Inhibiting Drivers of Non-mutational Drug Tolerance Is a Salvage Strategy for Targeted Melanoma Therapy

Abstract

Once melanomas have progressed with acquired resistance to mitogen-activated protein kinase (MAPK)-targeted therapy, mutational heterogeneity presents a major challenge. We therefore examined the therapy phase before acquired resistance had developed and discovered the melanoma survival oncogene MITF as a driver of an early non-mutational and reversible drug-tolerance state, which is induced by PAX3-mediated upregulation of MITF. A drug-repositioning screen identified the HIV1-protease inhibitor nelfinavir as potent suppressor of PAX3 and MITF expression. Nelfinavir profoundly sensitizes BRAF and NRAS mutant melanoma cells to MAPK-pathway inhibitors. Moreover, nelfinavir is effective in BRAF and NRAS mutant melanoma cells isolated from patients progressed on MAPK inhibitor (MAPKi) therapy and in BRAF/NRAS/PTEN mutant tumors. We demonstrate that inhibiting a driver of MAPKi-induced drug tolerance could improve current approaches of targeted melanoma therapy.

Graphical abstract

Figure 1

MITF Expression Is Upregulated in Response to Long-Term BRAF and MEK Inhibition (A) qRT-PCR for MITF expression (mean ± SD) in melanoma of patients undergoing treatment with vemurafenib (^∗) or a dabrafenib/trametinib combination. (B) qRT-PCR for MITF expression in Braf^V600E murine melanoma allografts from mice treated with vehicle or 25 mg/kg once daily (qd) PD184352 (MEKi) (each group: n = 5), and in A375 xenografts from mice treated with vehicle, 10 mg/kg qd selumetinib (MEKi) or 25 mg/kg qd PLX4720 (BRAFi) (each group: n = 3). Data show box plots indicating the upper/lower quartile and the median with whiskers from min to max values. (C) A375-GFP cells were isolated from xenografts grown in mice treated with vehicle (n = 4) or 100 mg/kg qd vemurafenib (BRAFi, n = 7) for 12 days. Mean relative tumor volume ± SEM and a phospho-ERK immunohistochemistry are shown; scale bars, 200 μm. (D) Western blot for MITF and ERK2 in A375-GFP cells after isolation from the indicated mice. (E) Dose-response curve (mean ± SEM) of A375-GFP cells treated with vemurafenib (BRAFi) for 48 hr. (F) A375-GFP cells isolated from a vemurafenib (BRAFi)-treated mouse were transfected with control or MITF-specific siRNAs and left in DMSO or cultured in the presence of vemurafenib (BRAFi) for 72 hr. Naive A375 cells were used as control. Relative cell numbers (mean ± SEM) and western blots are shown. (G) After treatment with vemurafenib (BRAFi) for 12 days as described in (C), three mice were kept off drug for another 12 days, before A375-GFP cells were isolated from xenografts (#611, #875). Mean relative tumor volume ± SEM and a western blot for MITF and ERK2 are shown. In parallel, A375-GFP cells isolated on treatment (#549, #026) were maintained with vemurafenib (BRAFi) or without drug (-BRAFi) for 12 days and analyzed for MITF. (H) Model describing a phase of non-mutational drug tolerance during which MAPK-pathway inhibition triggers adaptive signaling. For (B) and (F): ^∗p < 0.05; ^∗∗p < 0.01; ^∗∗∗p < 0.001. See also Figure S1 and Table S1.

Figure 2

A Drug Screen to Target PAX3 and MITF (A) Correlation of fold change in MITF and PAX3 expression (mean ± SD) in melanomas of patients (n = 9) undergoing treatment with vemurafenib or dabrafenib/trametinib combination. Shown is the mean (blue) with the 95% confidence interval (red dashed line). (B) qRT-PCR analysis for PAX3 and MITF expression (mean ± SEM, ^∗p < 0.05; ^∗∗p < 0.01; ^∗∗∗p < 0.001) in a panel of melanoma cell lines treated with DMSO, vemurafenib (B) or selumetinib (M) for 48 hr. (C) Western blot of WM266-4 cells untreated or treated with a control or PAX3-specific siRNA in the presence of DMSO or PD184352 (MEKi, 24 hr). The asterisk indicates an ERK-phosphorylated MITF form. (D) PAX3 expression analysis of the Barretina and Garnett datasets deposited in Oncomine. Data show scatter dot plots, indicating the mean ± SD. ^∗∗∗p < 0.001. (E) WM266-4 cells treated with an FDA-approved drug library were analyzed for PAX3 expression and viability. Values are presented as % relative cell number and Z score. Drugs with a survival score >90% and a Z score <−10 were nominated candidate drugs. (F) Drug screen for MITF expression as described in (E). Values are presented as % relative cell number and Z score. Drugs with a survival score >90% and a Z score <−5 were nominated candidate drugs. See also Figure S2.

