Year: 2015
Type: article
Abstract: It is critical to determine mechanisms underlying the ability of cancer cells to metastasize, given that tumor metastasis is responsible for most cancer-related deaths. While solid tumors shed only a minute fraction of cells capable of colonizing at distant tissues and forming a secondary tumor, those invasive cells represent a key obstacle for successful clinical management of most cancers, including melanoma. Among factors implicated in melanoma propensity to metastasize is oxidative stress. High intracellular ROS levels promote oxidative stress associated with cell death through oxidation of proteins, DNA, and lipids (Trachootham et al., 2009). Yet, as increased ROS levels reportedly have both pro- and antitumorigenic effects on tumor initiation and progression (Schumacker, 2015), further understanding the effect of oxidative stress on tumor metastasis remains an important need. A recent publication from Dr. Morrison's group demonstrates that metastatic melanoma cells can reprogram their metabolism to survive oxidative stress conditions in circulating tumor cells as in tumor cells found in nodules. In this study, human melanoma tumors were collected and xenografted into immune incompetent NSG mice, revealing that their metastatic capacity coincided with the presence of circulating melanoma cells. Interestingly, the authors found that metastatic melanoma cells adapt to their environment at the metastatic niche and exhibit more robust invasiveness when reinjected into new mice relative to cells that remained at the primary (subcutaneous) site. This capacity could be reversed (i.e., become less metastatic) when metastatic cells were first passaged subcutaneously before being subjected to a second round of injection. The authors conclude that these outcomes are not the consequence of acquired genetic events but rather are likely due to epigenetic or metabolic changes. Using liquid chromatography–tandem mass spectrometry, the authors then compared the metabolic content of subcutaneous primary tumors relative to derived visceral metastatic nodules. They observed a lower glutathione (GSH) to oxidized glutathione (GSSG) ratio in metastatic nodules than in subcutaneous cells, suggesting that while becoming invasive, cells use GSH and concomitantly produce GSSG to maintain their redox status as a means of surviving strong oxidative stress. Consistently, both cytoplasmic and mitochondrial ROS levels were elevated in tumor cells of metastatic nodules relative to the primary tumor. Treatment of xenografted mice with the antioxidant N-acetyl-cysteine increased circulating melanoma cells and metastatic burden, indicating that oxidative stress blocks melanoma cell invasiveness. Metastastic melanoma cells must generate sufficient levels of GSH to buffer the cell redox status. Further analysis revealed higher levels of NADPH and NADP in metastatic cells as compared to subcutaneous cells. NADPH is required to regenerate GSH from GSSG (Filomeni et al., 2002), suggesting a mechanism used by metastatic cells to regenerate GSH. NADPH is an important cellular energy source primarily generated in the oxidative pentose phosphate pathway. Although, the Piskounova study shows that in proliferating cells, the folate pathway, which normally functions in de novo purine synthesis, contributes equally to NADPH production. Their metabolomic analysis revealed an enrichment of purine intermediates in metastatic nodules, suggesting increased folate pathway activity (Figure 1). Serine, a major carbon source for folate-dependent NADPH production, is synthesized following degradation of glucose (Fan et al., 2014), providing a readout for folate pathway activity. To assess potential contribution of the folate pathway to the formation of metastatic tumors, Piskounova et al. injected 13C-labeled glucose into mice harboring xenografted tumors (primary tumors) and observed increased 13C-labeled serine in metastatic cells, indicating enhanced folate pathway activity, possibly to provide cells with NADPH. In vivo inhibition of the folate pathway via methotrexate administration strongly impaired metastasis without altering growth of the subcutaneous tumor. Among folate pathway proteins, the enzyme ALDH1L2, which regulates the GSH/GSSG ratio, was overexpressed in metastatic nodules relative to corresponding subcutaneous primary tumors. ALDH1L2 inhibition by shRNA strongly impaired metastatic development, an effect also observed upon knockdown of MTHFD1, another folate pathway enzyme. These results indicate overall that the folate pathway plays a key role in metastatic capability of melanoma cells and identify two enzymes that could be potential therapeutic targets. In all, this study elegantly demonstrates that metastatic melanoma cells adapt their metabolism to counteract the deleterious effects of oxidative stress. The study also raises broader questions. Strong but incomplete inhibition of metastasis following folate pathway inhibition reported here implies that other pathways also function in this adaptive mechanism. As NADPH generation is important to maintain high GSH levels, it would be interesting to investigate involvement of other NADPH producers such as the pentose phosphate pathway. In addition, although GSH is the major cellular antioxidant, other antioxidant pathways may be important in melanoma metastasis. Indeed, a recent paper from Dr. Mak's group implicates the thioredoxin (TXN) pathway in progression of a mammary tumor mouse model. They report that the GSH pathway is critical for tumor initiation, but that the TXN pathway takes charge of antioxidant function once a tumor is established. Further, inhibiting both pathways halts tumor growth (Harris et al., 2015). Of note, TXN and GST-pi, key players in GSSH/GSH levels and cellular redox balance, also regulate the stress kinases, p38 and JNK, respectively (Adler et al., 1999). Thus, the concerted action of antioxidant pathways likely contributes to different phases of melanoma development and progression. Of note, Dr. Morrison's findings suggest that melanoma tumors can be classified as efficient and inefficient in terms of metastasis. The former undergoes effective metabolic reprogramming to impair the antisurvival effect of oxidative stress, an adaptation that does not seem to be linked to BRAF or NRAS mutations. In addition to validating this human tumor classification, it would be interesting to determine whether any genetic event (mutation or polymorphism) intrinsically predisposes melanoma tumors to adapt to oxidative stress. The latter may highlight upstream regulators and possible predictive markers that can be taken into account while defining a therapeutic approach. The immediate implications of the current study are that inhibition of the folate pathway would reduce tumor cell survival within the circulation and thus antagonize metastasis. This would thereby constitute a novel therapeutic strategy that could be administered as part of a treatment regimen to prevent disease progression.
Source: Pigment Cell & Melanoma Research
DOI: 10.1111/pcmr.12440
Authors: Alex Cazes, Ze'ev A. Ronai
Language: English
Subfield: Cancer Research
Domain: Life Sciences
Open Access status: bronze
OpenAlex ID: W2179240489