Aspalathus linearis suppresses cell survival and proliferation of enzalutamide-resistant prostate cancer cells via inhibition of c-Myc and stability of androgen receptor

PLoS One. 2022 Jul 1;17(7):e0270803. doi: 10.1371/journal.pone.0270803. eCollection 2022.

Abstract

Enzalutamide, a nonsteroidal antiandrogen, significantly prolonged the survival of patients with metastatic castration-resistant prostate cancer (CRPC). However, patients receiving enzalutamide frequently develop drug resistance. Rooibos (Aspalathus linearis) is a shrub-like leguminous fynbos plant endemic to the Cedarberg Mountains area in South Africa. We evaluated the possibility of using a pharmaceutical-grade green rooibos extract (GRT, containing 12.78% aspalathin) to suppress the proliferation and survival of enzalutamide-resistant prostate cancer (PCa) cells. Treatment with GRT dose-dependently suppressed the proliferation, survival, and colony formation of enzalutamide-resistant C4-2 MDV3100r cells and PC-3 cells. Non-cancerous human cells were more resistant to GRT treatment. GRT suppressed the expression of proteins involved in phosphoinositide 3-kinase (PI3K)-Akt signaling, androgen receptor (AR), phospho-AR (Ser81), cyclin-dependent kinase 1 (Cdk1), c-Myc and Bcl-2 but increased the expression of apoptotic proteins. Overexpression of c-Myc antagonized the suppressive effects of GRT, while knockdown of c-Myc increased the sensitivity of PCa cells to GRT treatment. Expression level of c-Myc correlated to resistance of PCa cells to GRT treatment. Additionally, immunofluorescence microscopy demonstrated that GRT reduced the abundance of AR proteins both in nucleus and cytoplasm. Treatment with cycloheximide revealed that GRT reduced the stability of AR. GRT suppressed protein expression of AR and AR's downstream target prostate specific antigen (PSA) in C4-2 MDV3100r cells. Interestingly, we observed that AR proteins accumulate in nucleus and PSA expression is activated in the AR-positive enzalutamide-resistant PCa cells even in the absence of androgen. Our results suggested that GRT treatment suppressed the cell proliferation and survival of enzalutamide-resistant PCa cells via inhibition of c-Myc, induction of apoptosis, as well as the suppression of expression, signaling and stability of AR. GRT is a potential adjuvant therapeutic agent for enzalutamide-resistant PCa.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Aspalathus* / metabolism
  • Benzamides
  • Cell Line, Tumor
  • Cell Proliferation
  • Cell Survival
  • Humans
  • Male
  • Nitriles
  • Phenylthiohydantoin
  • Phosphatidylinositol 3-Kinases
  • Prostate-Specific Antigen / therapeutic use
  • Prostatic Neoplasms, Castration-Resistant* / pathology
  • Receptors, Androgen / genetics
  • Receptors, Androgen / metabolism

Substances

  • Benzamides
  • Nitriles
  • Receptors, Androgen
  • Phenylthiohydantoin
  • enzalutamide
  • Prostate-Specific Antigen

Grants and funding

This research was funded by Ministry of Science and Technology in Taiwan (MOST 109-2923-B-400-002-MY3), intramural grant from National Health Research Institutes (CS-110/111-PP-03), collaboration grant between National Health Research Institutes and Kaohsiung Medical University (NHRIKMU-111-I002), and the National Research Foundation (NRF) of South Africa (IRG-Taiwan/South African Research Cooperation Programme; Grant Number 98854). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.