Implications of Therapy-Induced Selective Autophagy on Tumor Metabolism and Survival

Accumulating evidence indicates that therapies designed to trigger apoptosis in tumor cells cause mitochondrial depolarization, nuclear damage, and the accumulation of misfolded protein aggregates, resulting in the activation of selective forms of autophagy. These selective forms of autophagy, including mitophagy, nucleophagy, and ubiquitin-mediated autophagy, counteract apoptotic signals by removing damaged cellular structures and by reprogramming cellular energy metabolism to cope with therapeutic stress. As a result, the efficacies of numerous current cancer therapies may be improved by combining them with adjuvant treatments that exploit or disrupt key metabolic processes induced by selective forms of autophagy. Targeting these metabolic irregularities represents a promising approach to improve clinical responsiveness to cancer treatments given the inherently elevated metabolic demands of many tumor types. To what extent anticancer treatments promote selective forms of autophagy and the degree to which they influence metabolism are currently under intense scrutiny. Understanding how the activation of selective forms of autophagy influences cellular metabolism and survival provides an opportunity to target metabolic irregularities induced by these pathways as a means of augmenting current approaches for treating cancer

When Cells Suffocate: Autophagy in Cancer and Immune Cells under Low Oxygen

Hypoxia is a signature feature of growing tumors. This cellular state creates an inhospitable condition that impedes the growth and function of all cells within the immediate and surrounding tumor microenvironment. To adapt to hypoxia, cells activate autophagy and undergo a metabolic shift increasing the cellular dependency on anaerobic metabolism. Autophagy upregulation in cancer cells liberates nutrients, decreases the buildup of reactive oxygen species, and aids in the clearance of misfolded proteins. Together, these features impart a survival advantage for cancer cells in the tumor microenvironment. This observation has led to intense research efforts focused on developing autophagy-modulating drugs for cancer patient treatment. However, other cells that infiltrate the tumor environment such as immune cells also encounter hypoxia likely resulting in hypoxia-induced autophagy. In light of the fact that autophagy is crucial for immune cell proliferation as well as their effector functions such as antigen presentation and T cell-mediated killing of tumor cells, anticancer treatment strategies based on autophagy modulation will need to consider the impact of autophagy on the immune system

When cells suffocate: autophagy in cancer and immune cells under low oxygen

Hypoxia is a signature feature of growing tumors. This cellular state creates an inhospitable condition that impedes the growth and function of all cells within the immediate and surrounding tumor microenvironment. To adapt to hypoxia, cells activate autophagy and undergo a metabolic shift increasing the cellular dependency on anaerobic metabolism. Autophagy upregulation in cancer cells liberates nutrients, decreases the buildup of reactive oxygen species, and aids in the clearance of misfolded proteins. Together, these features impart a survival advantage for cancer cells in the tumor microenvironment. This observation has led to intense research efforts focused on developing autophagy-modulating drugs for cancer patient treatment. However, other cells that infiltrate the tumor environment such as immune cells also encounter hypoxia likely resulting in hypoxia-induced autophagy. In light of the fact that autophagy is crucial for immune cell proliferation as well as their effector functions such as antigen presentation and T cell-mediated killing of tumor cells, anticancer treatment strategies based on autophagy modulation will need to consider the impact of autophagy on the immune system

Hypoxia activates autophagy in CD8 T cells.

<p>(A) A representative Western blot indicating protein expression of HIF-1α, p62, LC3-II and β-actin is shown for OT-I CD8 T cells cultured under 1.5% oxygen. Cobalt chloride (CoCl₂) treatment was used as a positive control for HIF-1α protein expression. (B) Autophagy induction is shown by quantification of decreasing p62 levels without chloroquine (CQ) treatment and (C) LC3-II accumulation with CQ treatment under normoxia (norm) or 1.5% oxygen (hyp). The mean and standard error of the mean for the fold change of 3 independent experiments is reported. Statistical significance was determined using a one-sample t-test on log transformed values compared to a hypothetical mean of zero. *<i>p</i><0.05.</p

Patient characteristics, follow-up time and survival characteristics for high-grade serous ovarian carcinoma cases.

