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Differential AKT dependency displayed by mouse models of BRAFV600E-initiated melanoma
Victoria Marsh Durban, … , Wayne Phillips, Martin McMahon
Victoria Marsh Durban, … , Wayne Phillips, Martin McMahon
Published November 8, 2013
Citation Information: J Clin Invest. 2013;123(12):5104-5118. https://doi.org/10.1172/JCI69619.
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Research Article Oncology

Differential AKT dependency displayed by mouse models of BRAFV600E-initiated melanoma

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Abstract

Malignant melanoma is frequently driven by mutational activation of v-raf murine sarcoma viral oncogene homolog B1 (BRAF) accompanied by silencing of the phosphatase and tensin homology (PTEN) tumor suppressor. Despite the implied importance of PI3K signaling in PTENNull melanomas, mutational activation of the gene encoding the catalytic subunit of PI3Kα (PIK3CA), is rarely detected. Since PTEN has both PI3-lipid phosphatase–dependent and –independent tumor suppressor activities, we investigated the contribution of PI3K signaling to BRAFV600E-induced melanomagenesis using mouse models, cultured melanoma cells, and PI3K pathway–targeted inhibitors. These experiments revealed that mutationally activated PIK3CAH1047R cooperates with BRAFV600E for melanomagenesis in mice. Moreover, pharmacological inhibition of PI3Ks prevented growth of BRAFV600E/PTENNull melanomas in vivo and in tissue culture. Combined inhibition of BRAFV600E and PI3K had more potent effects on the regression of established BRAFV600E/PTENNull melanomas and cultured melanoma cells than individual blockade of either pathway. Surprisingly, growth of BRAFV600E/PIK3CAH1047R melanomas was dependent on the protein kinase AKT; however, AKT inhibition had no effect on growth of BRAFV600E/PTENNull melanomas. These data indicate that PTEN silencing contributes a PI3K-dependent, but AKT-independent, function in melanomagenesis. Our findings enhance our knowledge of how BRAFV600E and PI3K signaling cooperate in melanomagenesis and provide preclinical validation for combined pathway–targeted inhibition of PI3K and BRAFV600E in the therapeutic management of BRAFV600E/PTENNull melanomas.

Authors

Victoria Marsh Durban, Marian M. Deuker, Marcus W. Bosenberg, Wayne Phillips, Martin McMahon

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Figure 1

Expression of PIK3CAH1047R cooperates with BRAFV600E in melanomagenesis.

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Expression of PIK3CAH1047R cooperates with BRAFV600E in melanomagenesis....
(A) Recombination was initiated in Tyr::CreER mice carrying BRafCA (BRAFV600E) or a combination of BRafCA with either Ptenlox4-5/lox4-5 (BRAFV600E/PTENNull) or Pik3calat-1047R/+ (BRAFV600E/PIK3CAH1047R) by topical 4-HT application, and melanoma initiation was assessed for up to 90 days. (B) Growth rate of BRAFV600E/PTENNull (n = 10) (dashed line) versus BRAFV600E/PIK3CAH1047R (n = 10) (solid line) melanomas. Average tumor sizes (mm3 ± SEM) were measured starting at 30 days following 4-HT induction. *P < 0.01, Student’s 1-tailed t test. (C) Kaplan-Meier survival analysis of mice bearing BRAFV600E/PTENNull (n = 37, dashed line) or BRAFV600E/PIK3CAH1047R melanomas (n = 39, solid line). Median survival times were 203 and 82 days after 4HT administration respectively (Mantel-Cox log-rank test, P < 0.01). (D) H&E staining revealed melanomas arising in BRAFV600E/PTENNull or BRAFV600E/PIK3CAH1047R mice to be histopathologically heterogeneous (top panels). Tumors of both genotypes comprised pigmented and nonpigmented cells (middle panels), and invasion through the musculature was frequently identified (bottom panels). (E) Tumor lysates of BRAFV600E/PTENNull or BRAFV600E/PIK3CAH1047R melanomas were probed with backbone- or phospho-specific antisera against the various indicated proteins. β-actin was used as a loading control. (F) Regional draining lymph nodes of mice bearing BRAFV600E/PTENNull or BRAFV600E/PIK3CAH1047R melanomas were found to be pigmented. H&E staining (middle panel) revealed the majority of pigmentation present was associated with melanophages; however, the structure of the lymph node was disrupted, suggesting the presence of tumor cells. Anti-S100 immunostaining (bottom panel) revealed clusters of positive cells. Scale bars: 100 μm. Scale bars rule (on whole-mount images) shows measurement in mm.
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