P1-03-03: Adaptive Exploitation of Stromal Cell Metabolism by Tumor Cells.
Authors: Patel B , Rattigan Y , Ackerstaff E , Koutcher J , Sukenick G , Glod J , Banerjee D 
Published: June 2012

Abstract
Tumor cells secrete cytokines to recruit and activate stromal cells in the tumor milieu leading to reciprocal paracrine support of tumor growth by stroma-derived growth factors. This is an important means by which tumors adapt their microenvironment to facilitate their growth. Indeed, breast cancer development and metastatic progression is highly dependent on stromal support, particularly from carcinoma associated fibroblasts (CAFs). As a result of aerobic glycolysis, tumor cells produce and secrete high levels of lactate, thought to be a toxic byproduct that needs to be extruded into the tumor milieu. Using CAFs generated by prolonged exposure to tumor conditioned medium, we have investigated the role that lactate may play in CAF-mediated support of tumor growth. In addition to extruding lactate as a byproduct of glycolysis, we suggest that tumor cells secrete it to recruit and subsequently exploit stromal cells to recycle lactate into utilizable metabolites, such as pyruvate, to fulfill metabolic demands of tumor cells. Our hypothesis is that lactate secreted by tumor cells is taken up by stromal cells and converted to metabolites critical for tumor growth and progression. Our studies indicate that (i) MDA-MB-231 breast cancer cells secrete significantly higher levels of lactate (3-fold more) under hypoxia, and that (ii) lactate recruits mesenchymal stem cells, the precursors of CAFs, towards tumor cells by activating signaling pathways to enhance migration. Lactate is transported by a family of proteins termed monocarboxylate transporters (MCTs); cells take up lactate via MCT1 and efflux it via MCT4. Expectedly, MDA-MB-231 breast carcinoma cells display low expression of MCT1 while exhibiting high expression of MCT4. However, CAFs show high expression of MCT1 while displaying low expression of MCT4, indicating that lactate extruded by the tumor cells is taken up by stromal cells. Our investigation further revealed that expression of lactate dehydrogenase B and pyruvate dehydrogenase are induced upon lactate exposure in CAFs, supporting the contention that the lactate taken up by stromal cells is metabolized. This implies that CAFs may be less glycolytic than MDA-MB-231s. Indeed the expression of pyruvate kinase isoform 2 (PKM2), which positively correlates with glycolysis, is greater in MDA-MMB-231s than in CAFs, while CAFs express significantly more PKM1, the isoform that correlates with a more oxidative metabolic state. Finally, 13C NMR spectroscopic analyses indicate that 13C-lactate is indeed metabolized via the TCA cycle in stromal cells, further supporting our hypothesis. Thus, stromal fibroblasts in the tumor microenvironment have the capacity to take up tumor-secreted lactate and use it as an energy source. To our knowledge this is the first in vitro model system demonstrating tumor/stroma metabolic coupling by which tumor cells exploit stromal cells.
A better understanding of the molecular mechanisms governing metabolic cooperation within the tumor milieu will potentially identify new targets for therapeutic intervention.
Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P1-03-03.