1<p>FIGO = Federation of Gynecology and Obstetrics.</p

TIL and markers of immune function are associated with improved patient outcome.

<p>Kaplan-Meier analysis of disease-specific survival of high-grade serous ovarian carcinoma patients categorized by CD31, VEGF or the indicated immune markers: (A) CD31 and FoxP3, (B) CD31 and TIA-1, (C) CD31 and CD8, (D) VEGF and Granzyme B (GzmB), (E) VEGF and CD8. The statistical significance was determined using a log-rank test.</p

Markers of T Cell Infiltration and Function Associate with Favorable Outcome in Vascularized High-Grade Serous Ovarian Carcinoma

<div><p>Background</p><p>When T cells infiltrate the tumor environment they encounter a myriad of metabolic stressors including hypoxia. Overcoming the limitations imposed by an inadequate tumor vasculature that contributes to these stressors may be a crucial step to immune cells mounting an effective anti-tumor response. We sought to determine whether the functional capacity of tumor infiltrating lymphocytes (TIL) could be influenced by the tumor vasculature and correlated this with survival in patients with ovarian cancer.</p><p>Methodology and Principal Findings</p><p>In 196 high-grade serous ovarian tumors, we confirmed that the tumor vascularity as measured by the marker CD31 was associated with improved patient disease-specific survival. We also found that tumors positive for markers of TIL (CD8, CD4 and forkhead box P3 (FoxP3)) and T cell function (granzyme B and T-cell restricted intracellular antigen-1 (TIA-1)) correlated significantly with elevated vascularity. <i>In vitro</i>, hypoxic CD8 T cells showed reduced cytolytic activity, secreted less effector cytokines and upregulated autophagy. Survival analysis revealed that patients had a significant improvement in disease-specific survival when FoxP3 expressing cells were present in CD31-high tumors compared to patients with FoxP3 expressing cells in CD31-low tumors [HR: 2.314 (95% CI 1.049–5.106); <i>p</i> = 0.0377]. Patients with high vascular endothelial growth factor (VEGF) expressing tumors containing granzyme B positive cells had improved survival compared to patients with granzyme B positive cells in VEGF-low tumors [HR: 2.522 (95% CI 1.097–5.799); <i>p</i> = 0.0294].</p><p>Significance</p><p>Overall, this data provides a rationale for developing strategies aimed at improving the adaptability and function of TIL to hypoxic tumor conditions.</p></div

High-grade serous ovarian tumors express the vasculature marker CD31.

<p>(A) An image of high (left panel) or low (right panel) CD31 immunohistochemistry staining scanned at 10× magnification. Scale bar = 50 µm. Inset images show high (left panel) or low (right panel) CD31 staining when scanned at 40× magnification. Arrowheads indicate brown regions of CD31 staining (B) Kaplan-Meier analysis of disease-specific survival in high-grade serous ovarian cancer patients. Statistical significance was assessed using a log-rank test. (C) Mean CD31 vascular density comparisons with patient age (split on the median) and disease stage (<i>p</i> = not significant). Statistical significance was assessed using a Mann Whitney rank-sum test. (D) Contingency analysis of CD31 and VEGF. Statistical significance was assessed by Fisher's exact test.</p

High-grade serous tumors containing TIL have higher vascular density scores than those without TIL.

<p>Mean CD31 vascular density scores were compared in relation to expression of (A) CD8, (B) CD4 and (C) FoxP3. Error bars indicate standard error of the mean. Statistical significance was assessed by Mann Whitney test. *<i>p</i><0.05, ** <i>p</i><0.01.</p

High-grade serous tumors containing markers of immune function have higher vascular density scores.

<p>Mean CD31 vascular density scores were compared in relation to the expression of (A) TIA-1 and (B) granzyme B (GzmB) tumor infiltrates in high-grade serous tumors. Error bars indicate standard error of the mean. Statistical significance was assessed by Mann Whitney test. ** <i>p</i><0.01, ***<i>p</i><0.001.</p