Autophagy modulation: role in anti-cancer therapy

Abstract

Autophagy is a conserved lysosomal degradation process characterized by cellular self-digestion. Autophagy results in turnover of the cytoplasm allowing for metabolic maintenance and organelle quality control, particularly during cell stress. These aspects of autophagy can facilitate tumor cell survival and resistance. As such, autophagy inhibition is being explored in clinical trials as a novel approach to chemosensitization. However, there are still a number of unresolved concerns in regards to the use of autophagy inhibition as a therapy. It is still unclear how autophagy functions in metastasis development. Therefore, we investigated the role of autophagy in metastasis by modulating autophagy in different mouse models and cell based assays that reflect the steps of metastatic development. We found that autophagy was not required for tumor cell colonization within the site of metastasis nor did autophagy alter the metastatic capabilities of the cells. Rather, autophagy appeared to impact the pre-metastatic environment through effects on bone marrow derived cell number which mediate the establishment of the metastatic niche. Stimulating autophagy, before tumor cells disseminated, could speed metastatic development and increase the number of these cells within circulation and eventual sites of metastasis. Correspondingly, inhibiting autophagy could delay metastasis and reduce circulating bone marrow derived cells. These studies suggest that autophagy is most critical in the stages prior to tumor cell arrival at the site of metastasis, by influencing the metastatic microenvironment. While increased autophagy is often considered to be a common tumor adaptation, it is now apparent that some tumor types are more dependent on autophagy than others. However, it is not well understood which tumors these are. Triple negative, Stat3 activated breast cancers were identified as autophagy dependent by collaborator Dr. Paola Maycotte. We tested the efficacy of autophagy inhibitor chloroquine (CQ) in xenograft models of triple negative and estrogen receptor positive breast cancer. CQ was only efficacious in the triple negative tumors. As some canine osteosarcomas also have constitutive Stat3 activity, we assessed the relationship of Stat3 activity and CQ sensitivity. Unlike in breast cancer, Stat3 phosphorylation did not indicate increased sensitivity to CQ in canine osteosarcoma. However, all the osteosarcoma cell lines responded to treatment. Using microarray analysis we identified potential compensatory pathways that have been previously reported to work in concert with autophagy in other cell types and may serve as useful combinational therapies. Currently, the only autophagy inhibitor available clinically is CQ or derivative hydroxycholorquine (HCQ). It is still uncertain whether these drugs can actually achieve autophagy inhibition in patients. Dogs serve as a good model for human cancer and there is an unmet need for novel therapies in the treatment of canine lymphoma. Thus we conducted a phase I clinical trial in canine lymphoma patients with the goals of finding a maximum tolerated dose in combination with doxorubicin (DOX) and the relationship of HCQ concentration and autophagy inhibition. We found that this combination can be well tolerated with a 20% reduction in DOX. HCQ can achieve autophagy inhibition in patients, but not consistently. There appears to be a threshold requirement of HCQ needed in order to effectively inhibit autophagy. There was a suggestion of efficacy as response rate was superior to historical data employing DOX alone. Therefore autophagy inhibition warrants further clinical study as an anti-cancer therapy.