MERTK receptor tyrosine kinase is a therapeutic target in melanoma
Jennifer Schlegel, Maria J. Sambade, Susan Sather, Stergios J. Moschos, Aik-Choon Tan, Amanda Winges, Deborah DeRyckere, Craig C. Carson, Dimitri G. Trembath, John J. Tentler, S. Gail Eckhardt, Pei-Fen Kuan, Ronald L. Hamilton, Lyn M. Duncan, C. Ryan Miller, Nana Nikolaishvili-Feinberg, Bentley R. Midkiff, Jing Liu, Weihe Zhang, Chao Yang, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Janiel M. Shields, Douglas K. Graham
Jennifer Schlegel, Maria J. Sambade, Susan Sather, Stergios J. Moschos, Aik-Choon Tan, Amanda Winges, Deborah DeRyckere, Craig C. Carson, Dimitri G. Trembath, John J. Tentler, S. Gail Eckhardt, Pei-Fen Kuan, Ronald L. Hamilton, Lyn M. Duncan, C. Ryan Miller, Nana Nikolaishvili-Feinberg, Bentley R. Midkiff, Jing Liu, Weihe Zhang, Chao Yang, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Janiel M. Shields, Douglas K. Graham
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Research Article Oncology

MERTK receptor tyrosine kinase is a therapeutic target in melanoma

Abstract

Metastatic melanoma is one of the most aggressive forms of cutaneous cancers. Although recent therapeutic advances have prolonged patient survival, the prognosis remains dismal. C-MER proto-oncogene tyrosine kinase (MERTK) is a receptor tyrosine kinase with oncogenic properties that is often overexpressed or activated in various malignancies. Using both protein immunohistochemistry and microarray analyses, we demonstrate that MERTK expression correlates with disease progression. MERTK expression was highest in metastatic melanomas, followed by primary melanomas, while the lowest expression was observed in nevi. Additionally, over half of melanoma cell lines overexpressed MERTK compared with normal human melanocytes; however, overexpression did not correlate with mutations in BRAF or RAS. Stimulation of melanoma cells with the MERTK ligand GAS6 resulted in the activation of several downstream signaling pathways including MAPK/ERK, PI3K/AKT, and JAK/STAT. MERTK inhibition via shRNA reduced MERTK-mediated downstream signaling, reduced colony formation by up to 59%, and diminished tumor volume by 60% in a human melanoma murine xenograft model. Treatment of melanoma cells with UNC1062, a novel MERTK-selective small-molecule tyrosine kinase inhibitor, reduced activation of MERTK-mediated downstream signaling, induced apoptosis in culture, reduced colony formation in soft agar, and inhibited invasion of melanoma cells. This work establishes MERTK as a therapeutic target in melanoma and provides a rationale for the continued development of MERTK-targeted therapies.

Authors

Jennifer Schlegel, Maria J. Sambade, Susan Sather, Stergios J. Moschos, Aik-Choon Tan, Amanda Winges, Deborah DeRyckere, Craig C. Carson, Dimitri G. Trembath, John J. Tentler, S. Gail Eckhardt, Pei-Fen Kuan, Ronald L. Hamilton, Lyn M. Duncan, C. Ryan Miller, Nana Nikolaishvili-Feinberg, Bentley R. Midkiff, Jing Liu, Weihe Zhang, Chao Yang, Xiaodong Wang, Stephen V. Frye, H. Shelton Earp, Janiel M. Shields, Douglas K. Graham

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Figure 4

MERTK inhibition by shRNA diminishes MERTK-mediated downstream signaling, colony-forming potential, and tumorigenesis.

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MERTK inhibition by shRNA diminishes MERTK-mediated downstream signaling...
(A) MERTK protein expression in cell lines stably expressing shRNA against MERTK. Whole-cell lysates from indicated cell lines were analyzed by Western blot for MERTK protein expression. Actin was detected as a loading control. (B) GAS6-mediated signaling in MERTK knockdown cell lines. shControl and MERTK knockdown cell lines were treated with GAS6 and downstream signaling was evaluated by Western blot. (C) Anchorage-independent colony-forming assays. Cell lines were grown in soft agar for 21 days (HMCB) or 14 days (G361) then stained and counted. Images from a representative experiment are shown. Mean values and standard errors were derived from at least 3 independent experiments. Statistically significant differences compared with shControl cells were determined using a Student’s 2-tailed t test (*P < 0.05; **P < 0.01; ***P < 0.001). (D) Tumor growth in HMCB xenograft mouse model. HMCB shControl and shMERTK1 cells were injected ectopically in mice. Tumor volume was monitored on the indicated days via caliper measurement, and the average tumor volume with standard deviation is plotted against the days after xenograft (*P < 0.05, n = 6).