Stromal-epithelial metabolic coupling in cancer: integrating autophagy and metabolism in the tumor microenvironment

Abstract

Cancer cells do not exist as pure homogeneous populations in vivo. Instead they are embedded in "cancer cell nests" that are surrounded by stromal cells, especially cancer associated fibroblasts. Thus, it is not unreasonable to suspect that stromal fibroblasts could influence the metabolism of adjacent cancer cells, and visa versa. In accordance with this idea, we have recently proposed that the Warburg effect in cancer cells may be due to culturing cancer cells by themselves, out of their normal stromal context or tumor microenvironment. In fact, when cancer cells are co-cultured with fibroblasts, then cancer cells increase their mitochondrial mass, while fibroblasts lose their mitochondria. An in depth analysis of this phenomenon reveals that aggressive cancer cells are "parasites" that use oxidative stress as a "weapon" to extract nutrients from surrounding stromal cells. Oxidative stress in fibroblasts induces the autophagic destruction of mitochondria, by mitophagy. Then, stromal cells are forced to undergo aerobic glycolysis, and produce energy-rich nutrients (such as lactate and ketones) to "feed" cancer cells. This mechanism would allow cancer cells to seed anywhere, without blood vessels as a food source, as they could simply induce oxidative stress wherever they go, explaining how cancer cells survive during metastasis. We suggest that stromal catabolism, via autophagy and mitophagy, fuels the anabolic growth of tumor cells, promoting tumor progression and metastasis. We have previously termed this new paradigm "The Autophagic Tumor Stroma Model of Cancer Metabolism", or the "Reverse Warburg Effect". We also discuss how glutamine addiction (glutaminolysis) in cancer cells fits well with this new model, by promoting oxidative mitochondrial metabolism in aggressive cancer cells.

Figure 1. The Autophagic Tumor Stroma Model of Cancer

In this model, cancer cells use oxidative stress as a “weapon” to extract recycled nutrients from cancer associated fibroblasts, via the induction of autophagy. Stromal autophagy, in turn, provides energy-rich recycled nutrients (such as lacatate, ketones, and glutamine) to fuel oxidative mitochondrial metabolism in cancer cells. Hypoxia, HIF1, and NFkB activation help drive autophagy in the tumor micronenvironment, while the upregulation of TIGAR (TP53-induced glycolysis and apoptosis regulator) in cancer cells protects them against apoptosis and confers autophagy resistance. TIGAR is a known inhibitor of both autophagy and apoptosis, and functionally shifts cells away from aerobic glycolysis, towards oxidative mitochondrial metabolism. [–27]. This scenario allows for the vectorial and unilateral transfer of energy from the tumor stroma (catabolism) to cancer cells, thereby fueling anabolic tumor growth via oxidative mitochondrial metabolism in cancer cells. Modified with permission from [22, 23]. CAFs, cancer associated fibroblasts; ROS, reactive oxygen species.

Figure 2. Fibroblasts Induce the Upregulation of TIGAR in Cancer Cells, Thereby Protecting Cancer Cells Against Apoptosis and Autophagy

Fibroblasts or breast cancer cells (MCF7), were cultured separately or co-cultured. Note the fibroblast-induced upregulation of TIGAR selectively in the cancer cells, protects cancer cells against apoptosis and autophagy (shown in red). Cancer cells are labeled with anti-keratin antibodies (shown in green). All cell nuclei are also labeled (shown in blue), to allow visualization of the keratin-negative fibroblasts. Modified with permission from [22, 23]. TIGAR is a known inhibitor of both autophagy and apoptosis, and functionally shifts cells away from aerobic glycolysis, towards oxidative mitochondrial metabolism. [–27].

Figure 3. Visualizing the “Reverse Warburg Effect”

Autophagy/mitophagy in fibroblasts promotes mitochondrial biogenesis in adjacent cancer cells. (Upper) Homotypic cultures of MCF7 cells and hTERT-fibroblasts were immunostained with a mitochondrial membrane antibody (Red). Mitochondrial mass is lower in monocultures of MCF7 cells, as compared to fibroblasts. (Middle) Co-culture with fibroblasts induces a significant increase in mitochondrial mass in the “central MCF7 cell colony”, encircled by a white oval. Conversely, mitochondrial mass decreases in co-cultured fibroblasts. (Lower) Lactate treatment increases mitochondria mass in MCF7 cells, thus simulating the co-culture with fibroblasts. Reproduced with permission from Figures 9 and 12, in the following reference [23].

Figure 4. Hypoxia Induces a Loss of Stromal Caveolin-1 via Autophagy

Fibroblasts were cultured under conditions of normoxia (upper panels) or hypoxia (lower panels). Note that hypoxia (lower panels) induces a loss of Cav-1 (shown in green), and the upregulation of autophagy markers (LC3, ATG16L, BNIP3, and BNIP3L; shown in red). Thus, hypoxia is sufficient to confer the cancer associated fibroblast phenotype. Modified with permission from [22, 23].

Figure 5. Mitochondrial Glutamine Metabolism in Cancer Cells Provides Another Related Mechanism for Maintaining Autophagy in the Tumor Stroma

Recent studies have shown that oxidative mitochondrial metabolism of glutamine in cancer cells produces ammonia. Ammonia is known to be sufficient to induce autophagy. Here, we propose that autophagy in cancer associated fibroblasts would provide cancer cells with an abundant source of glutamine. The ammonia produced would, in turn, help to maintain the autophagic phenotype of the cancer associated fibroblasts. CAFs, cancer associated fibroblasts.