Metabolic reprogramming and two-compartment tumor metabolism Opposing role(s) of HIF1α and HIF2α in tumor-associated fibroblasts and human breast cancer cells

Hypoxia-inducible factor (HIF) 1 alpha and 2 alpha are transcription factors responsible for the cellular response to hypoxia. The functional roles of HIF1 alpha and HIF2 alpha in cancer are distinct and vary among different tumor types. The aim of this study was to evaluate the compartment-specific role(s) of HIF1 alpha and HIF2 alpha in breast cancer. To this end, immortalized human fibroblasts and MDA-MB-231 breast cancer cells carrying constitutively active HIF1 alpha or HIF2 alpha mutants were analyzed with respect to their metabolic function(s) and ability to promote tumor growth in an in vivo setting. We observed that activation of HIF1 alpha, but not HIF2 alpha, in stromal cells promotes a shift toward aerobic glycolysis, with increased L-lactate production and a loss of mitochondrial activity. In a xenograft model, HIF1 alpha-activated fibroblasts promoted the tumor growth of co-injected MDA-MB-231 cells without an increase in angiogenesis. Conversely, HIF2 alpha-activated stromal cells did not favor tumor growth and behaved as the empty vector controls. Similarly, activation of HIF1 alpha, but not HIF2 alpha, in MDA-MB-231 cells promoted a shift toward aerobic glycolysis, with increased glucose uptake and L-lactate production. In contrast, HIF2 alpha activation in cancer cells increased the expression of EGFR, Ras and cyclin D1, which are known markers of tumor growth and cell cycle progression. In a xenograft model, HIF1 alpha activation in MDA-MB-231 cells acted as a tumor suppressor, resulting in an almost 2-fold reduction in tumor mass and volume. Interestingly, HIF2 alpha activation in MDA-MB-231 cells induced a significant similar to 2-fold-increase in tumor mass and volume. Analysis of mitochondrial activity in these tumor xenografts using COX (cytochrome C oxidase) staining demonstrated elevated mitochondrial oxidative metabolism (OXPHOS) in HIF2 alpha-tumors. We conclude that the role(s) of HIF1 alpha and HIF2 alpha in tumorigenesis are compartment-specific. HIF1 alpha acts as a tumor promoter in stromal cells but as a tumor suppressor in cancer cells. Conversely, HIF2 alpha is a tumor promoter in cancer cells. Mechanistically, HIF1 alpha-driven aerobic glycolysis in stromal cells supports cancer cell growth via the paracrine production of nutrients (such as L-lactate) that can "feed" cancer cells. However, HIF1 alpha-driven aerobic glycolysis in cancer cells inhibits tumor growth. Finally, HIF2 alpha activation in cancer cells induces the expression of known pro-oncogenic molecules and promotes the mitochondrial activity of cancer cells.