We have previously demonstrated that enhanced aerobic glycolysis and/or 
autophagy in the tumor stroma supports epithelial cancer cell growth and 
aggressive behavior, via the secretion of high-energy metabolites. These 
nutrients include lactate and ketones, as well as chemical building blocks, such 
as amino acids (glutamine) and nucleotides. Lactate and ketones serve as fuel 
for cancer cell oxidative metabolism, and building blocks sustain the anabolic 
needs of rapidly proliferating cancer cells. We have termed these novel concepts 
the "Reverse Warburg Effect," and the "Autophagic Tumor Stroma Model of Cancer 
Metabolism." We have also identified a loss of stromal caveolin-1 (Cav-1) as a 
marker of stromal glycolysis and autophagy. The aim of the current study was to 
provide genetic evidence that enhanced glycolysis in stromal cells favors 
tumorigenesis. To this end, normal human fibroblasts were genetically-engineered 
to express the two isoforms of pyruvate kinase M (PKM1 and PKM2), a key enzyme 
in the glycolytic pathway. In a xenograft model, fibroblasts expressing PKM1 or 
PKM2 greatly promoted the growth of co-injected MDA-MB-231 breast cancer cells, 
without an increase in tumor angiogenesis. Interestingly, PKM1 and PKM2 promoted 
tumorigenesis by different mechanism(s). Expression of PKM1 enhanced the 
glycolytic power of stromal cells, with increased output of lactate. Analysis of 
tumor xenografts demonstrated that PKM1 fibroblasts greatly induced tumor 
inflammation, as judged by CD45 staining. In contrast, PKM2 did not lead to 
lactate accumulation, but triggered a "pseudo-starvation" response in stromal 
cells, with induction of an NFκB-dependent autophagic program, and increased 
output of the ketone body 3-hydroxy-buryrate. Strikingly, in situ evaluation of 
Complex IV activity in the tumor xenografts demonstrated that stromal PKM2 
expression drives mitochondrial respiration specifically in tumor cells. 
Finally, immuno-histochemistry analysis of human breast cancer samples lacking 
stromal Cav-1 revealed PKM1 and PKM2 expression in the tumor stroma. Thus, our 
data indicate that a subset of human breast cancer patients with a loss of 
stromal Cav-1 show profound metabolic changes in the tumor microenvironment. As 
such, this subgroup of patients may benefit therapeutically from potent 
inhibitors targeting glycolysis, autophagy and/or mitochondrial activity (such 
as metformin).