The Pancreatic Tumor Microenvironment Compensates for Loss of GOT2

Abstract

The tumor microenvironment (TME) in pancreatic ductal adenocarcinoma (PDA) restricts vascularization and consequently nutrient and oxygen access and drug delivery. Concurrently, recent work also demonstrates that the TME directly provides metabolites that facilitate cancer cell survival, growth, and therapeutic resistance. Redox imbalance is another restraint on cellular proliferation, yet it is unknow how the TME contributes to the maintenance redox homeostasis in PDA cells. Here, we demonstrate that the loss of mitochondrial glutamate-oxaloacetate transaminase 2 (GOT2), a component in the malate-aspartate shuttle (MAS), disturbs redox homeostasis and halts proliferation of PDA cells in vitro. Surprisingly, we found GOT2 knockdown has no effect on in vivo tumor growth. We demonstrate that this discrepancy is explained by heterocellular pyruvate exchange from the TME, including from cancer associated fibroblasts (CAF). More broadly, pyruvate similarly confers resistance to inhibitors of mitochondrial respiration. Blocking pyruvate uptake through genetic or pharmacologic inhibition of monocarboxylate transporter 1 (MCT1) abrogated pyruvate-mediated restoration of redox homeostasis. In sum, this work describes a potential resistance mechanism mediated by metabolic crosstalk within the pancreatic TME. These findings have important implications for metabolic treatment strategies since several mitochondrial inhibitors are currently in clinical trials for PDA and other cancers.

Interactions with stromal cells promote a more oxidized cancer cell redox state in pancreatic tumors

“…We next explored which metabolites in PSC- and cancer cell-conditioned media could be promoting the increase in pancreatic cancer organoid number. Since alanine has been shown to be secreted by PSCs 7 , and pyruvate has been shown to promote proliferation of multiple cancer cells types including pancreatic cancer 23,24,57,58 , we first examined whether the organoid growth-promoting role of PSCs could be replaced by addition of exogenous pyruvate or alanine to the media of organoid monocultures (Supplementary Figure 1). Strikingly, when grown in monoculture, pancreatic cancer cell organoid number was approximately doubled in the presence of exogenous pyruvate (Figure 1c, Supplementary Figure 2a-c).…”

“…Consistent with this notion, we find cancer cells in both organoid and mouse tumor PDAC models are more reduced than surrounding non-cancer cells. Gaining access to exogenous electron acceptors, such as pyruvate, can overcome this redox limitation 23,24 , and secretion of pyruvate by PSCs is one mechanism by which stromal cells might help cancer cells proliferate 58 . However, the concentration of pyruvate in interstitial fluid isolated from pancreatic tumors is lower than the concentration of pyruvate found in plasma, suggesting that some cells in pancreatic tumors may not have extensive access to this metabolite in vivo 64 .…”

Cancer-associated fibroblasts: The chief architect in the tumor microenvironment

“…Aside from lactate, recent reports indicate pyruvate to be the major contribution of CAFs to the metabolome of the stroma as well. Significantly high secretion from breast CAFs and PDAC-CAF conditioned media substantiated the claim for a pyruvate-rich metabolome (Becker et al, 2020;Kerk et al, 2020). Kerk et al (2020) described that the CAF-secreted extracellular pyruvate maintained redox homeostasis and was beneficial in developing resistance against mitochondrial inhibitors in pancreatic cancer.…”

“…Significantly high secretion from breast CAFs and PDAC-CAF conditioned media substantiated the claim for a pyruvate-rich metabolome (Becker et al, 2020;Kerk et al, 2020). Kerk et al (2020) described that the CAF-secreted extracellular pyruvate maintained redox homeostasis and was beneficial in developing resistance against mitochondrial inhibitors in pancreatic cancer. Strategic aid from pyruvate was again validated by the reports of lymphoma cell survival and ECM remodeling by metastatic breast cancer cells (Sakamoto et al, 2019).…”