Figure 3

Nelfinavir Suppresses PAX3 and MITF Expression in Melanoma Cells (A) Immunofluorescence analysis for PAX3 and MITF in WM266-4 cells left untreated or treated with 10 μM nelfinavir for 24 hr; scale bars, 50 μm. (B) Western blot of the indicated cell lines treated with 10 μM nelfinavir for 24 hr for PAX3, MITF, and ERK2. (C) Dose-response curve (mean ± SEM) for A375 or A375-T cells treated with 7 μM nelfinavir for 24 hr followed by 48 hr selumetinib (MEKi) treatment. (D) Melanoma cells ectopically expressing GFP or MITF were treated with 10 μM nelfinavir and selumetinib (MEKi) alone or in combination for 72 hr before cell number analysis (mean ± SEM). A MITF, pERK, and ERK2 western blot is shown. (E) Colony survival analysis after 3 weeks of nelfinavir/selumetinib (MEKi) treatment using the indicated cell lines transfected with an empty vector or a PAX3- or MITF-expressing vector. Data show box plots indicating the upper/lower quartile and the median with whiskers from min to max values. (F) qRT-PCR analysis for MITF expression (mean ± SEM) in melanoma cell lines treated with DMSO or with 10 μM nelfinavir. (G) qRT-PCR analysis for PAX3 expression (mean ± SEM) in the samples used in (F). For all panels: ^∗p < 0.05; ^∗∗p < 0.01; ^∗∗∗p < 0.001. See also Figure S3.

Figure 4

Nelfinavir Suppresses PAX3 through a SMAD2/SMAD4/SKI Complex (A) Immunofluorescence analysis for phospho-SMAD2 in melanoma cells treated with nelfinavir; scale bars, 10 μm. (B) Western blot for MITF, PAX3, SMAD2, pSMAD2, and ERK2 in melanoma cells treated with nelfinavir. (C) SKI immunoprecipitates from melanoma cells were analyzed for the presence of SKI, SMAD2, and pSMAD2. (D) SKI immunoprecipitates from untreated or nelfinavir (10 μM)-treated melanoma cells were analyzed for the presence of SMAD2 and SMAD4. (E) Chromatin immunoprecipitation analysis from A375 cells treated with selumetinib (MEKi) or nelfinavir (10 μM) for 24 hr alone or in combination using SKI antibodies or non-specific antibodies. The region of the PAX3 promoter spanning the SMAD binding site (−135/−98) was amplified. Relative binding in DMSO was set 1; shown are mean values ± SEM A region in the PAX3 intron2 was used as control. (F) Melanoma cells transfected with a vector control or an SKI or SMAD2 expression plasmid were analyzed for PAX3 by western blotting. (G) qRT-PCR and western blot for PAX3 in cells treated with control or SKI-specific siRNAs (using an SMART-pool [SKI-p] of four siRNAs). (H) Melanoma cells transfected with control or different SMAD4-specific siRNAs were left untreated or treated with nelfinavir for 24 hr and analyzed for indicated proteins by western blotting. All error bars are ± SEM from the mean. ^∗∗p < 0.01; ^∗∗∗p < 0.00. See also Figure S4.

Figure 5

MEK Suppresses PAX3 through SKI (A) Western blot of WM266-4 cells treated for 24 hr with DMSO, PD184352 (MEKi), or vemurafenib (BRAFi) for SKI, pERK, and ERK2. (B) qRT-PCR for SKI expression (mean ± SEM) in the indicated melanoma cell lines treated with vemurafenib (BRAFi) or selumetinib (MEKi) for 48 hr. (C) Western blot of WM266-4 cells transfected with a control or SKI expression plasmid for SKI, PAX3, pERK, and ERK2. Cells were treated for 24 hr with DMSO or PD184352 (MEKi). (D) qRT-PCR analysis for PAX3 (mean ± SD) in WM266-4 treated as in (C). (E) qRT-PCR analysis for SKI, PAX3, and MITF expression (mean ± SEM) in A375 melanoma xenografts from mice treated with selumetinib (MEKi) for 4 weeks. (F) qRT-PCR analysis of SKI, PAX3, and MITF (mean ± SD) in patients on treatment (2 weeks) with vemurafenib (^∗) or dabrafenib/trametinib inhibitor combination. (G) Analysis of publicly available gene expression datasets GEO: GSE50509 (21 patients) and GEO: GSE61992 (9 patients) as well as our dataset (11 patients) for fold changes in PAX3 and MITF expression. In total 41 pre-treatment, 14 on treatment, and 43 progressed samples were analyzed; % changes are indicated. For all graphs: ^∗p < 0.05; ^∗∗p < 0.01; ^∗∗∗p < 0.001. See also Figure S5.

