistance