Bladder cancer, with over 600,000 global diagnoses in 2022, poses a substantial societal burden (Bray et al., 2024). While 75% of these diagnoses are non-muscle-invasive bladder cancer (NMIBC), 51% of these patients experience recurrence, with 19% progressing to muscle-invasive bladder cancer (MIBC) (Cambier et al., 2016, Gontero et al., 2015, Patel et al., 2020). MIBC is an aggressive subtype with a 5-year survival rate that has remained stagnant over the past decade (Edge and Cancer, 2010, Patel et al., 2020). The primary treatment for MIBC is the systemic administration of cisplatin-based chemotherapy. However, the response rate in patients is less than 50% due to inherent or acquired resistance to cisplatin, leading to treatment failure, increased metastasis, higher recurrence rates, and ultimately worsened overall survival (Kamat et al., 2016, Shi et al., 2022). The concerted effort in addressing cisplatin resistance has revealed the complexity of the underlying mechanisms involving intracellular, tumor microenvironment-associated, and metabolic-regulated pathways.
Understanding the molecular mechanisms underlying cisplatin resistance is crucial for identifying potential therapeutic targets. Various methods, including genomic, genetic, and epigenetic approaches, have been employed to detect alterations in pathways, gene expression, and transcription factors between normal, cancerous, or resistant cells. However, these analyses often fail to distinguish between promoter, suppressor, or incidental resistance markers and rarely provide specific mechanisms for irregular gene expression, highlighting the need for detailed mechanistic investigations into chemoresistance.
MicroRNAs (miRNAs) have emerged as significant regulators of gene expression, with alterations in miRNA profiles frequently observed in cancer. Dysregulation of miRNAs can lead to aberrant gene expression, affecting various cellular processes such as drug influx/efflux, cell cycle progression, differentiation, proliferation, and apoptosis—all critical factors in cancer progression and chemotherapy response (Cai et al., 2019). miRNAs play significant roles in cisplatin resistance, acting as either promoters or suppressors depending on the specific genes or pathways they modulate. Combination therapies that utilize both cisplatin and miRNA modulation have shown promising results in increasing cisplatin efficacy in vitro and in vivo across several cancer types, including breast, colorectal, gastric, liver, lung, ovarian, prostate, testicular, and thyroid cancers, suggesting a potential therapeutic approach (Wang et al., 2020). Given the tissue-specific expression of miRNAs compared to mRNA, especially in tumors, targeting miRNAs may be more therapeutically effective than targeting a single oncogene (Cai et al., 2019, Li et al., 2019). Although miRNA can simultaneously regulate multiple pathways that contribute to cisplatin resistance, the lack of non-toxic miRNA modulators that can remain effective in vivo for a long period continues to hinder the advancement of miRNA-chemotherapy combination therapies, making it challenging for these approaches to progress to clinical trials. Nevertheless, with more research tools available, simultaneous studies can be conducted to evaluate both the efficacy and safety/toxicity, as well as to further investigate the molecular pathways involving miRNA (Wang et al., 2020).
Recently, dysregulation of the cholesterol biosynthesis pathway, also known as the mevalonate pathway, has been linked to multidrug resistance in several cancer types (Yan et al., 2020). Farnesyl-diphosphate farnesyltransferase 1 (FDFT1), a key gene downstream of the mevalonate pathway, has increasingly been implicated in tumor progression and resistance to anticancer drugs (Said et al., 2014). FDFT1 encodes the squalene synthase (SQS) enzyme, which catalyzes the conversion of the metabolite farnesyl pyrophosphate (FPP) to squalene, marking the first committed step in sterol biosynthesis (Fig. 1). Thus, FDFT1 plays a critical role in directing the flow of FPP utilization toward either the sterol or non-sterol branches of the pathway (Do et al., 2009, Ha and Lee, 2020). The sterol branch leads to cholesterol synthesis, while the non-sterol branch results in the prenylation of isoprenoids and the activation of Ras and Rho family proteins. Both cholesterol and Ras/Rho proteins have been associated with carcinogenesis and resistance to chemotherapeutic drugs (Yan et al., 2020), highlighting the crucial role of FDFT1 as the downstream regulator of the pathway in cancer progression and chemoresistance.
In our previous multiparametric siRNA library screening for bladder cancer metastasis modulators, FDFT1 was identified as a tumor suppressor gene. Subsequent bioinformatics analyses of public data sets demonstrated its role as a prognostic indicator for responsiveness to various chemotherapeutic agents (Said et al., 2014). Indeed, FDFT1 was highly expressed in NMIBC tissue samples compared to MIBC (Dyrskjøt et al., 2003). FDFT1-associated molecular identification has also been proposed as a potential strategy for predicting chemoresistant bladder cancer (Kanmalar et al., 2022). Moreover, our previous metabolite characterization using Raman spectroscopy on bladder tissue samples revealed higher levels of FDFT1-associated metabolites in chemotherapy-sensitive tissue compared to resistant tissue (Kanmalar et al., 2024). However, the regulation of FDFT1 and its association with the chemotherapeutic response in bladder cancer have not been mechanistically and functionally elucidated.
In this study, we explored the mechanistic role of FDFT1 in mediating the chemosensitivity of bladder cancer cells to cisplatin. We report that FDFT1 is downregulated in cisplatin-resistant bladder cancer cells and mediates the sensitivity of these cells to cisplatin treatment. Notably, we found that miR-146b-5p directly targets FDFT1, reducing cisplatin sensitivity in bladder cancer cells—an effect that can be reversed by ectopic expression of FDFT1. Furthermore, FDFT1 modulation resulted in alterations in downstream components of the cholesterol biosynthesis pathway, suggesting a redirection of FPP utilization. This shift in the cholesterol pathway potentially contributes to resistance in the bladder cancer cells, establishing a miR-146b-5p/FDFT1 axis that regulates the response of bladder cancer cells to cisplatin treatment.