Mutations in the BRAF serine/threonine kinase gene are frequently found in cutaneous melanomas. Activation of hypoxia inducible factor-1α (HIF-1α) in response to both hypoxic stress and oncogenic signals has important implications in cancer development and progression. Here, we report that mutant BRAFV600E increases HIF-1α expression in melanoma cells. Our microarray profiling data in 35 melanoma and melanocyte cell lines showed that HIF-1α gene expression was significantly increased in melanomas harboring BRAFV600E mutation. Stable suppression of mutant BRAFV600E or both wild-type and mutant BRAFV600E by RNA interference in melanoma cells resulted in significantly decreased HIF-1α expression. Knockdown of mutant BRAFV600E induced significant reduction of cell survival and proliferation under hypoxic conditions, whereas knockdown of both wild-type and mutant BRAFV600E resulted in further reduction. The effects of BRAF knockdown can be rescued by reintroducing BRAFV600E into tumor cells. Transfection of BRAFV600E into melanoma cells with wild-type BRAF induced significantly more hypoxic tolerance. Knockdown of HIF-1α in melanoma cells resulted in decreased cell survival under hypoxic conditions. Pharmacologic inhibition of BRAF by BAY 43-9006 also resulted in decreased HIF-1α expression. Although HIF-1α translational rate was not changed, the protein was less stable in BRAF knockdown cells. In additional, von Hippel-Lindau protein expression was significantly increased in BRAF knockdown cells. Our data show for the first time that BRAFV600E mutation increases HIF-1α expression and melanoma cell survival under hypoxic conditions and suggest that effects of the oncogenic V600E BRAF mutation may be partially mediated through the HIF-1α pathway. [Cancer Res 2007;67(7):3177–84]
Genetic alterations are common in both familial and sporadic melanomas; perturbations in the mitogen-activated protein kinase (MAPK) signaling pathway have been implicated in the development of 60% to 90% of melanomas, with activating BRAF mutations playing the most prominent role in this process (1). Activation of the BRAF/MAPK kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway has been shown to increase tumor cell proliferation, survival, invasion, and tumor angiogenesis (2). Somatic mutations in BRAF have been reported in 70% of melanomas (3, 4). However, these mutations are also present in banal or dysplastic nevi and may thereby implicate BRAF activation as an initiating event in tumorigenesis (5). Nevertheless, how BRAF mutations contribute to melanocyte transformation and melanoma progression is largely unknown.
The RAF family is composed of three members [ARAF, BRAF, and CRAF (RAF1)] that exhibit a high degree of homology within three conserved regions. RAF family members are intermediate molecules in the MAPK pathway, which is a cellular signal transduction pathway that conveys extracellular signals from the cell membrane to nucleus through a series of phosphorylation events (6, 7) and leads to the expression of genes associated with cell growth and survival (8). Suppression of BRAF expression in cultured melanoma cells inhibits the MAPK cascade and leads to growth arrest and promotes apoptosis in vitro and inhibits tumor development in animals (9, 10).
The rapid proliferation of cancer cells often outgrow blood supply in solid tumors, resulting in a reduction in oxygen tension to drop below physiologic levels. Hypoxic areas are common features of rapidly growing malignant tumors and their metastasis. Tissue hypoxia due to inadequate blood supply typically occurs very early during tumor development beginning at a tumor diameter of a few millimeters (11). Hypoxia-inducible factor-1 (HIF-1) is a master mediator of cellular responses to hypoxia, which is composed of two subunits (HIF-1α and HIF-1β). HIF-1α is an oxygen-regulated subunit, and its expression is stabilized under hypoxic conditions (12). Under normoxic conditions, hydroxylation of key proline residues within the regulatory oxygen-dependent domain of the HIF-1α subunit facilitates von Hippel-Lindau protein (VHL) binding, which in turn allows ubiquitination and subsequent proteasome-targeted degradation (13). Under limiting O2 conditions, proline hydroxylation is inhibited, thereby stabilizing HIF-1α subunits, which can then translocate into the nucleus and bind to constitutively stabilized HIF-1β subunits, forming the active HIF-1 complex (14). HIF-1 activates a multitude of O2-responsive genes, such as vascular endothelial growth factor (VEGF) and erythropoietin, which are involved in various normal cell functions such as survival, apoptosis, glucose metabolism, and angiogenesis (15, 16).
HIF-1α expression is regulated by the MAPK/ERK and Akt/phosphatidylinositol 3-kinase (PI3K) pathways (11, 17). It has been shown that Ras regulates HIF-1α expression via the RAF/MEK/ERK pathway, and that both phorbol ester and epidermal growth factor induce HIF-1α via the same pathway (18, 19). BRAF is essential for ERK activation and for vascular development in the placenta (20). BRAF knockout mice suffer from an impaired development of the vascular system (21), which is similar to mice lacking the HIF-1β (22). In this study, we show for the first time that mutant V600E BRAF increases HIF-1α expression and melanoma cell survival under hypoxic conditions.
Materials and Methods
Cell culture. The melanoma cell lines used in this study were derived from human melanomas representing all stages of melanoma progression [3 radial growth phase (RGP), 10 vertical growth phase (VGP), and 17 metastatic melanomas]. Melanoma cells were cultured under 37°C humidified atmosphere containing 5% CO2 and grown in MCDB153/L15 medium (4:1, v/v) supplemented with 2% fetal bovine serum, insulin (5 units/mL), CaCl2 (2 mmol/L), 100 units/mL penicillin, and 100 mg/mL streptomycin. Five melanocyte cell lines were generated from neonatal foreskin specimens, and they were maintained in melanocyte medium.3
http://www.wistar.org/herlyn/resource_culture.htm for details.
