BRAF Inhibition Stimulates Melanoma-Associated Macrophages to Drive Tumor Growth

Clin Cancer Res. 2015 Apr 1;21(7):1652-64. doi: 10.1158/1078-0432.CCR-14-1554. Epub 2015 Jan 23.

Abstract

Purpose: To investigate the roles of melanoma-associated macrophages in melanoma resistance to BRAF inhibitors (BRAFi).

Experimental design: An in vitro macrophage and melanoma cell coculture system was used to investigate whether macrophages play a role in melanoma resistance to BRAFi. The effects of macrophages in tumor resistance were examined by proliferation assay, cell death assay, and Western blot analyses. Furthermore, two mouse preclinical models were used to validate whether targeting macrophages can increase the antitumor activity of BRAFi. Finally, the number of macrophages in melanoma tissues was examined by immunohistochemistry.

Results: We demonstrate that in BRAF-mutant melanomas, BRAFi paradoxically activate the mitogen-activated protein kinase (MAPK) pathway in macrophages to produce VEGF, which reactivates the MAPK pathway and stimulates cell growth in melanoma cells. Blocking the MAPK pathway or VEGF signaling then reverses macrophage-mediated resistance. Targeting macrophages increases the antitumor activity of BRAFi in mouse and human tumor models. The presence of macrophages in melanomas predicts early relapse after therapy.

Conclusions: Our findings demonstrate that macrophages play a critical role in melanoma resistance to BRAFi, suggesting that targeting macrophages will benefit patients with BRAF-mutant melanoma.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Blotting, Western
  • Coculture Techniques
  • Drug Resistance, Neoplasm / immunology*
  • Enzyme-Linked Immunosorbent Assay
  • Female
  • Flow Cytometry
  • Humans
  • Immunoblotting
  • Immunohistochemistry
  • Macrophages / drug effects
  • Macrophages / immunology
  • Macrophages / metabolism*
  • Melanoma / immunology
  • Melanoma / pathology*
  • Mice
  • Mice, Inbred BALB C
  • Molecular Targeted Therapy / adverse effects
  • Protein Kinase Inhibitors / pharmacology
  • Proto-Oncogene Proteins B-raf / antagonists & inhibitors*
  • RNA, Small Interfering
  • Signal Transduction / drug effects*
  • Transfection
  • Tumor Microenvironment / immunology
  • Xenograft Model Antitumor Assays

Substances

  • Protein Kinase Inhibitors
  • RNA, Small Interfering
  • Proto-Oncogene Proteins B-raf