Reply to comment on “GRP94 promotes brain metastasis by engaging pro-survival autophagy”
We thank Yoshida1 for his interest in our recently published paper by Santana-Codina et al.2 We appreciate his positive feedback and valuable comments relative to our study on the role of glucose-regulated protein 94 (GRP94) in brain metastases (BM). Here, we describe a metabolic switch consisting of activation of the unfolded protein response and pro-survival autophagy to relieve metabolic stress in low glucose conditions, facilitating BM cell survival.2
Yoshida1 emphasizes the importance of evaluating p62/SQSTM1 in metastatic tumor cells located in the tumors’ invasive front and perinecrotic regions. We agree that p62 levels could help clarify if there is increased or impaired autophagic flux in hypoxia-inducible factor α–positive cells. BM cells show strong dependence on autophagy, and in fact inhibiting autophagic flux with chloroquine impaired BM development. Despite the well-known dual role of autophagy,3 we show that, in our model, autophagy is a pro-survival mechanism mediating adaptation to low nutrient conditions. Since cells expressing LC3B are close to necrotic areas in BM, it is tempting to speculate that autophagy might also be implicated in sheltering subpopulations in the hypoxic microenvironment.
Yoshida references edelfosine, a new therapy in phase II clinical trials as a promoter of autophagic cell death in triple negative breast cancer (TNBC). Edelfosine is an apoptotic inducer in cancer cells by promoting mitochondrial membrane permeability independent of reactive oxygen species, and lipid rafts redistribution from the plasma membrane to mitochondria.4 The mechanisms by which edelfosine induces autophagic cell death or apoptosis are complex and likely dose dependent.5 Moreover, autophagy has a dynamic role that can either suppress or promote tumor growth at different stages of tumor progression. In fact, our data are in accordance with previous studies showing a critical temporal dependence on autophagy, especially at initial stages of the metastatic establishment, where autophagy would promote the outgrowth of dormant metastatic cells.6 We agree with Yoshida that targeting brain adaptive cancer stem cells (CSCs) is a promising field to prevent or treat TNBC-BM, either with edelfosine or with other autophagy preventive drugs.
Finally, we agree with Yoshida on the importance of evaluating the effect of GRP94 on mitophagy in brain CSCs. In this regard, we have shown that BM cells present elongated mitochondria,7 a phenotype observed in response to starvation and autophagy induction in order to maintain ATP production and cell viability.8 We showed that BM cells exert a high ability to oxidize fatty acid and compensate hypoglycemic stress due to an overexpression of proteins involved in fatty acid synthesis and degradation, whereas GRP94 ablation restored glucose dependence and decreased tumorigenicity in vivo.7 Activation of autophagy (and likely mitophagy) might be concurrent events under starvation conditions to maintain energy production and mitochondrial integrity. Further studies are required to understand the interplay between mitophagy, bulk autophagy, and metabolic dependencies in BM cells.
In conclusion, we appreciate Yoshida’s comments on our work. We agree that continued research on GRP94 and its modulation of mitophagy and metabolic adaptation is a promising and exciting field that may lead to new therapeutic opportunities in BM.
Funding
This study was supported by grants from the Spanish Ministry of Health and Consumer Affairs, FIS-PI14/00336 and FIS-PI18/00916, from the I+D+I National Plan with the financial support from ISCIII-Subdirección General de Evaluación and the Fondo Europeo de Desarrollo Regional (FEDER), and by grants from Generalitat de Catalunya 2017_SGR_1305.
Conflict of interest statement
A.S. is inventor of the patent US 9,645,150 B2 and EP 2440931 named “Methods for determining the risk for developing brain metastasis, and a kit to carry out said method”, which refers to GRP94’s potential as biomarker to predict brain metastasis and to target therapies. All other authors have declared that no conflicts of interest exist.
References
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