Lysosomal Zn2+ release triggers rapid, mitochondria-mediated, non-apoptotic cell death in metastatic melanoma

Highlights

• •TRPML1 is dramatically upregulated in metastatic melanoma cells
• •Activation of TRPML1, instead of inhibition, induces selective melanoma cell death
• •TRPML-specific synthetic agonists (ML-SAs) trigger a distinctive form of cell death
• •ML-SAs exhibit potent in vivo therapeutic efficacy in advanced melanoma mouse models

Summary

During tumor progression, lysosome function is often maladaptively upregulated to match the high energy demand required for cancer cell hyper-proliferation and invasion. Here, we report that mucolipin TRP channel 1 (TRPML1), a lysosomal Ca²⁺ and Zn²⁺ release channel that regulates multiple aspects of lysosome function, is dramatically upregulated in metastatic melanoma cells compared with normal cells. TRPML-specific synthetic agonists (ML-SAs) are sufficient to induce rapid (within hours) lysosomal Zn²⁺-dependent necrotic cell death in metastatic melanoma cells while completely sparing normal cells. ML-SA-caused mitochondria swelling and dysfunction lead to cellular ATP depletion. While pharmacological inhibition or genetic silencing of TRPML1 in metastatic melanoma cells prevents such cell death, overexpression of TRPML1 in normal cells confers ML-SA vulnerability. In the melanoma mouse models, ML-SAs exhibit potent in vivo efficacy of suppressing tumor progression. Hence, targeting maladaptively upregulated lysosome machinery can selectively eradicate metastatic tumor cells in vitro and in vivo.

ntroduction

Once metastasized, melanoma skin cancer responds poorly to the traditional anti-cancer options, calling for a need to develop novel treatment strategies (Narayana et al., 2013; Welsh et al., 2016). Lysosomes, the cellular hub in metabolism, protein degradation, and nutrient sensing, play an indispensable role in cell survival and growth in normal physiology (Lawrence and Zoncu, 2019). In the metastatic cancer, lysosomes are often “transformed” to be hypertrophic (Katheder et al., 2017; Kroemer and Jäättelä, 2005), actively contributing to tumor progression, not only in cancer cell proliferation and survival by providing nutrients via macromolecule degradation but also in cancer invasion and metastasis by secreting lysosomal hydrolases to digest the extracellular matrix (Finicle et al., 2018; Hämälistö and Jäättelä, 2016). Due to a high energy demand of cancer cells, the function of lysosomes is often adaptively or maladaptively upregulated to meet the metabolic requirement of cancer cells (Kallunki et al., 2013; Piao and Amaravadi, 2016). Given the heavy dependence on lysosome machinery for hyper-proliferation and invasion, cancer cells are particularly sensitive to lysosome disruptions (Serrano-Puebla and Boya, 2018). Lysosome inhibitors and/or inducers of lysosome membrane permeability are reportedly effective in triggering cancer cell death (McAfee et al., 2012; Petersen et al., 2013; Tardy et al., 2006). However, broad-spectrum lysosome inhibition, such as inhibitors of vacuolar type ATPase (v-ATPase), may inevitably compromise the functions of normal cells and tissues (Pérez-Sayáns et al., 2009). Hence, it is more desirable to target specific lysosomal machineries that operate in normal physiology but are maladaptively upregulated in cancer.Mucolipin TRP channel 1 (TRPML1/MCOLN1; ML1) is a Ca²⁺ and Zn²⁺/Fe²⁺ dually permeable cation channel predominantly localized on the membranes of late endosomes and lysosomes (LELs) in all mammalian cell types (Cheng et al., 2010; Dong et al., 2008). The related ML2 and ML3 channels are also permeable to Ca²⁺, as well as heavy metal ions, but are more restrictively expressed (Cheng et al., 2010; Li et al., 2019). Using TRPML-specific synthetic agonists (ML-SAs) and synthetic inhibitors (ML-SIs), which bind directly to TRPML proteins in the atomic-resolution co-structures (Schmiege et al., 2017; Schmiege et al., 2021), we and others have demonstrated that TRPML channels, especially ML1, play essential roles in various lysosomal functions, which include lysosome movement, membrane trafficking, lysosomal exocytosis, lysosome biogenesis, and heavy metal homeostasis (Dong et al., 2008; Li et al., 2016; Minckley et al., 2019; Peng et al., 2020; Samie et al., 2013; Yu et al., 2020).While a high level of ML1 expression may serve as a favorable prognostic marker for several types of cancer, RNA sequencing (RNA-seq) analysis revealed that the highest expression among them is in melanoma (WebLink, 2019). Although several recent loss-of-function studies suggest that ML1 inhibition or knock down could reduce cancer cell proliferation, the effects were marginal even over time courses of days (Jung et al., 2019; Shekoufeh Almasi et al., 2020; Xu et al., 2019). Whereas no gross growth phenotype is seen in Trpml1 knockout (KO) mice and type IV mucolipindosis patients (Venugopal et al., 2007), the Genome-wide Cancer Dependence Map showed that ML1 is dispensable for cancer cell survival (DepMap, https://depmaporg/portal/gene/MCOLN1?tab=overview). Consistent with these observations, we found that prolonged inhibition of ML1 using ML-SIs that are much more potent than the published ones barely inhibited the growth of multiple cancer cell lines (unpublished data). In the current study, we found that small-molecule activation, but not inhibition of ML1, induced selective cell death of metastatic melanoma in vitro and in vivo…