Tumor microenvironment derived exosomes pleiotropically modulate cancer cell metabolism Cancer-associated fibroblasts release exosomes that both deliver nutrients to cancer cells and inhibit oxidative phosphorylation. Exosomes from fibroblasts can supply an amino acid called glutamine and other nutrients to cancer cells. A shortage of glutamine can limit the growth of pancreatic and perhaps other cancers Adding exosomes to prostate or pancreatic cancer cells both promotes glycolysis and blocks oxidative metabolism. It is likely that the increase in glycolysis is caused by the reduction in oxidative phosphorylation so, in this respect, the exosomes trigger glycolysis in the way initially envisioned by Warburg. These results call for a re-examination of the contributions of both processes to energy generation in cancer cells that are still associated with their neighbors.