Autophagy in cancer associated fibroblasts promotes tumor cell survival

Abstract

RepoRtRecently, using a co-culture system, we demonstrated that MCF7 epithelial cancer cells induce oxidative stress in adjacent cancerassociated fibroblasts, resulting in the autophagic/lysosomal degradation of stromal caveolin-1 (Cav-1). However, the detailed signaling mechanism(s) underlying this process remain largely unknown. Here, we show that hypoxia is sufficient to induce the autophagic degradation of Cav-1 in stromal fibroblasts, which is blocked by the lysosomal inhibitor chloroquine. Concomitant with the hypoxia-induced degradation of Cav-1, we see the upregulation of a number of well-established autophagy/mitophagy markers, namely LC3, AtG16L, BNIp3, BNIp3L, HIF-1α and NFκB. In addition, pharmacological activation of HIF-1α drives Cav-1 degradation, while pharmacological inactivation of HIF-1 prevents the downregulation of Cav-1. Similarly, pharmacological inactivation of NFκB-another inducer of autophagy-prevents Cav-1 degradation. Moreover, treatment with an inhibitor of glutathione synthase, namely BSo, which induces oxidative stress via depletion of the reduced glutathione pool, is sufficient to induce the autophagic degradation of Cav-1. thus, it appears that oxidative stress mediated induction of HIF1-and NFκB-activation in fibroblasts drives the autophagic degradation of Cav-1. In direct support of this hypothesis, we show that MCF7 cancer cells activate HIF-1α-and NFκB-driven luciferase reporters in adjacent cancer-associated fibroblasts, via a paracrine mechanism. Consistent with these findings, acute knockdown of Cav-1 in stromal fibroblasts, using an siRNA approach, is indeed sufficient to induce autophagy, with the upregulation of both lysosomal and mitophagy markers. How does the loss of stromal Cav-1 and the induction of stromal autophagy affect cancer cell survival? Interestingly, we show that a loss of Cav-1 in stromal fibroblasts protects adjacent cancer cells against apoptotic cell death. thus, autophagic cancer-associated fibroblasts, in addition to providing recycled nutrients for cancer cell metabolism, also play a protective role in preventing the death of adjacent epithelial cancer cells. We demonstrate that cancer-associated fibroblasts upregulate the expression of tIGAR in adjacent epithelial cancer cells, thereby conferring resistance to apoptosis and autophagy. Finally, the mammary fat pads derived from Cav-1 (-/-) null mice show a hypoxia-like response in vivo, with the upregulation of autophagy markers, such as LC3 and BNIp3L. taken together, our results provide direct support for the "autophagic tumor stroma model of cancer metabolism", and explain the exceptional prognostic value of a loss of stromal Cav-1 in cancer patients. thus, a loss of stromal fibroblast Cav-1 is a biomarker for chronic hypoxia, oxidative stress and autophagy in the tumor microenvironment, consistent with its ability to predict early tumor recurrence, lymph node metastasis and tamoxifen-resistance in human breast cancers. our results imply that cancer patients lacking stromal Cav-1 should benefit...

Loss of stromal caveolin-1 expression in malignant melanoma metastases predicts poor survival

“…31 It was previously demonstrated in breast cancer that loss of Cav-1 in stromal fibroblasts induces the activation of hypoxia-inducible factor-1α (HIF-1α) in the tumor stroma. 32 Activation of HIF-1α induces the generation of a glycolytic and autophagic microenvironment directly sustaining tumor growth and progression. 33,34 Thus, it is possible that in melanoma, a loss of stromal Cav-1 similarly supports metastatic dissemination by generating a lethal microenvironment.…”

Is cancer a metabolic rebellion against host aging? In the quest for immortality, tumor cells try to save themselves by boosting mitochondrial metabolism

“…Cancer cells first secreted hydrogen peroxide (H 2 O 2 ) to induce oxidative stress in adjacent fibroblasts as a form of accelerated aging; [30][31][32] at the same time, the cancer cells mounted an antioxidant defense by upregulating antioxidant proteins, such as TIGAR and peroxiredoxins. 29,33,34 Oxidative stress in the cancer-associated fibroblasts increased stromal ROS production, activating two major transcription factors, namely, HIF1a and NFκB, which both function as master regulators of autophagy, mitophagy, aerobic glycolysis as well as inflammation. 30,[33][34][35][36] As a consequence, the stromal fibroblasts would produce high-energy nutrients (L-lactate, ketones and glutamine).…”

CAF-like state in primary skin fibroblasts with constitutional BRCA1 epimutation sheds new light on tumor suppressor deficiency-related changes in healthy tissue

“…[40][41][42] This process is crucially regulated by elevation of HIF-1a, which, in turn, is depending on BRCA1. 43,44 As BRCA1 seems to be an emerging regulator of mitochondrial integrity, we hypothesize that its disruption in primary fibroblasts of persons with germline or somatic alterations also may induce CAF-like metabolic changes in the absence of cancer cells. This is supported by experimental data from a stable knockdown of BRCA1 in immortalized fibroblasts that results in upregulation of HIF-1a, autophagy, and ketone body production.…”