Cellular Responses to Lactic Acidosis in Human Cancers
The physiology of the tumor microenvironment is characterized by lower oxygen (hypoxia), higher lactate, extracellular acidosis and glucose starvation. We examined the global, transcriptional cellular responses to each of these microenvironmental stresses in vitro, projected them onto clinical breast cancer patients' samples in vivo, and returned to perform further in vitro experiments to investigate the potential mechanisms involved in these stress responses. The reciprocal exchange of information was critical and advanced our understanding of the potential clinical relevance of cellular responses.
Our expression array result showed that lactic acidosis induces a strong response, distinct from that of hypoxia in human mammalian epithelial cells (HMECs), indicating lactic acidosis is not only a by-product of hypoxia but has a unique role as a stimulant to cells in the tumor microenvironment. Cellular responses to lactosis and acidosis further demonstrated that acidosis was the main driving force in the lactic acidosis response. These responding gene signatures were then statistically projected into clinical breast cancer patients' expression data sets. The hypoxia response, as reported previously, was associated with bad prognosis, where as the lactic acidosis and acidosis responses, were associated with good prognosis. Additionally, the acidosis response could be used to separate breast tumors with high versus low aggressiveness based on its inversed correlation with metastatic character. We further discovered that lactic acidosis, in contrast to hypoxia, abolished Akt signaling. Moreover, it downregulated glycolysis and shifted energy utilization towards aerobic respiration.
We continued to examine the cellular response to lactic acidosis temporally in MCF7 cells, a breast cancer cell line. The lactic acidosis response of MCF7 cells also showed the prognostic result of better clinical outcomes in datasets of breast cancer patients. The lactic acidosis responses of HMEC and MCF cells were highly correlated. Strikingly in MCF7 cells, lactic acidosis and glucose deprivation actually induced similar transcriptional profiles, with only a few genes being oppositely regulated. Furthermore, lactic acidosis, similar to glucose starvation, induced AMPK signaling and abolished mTOR. However, lactic acidosis and glucose deprivation induced opposite glucose uptake phenotypes. Lactic acidosis significantly repressed glucose uptake whereas glucose deprivation significantly induced it. Among the genes differentially regulated by these two stresses, thioredoxin-interacting protein (TXNIP) was among the most different. The negative regulatory role of TXNIP on glucose uptake has been demonstrated previously. In the cancer research field, TXNIP is recognized as a tumor suppressor gene. We observed that lactic acidosis induced TXNIP strongly and most importantly, TXNIP played a critical role in regulating glucose uptake in cells under lactic acidosis. Furthermore, MondoA, the transcription factor and glucose sensor previously reported to regulate TXNIP induction upon glucose exposure, was also responsible for regulating TXNIP under lactic acidosis. We demonstrated that TXNIP not only plays an important role in the lactic acidosis response but also has strong prognostic power to separate breast cancer patients based on survival.
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