Figure 6

Nelfinavir Sensitizes BRAF Mutant Melanoma to MAPK Inhibition (A) Western blot of indicated cell lines incubated with nelfinavir (A375 7 μM, others 10 μM) 24 hr prior to a 48-hr treatment with DMSO or selumetinib (MEKi) for the indicated proteins. The asterisk indicates an ERK-phosphorylated form of MITF. (B) Indicated cells lines were treated with 10 μM nelfinavir 24 hr prior to treatment with DMSO, selumetinib (MEKi), or vemurafenib (BRAFi). Forty-eight hours later cells were quantified. (C) A375-GFP cells isolated from mice treated as shown in Figure 1C were cultured in the presence of vemurafenib (BRAFi) or selumetinib (MEKi) alone or in combination with nelfinavir for 3 weeks before quantification. Data show box plots indicating the upper/lower quartile and the median with whiskers from min to max values. (D) GFP-expressing A375 cells (false colored in red) were injected into zebrafish larvae; the larvae were treated with DMSO, PD184352 (MEKi), or nelfinavir alone or in combination. Three days after drug addiction the xenografts were imaged (scale bars, 100 μm) and the volume was quantified using Volocity software. Data show scatter dot plots, indicating the mean ± SD ^∗p < 0.05. (E) MITF immunofluorescence analysis of A375 tumors from mice treated with vehicle, nelfinavir (25 mg/kg qd) or PLX4720 (BRAFi, 25 mg/kg qd) alone or in combination for 21 days; scale bars, 200 μm. (F) qRT-PCR for PAX3 and MITF expression in the individual tumors; mean expression ± SEM relative to vehicle control, ^∗∗p < 0.01; ^∗∗∗p < 0.001. (G) Mean tumor volumes ± SEM (n = 8) and a phospho-ERK IHC for a vehicle tumor and PLX4720 (BRAFi; 25 mg/kg)-treated tumor; scale bars, 200 μm. ^∗∗p < 0.01; ^∗∗∗p < 0.001. See also Figure S6.

Figure 7

Nelfinavir Sensitizes NRAS Mutant Melanoma to MEK Inhibition (A) Summary of survival of indicated cell lines treated with MITF siRNA nelfinavir (10 μM for 24 hr) or DMSO, followed by 48-hr incubation with selumetinib (MEKi). Cells were quantified using crystal violet. (B) Western blot of NRAS mutant melanoma cells treated with DMSO or 10 μM nelfinavir for 24 hr for PAX3, MITF pERK, and ERK2. MEL-JUSO samples for PAX3, MITF, and pERK detection corresponding to the loading control ERK2 were run on a separate gel. (C) M130219 and M130429 cells treated with vemurafenib (BRAFi) or selumetinib (MEKi) or RAF265 for 48 hr were analyzed by western blotting for pERK and ERK2. (D) Dose-response curves (mean ± SEM) for M130219 and M130429 cells treated with MEKi (selumetinib) in the presence or absence of 5 μM nelfinavir for 72 hr. See also Table S2.

Figure 8

Nelfinavir Overcomes NRAS-Driven Acquired Resistance (A) Dose-response curve (mean ± SEM) for M121224 cells treated for 48 hr with vemurafenib (BRAFi) or selumetinib (MEKi) in the presence or absence of 5 μM nelfinavir for 72 hr. DMSO-treated cells were set 100%. Western blot for pERK and ERK2 of M121224 cells treated as indicated above. (B) Western blot of M11224 cells incubated with nelfinavir (10 μM) alone or 48 hr in combination with selumetinib (MEKi) for the indicated proteins. (C) M249 and M249-R4 cells treated with vemurafenib (BRAFi) or selumetinib (MEKi) for 48 hr alone or in the presence of nelfinavir (10 μM) were analyzed by western blotting for pERK and ERK2 (D) qRT-PCR analysis for PAX3 and MITF expression (mean ± SEM) in M249 and M249-R4 cells. (E) qRT-PCR analysis for PAX3 and MITF expression (mean ± SEM) in M249-R4 cells treated with DMSO or trametinib (MEKi) for 24 hr. (F) Western blot for PAX3, MITF, cleaved caspase3, pERK, and ERK2 of M249-R4 cells treated with selumetinib (MEKi) or nelfinavir (10 μM) for 48 hr alone or in combination. (G) Dose-response curves (mean ± SEM) for M249 and M249-R4 cells treated with vemurafenib (BRAFi) or selumetinib (MEKi) in the presence or absence of 5 μM nelfinavir for 72 hr. DMSO-treated cells were set 100%. (H) Nude mice bearing tumors from M249-R4 cells were treated with vehicle, nelfinavir (25 mg/kg qd) or selumetinib (25 mg/kg qd) alone or in combination for 33 consecutive days. The results show mean tumor volumes ± SEM (n = 6). (I) qRT-PCR for PAX3 and MITF expression in the individual tumors. Data show box plots indicating the upper/lower quartile and the median with whiskers from min to max values. For all graphs: ^∗p < 0.05; ^∗∗p < 0.01; ^∗∗∗p < 0.001. See also Figure S7.