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Published on: September 8, 2021
Role of Rho/MRTF in Aggressive Vemurafenib-Resistant Murine Melanomas and Immune Checkpoint Upregulation
Bardees M Foda1,2, Richard R Neubig1,3
1Department of Pharmacology and Toxicology, Michigan State University, East Lansing, MI 48823, USA.
Abstract:
Cutaneous melanoma is the deadliest skin cancer. Most have Ras-MAPK pathway (BRAFV600E or NRAS) mutations and highly effective targeted therapies exist; however, they and immune therapies are limited by resistance, in part driven by small GTPase (Rho and Rac) activation. To facilitate preclinical studies of combination therapies to provide durable responses, we describe the first mouse melanoma lines resistant to BRAF inhibitors. Treatment of mouse lines, YUMM1.7 and YUMMER, with vemurafenib (Vem), the BRAFV600E-selective inhibitor, resulted in high-level resistance (IC50 shifts 20-30-fold). Resistant cells showed enhanced activation of Rho and the downstream transcriptional coactivator, myocardin-related transcription factor (MRTF). Resistant cells exhibited increased stress fibers, nuclear translocation of MRTF-A, and an increased MRTF-A gene signature. Pharmacological inhibition of the Rho/MRTF pathway using CCG-257081 reduced viability of resistant lines and enhanced sensitivity to Vem. Remarkably, co-treatment of parental lines with Vem and CCG-257081 eliminated resistant colony development. Resistant cells grew more slowly in vitro, but they developed highly aggressive tumors with a shortened survival of tumor-bearing mice. Increased expression of immune checkpoint inhibitor proteins (ICIs) in resistant lines may contribute to aggressive in vivo behavior. Here, we introduce the first drug-resistant mouse melanoma models for assessing combinations of targeted and immune therapies.
Insights
Researchers developed new BRAF inhibitor-resistant melanoma mouse models. Targeting the Rho/myocardin-related transcription factor (MRTF) pathway with CCG-257081 resensitized melanoma cells to BRAF inhibitors, offering potential for durable cancer therapies.
Area of Science:
- Oncology
- Dermatology
- Molecular Biology
Background:
- Cutaneous melanoma, the deadliest skin cancer, often harbors Ras-MAPK pathway mutations (BRAFV600E or NRAS).
- Targeted therapies and immunotherapies for melanoma show efficacy but are limited by acquired resistance, partly mediated by small GTPase activation (Rho and Rac).
- Preclinical models are crucial for developing combination therapies to overcome resistance and achieve durable responses.
Purpose of the Study:
- To establish and characterize the first mouse melanoma models resistant to BRAF inhibitors.
- To investigate the mechanisms driving BRAF inhibitor resistance, focusing on small GTPase pathways.
- To evaluate combination therapies involving BRAF inhibitors and Rho/MRTF pathway inhibitors for overcoming resistance.
Main Methods:
- Generation of BRAF inhibitor-resistant mouse melanoma cell lines (YUMM1.7 and YUMMER) via vemurafenib (Vem) treatment.
- Analysis of Rho/MRTF pathway activation, including stress fiber formation, MRTF-A nuclear translocation, and gene signature.
- Assessment of pharmacological inhibition of the Rho/MRTF pathway using CCG-257081 on resistant cells and in combination with Vem.
Main Results:
- Vem treatment induced high-level resistance (20-30-fold IC50 shift) in YUMM1.7 and YUMMER lines.
- Resistant cells exhibited enhanced Rho/MRTF pathway activation, increased stress fibers, and nuclear MRTF-A.
- CCG-257081 treatment reduced viability of resistant cells, enhanced Vem sensitivity, and prevented resistant colony formation when co-administered with Vem to parental cells.
- Resistant melanoma tumors were aggressive in vivo, associated with shortened mouse survival and increased immune checkpoint inhibitor (ICI) expression.
Conclusions:
- The developed mouse melanoma models represent the first models of BRAF inhibitor resistance.
- Rho/MRTF pathway activation is a key mechanism driving BRAF inhibitor resistance in melanoma.
- Combination therapy targeting BRAF and the Rho/MRTF pathway shows promise for overcoming resistance and improving treatment outcomes in melanoma.
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