DNA constructs and transfection. BRAF shRNAs in pSUPER.retro vectors (Oligoengine, Seattle, WA) were kindly provided to us by Dr. David Tuveson at University of Pennsylvania. The Com-4 construct (designated as BRAFsiRNA) targets sequences common to both wild-type and BRAFV600E mutant alleles, and Mu-A construct (designated as BRAFmutsiRNA) targets only mutated alleles, which can essentially completely abolished BRAFV600E expression while keeping wild-type BRAF expression intact. The specificity of these constructs has been extensively studied by a previous study (9).
The myc-tagged BRAFV600E cDNA in the pEFP expression vector was kindly provided by Dr. Jeffrey Knauf (Memorial Sloan-Kettering Cancer Center, New York). For transient expression of BRAFV600E protein, melanoma cells (1 × 105 per well in six-well plate) were seeded and incubated overnight. The cells were transfected with 200 to 2,000 ng of plasmids using LipofectAMINE (Invitrogen, Carlsbad, CA) as described previously (15).
Expression profiling. Total RNA was extracted from cells grown to 70% to 80% confluence using the RNeasy Mini kit (Qiagen, Inc., Valencia, CA) according to the manufacturer's protocol. cDNA was synthesized from 10 μg total RNA using the Superscript Choice System for cDNA Synthesis (Invitrogen). In vitro transcription reactions to synthesize biotinylated cRNA were done with the Bioarray High Yield RNA Transcript Labeling Kit (T7; Enzo Life Sciences, Farmingdale, NY). Labeled cRNA was fragmented and hybridized to the Human Genome U133A Array (Affymetrix, Santa Clara, CA) that contains 14,500 genes. Hybridized gene chips were scanned, and data were normalized using Microarray Suite Software (v5.0). Data sets were analyzed using Genesifter software (VizX Labs, Seattle, WA).
RNA interference. A VGP melanoma cell line with BRAFV600E mutation (WM793) was used for transfection. BRAFsiRNA, BRAFmutsiRNA, or vector control (pSUPER) plasmids were added to 1 × 106 cells, which were previously washed in 1× PBS and resuspended in 100 μL of Nucleofector solution V (Amaxa, Inc., Gaithersburg, MD). For nucleofection, we used the built-in T-20 program in the nucleofector device (Amaxa) and immediately transfected into prewarmed 2% MCDB tumor medium. Transfected cells were seeded into six-well plates and incubated for 24 h under standard conditions. Twenty-four hours after transfection, cells were selected in media containing 5 μg/mL Puromycin for 4 to 5 days, and well-formed colonies were selected and expanded for biochemical assays.
Transient transfection with HIF-1α small interfering RNA (siRNA) was done using the HiperFect Reagent (Qiagen) according to the manufacturer's specifications. Briefly, all of these experiments were done in six-well plates using melanoma cell lines Sbcl2 (with wild-type BRAF) and WM793 (with BRAFV600E). Tumor cells were incubated with HIF-1α siRNA (5 nmol/L, Hs_HIF-1A_5_HP validated siRNA, NM_001530; Qiagen), or scrambled control siRNA (5 nmol/L; Qiagen), mixed with HiPerFect transfection reagent (12 μL) in a 100 μL of serum-free MCDB medium. Twenty-four hours after transfection, the medium was changed, and cells were harvested 48 h after transfection.
Western blot. Adherent monolayer cells were washed with ice-cold PBS and lysed in-place for the analysis of total proteins. These melanoma cells were washed with ice-cold 1× PBS and lysed in Tissue Protein Extraction Reagent (T-PER, Pierce, Rockford, IL) with (1×) Protease Inhibitor cocktail (Sigma, St. Louis, MO) and 1 mmol/L phenylmethylsulfonyl fluoride (Sigma), subsequently homogenized on ice, and centrifuged at 10,000 rpm for 5 min. Whole-cell lysates were normalized for protein concentration. Fifty micrograms of proteins were separated in Nu PAGE 4-12% Bis-Tris Gel (Invitrogen) and transferred to a polyvinylidene difluoride (PVDF) membrane (Hybond-P, Amersham Biosciences, Little Chalfont, England). Membranes were blocked and incubated with primary antibodies (HIF-1α, BRAF, VHL, and VEGF; Santa Cruz Biotechnology, Santa Cruz, CA; 1:1,000 to 1:10,000) in 5% milk TBST buffer [150 mmol/L Tris-HCl (pH 8), 150 mmol/L NaCl, 5% nonfat dry milk, and 0.1% Tween 20]. The membranes were washed thrice with wash buffer for 5 min and incubated with horseradish peroxidase–conjugated secondary antibodies and washed again before being processed with chemiluminescence (ECL Western Blotting Detection System, Amersham Biosciences). Bands were scanned and quantified using a ChemiDoc XRS system (Bio-Rad Laboratories, Hercules, CA).
MG132 and cycloheximide treatment. MG132 is a proteasomal inhibitor, and it is known to block the degradation of HIF-1α. To investigate the translational rate of HIF-1α, we seeded WM793 cells (5 × 105) that were stably transfected with pSUPER or BRAFsiRNA plasmids and incubated at 37°C overnight, aspirated the medium, and fed serum-free medium with MG132 (10 μmol/L) in each plate. The plates were kept in 37°C for up to 6 h, then the cells were lysed in Laemmli protein lysis buffer (200 μL) and boiled immediately. CoCl2 is a hypoxia mimic and is known to stabilize HIF-1α in the cells. For CoCl2 treatment, melanoma cells were subjected to 100 μmol/L CoCl2 for 24 h in a CO2 incubator. For cycloheximide treatment, the above mentioned melanoma cells were subjected to 10 μmol/L cycloheximide. The cells were harvested at 0, 5, and 60 min after cycloheximide treatment. Ten microliters of protein samples from each plates were separated in Nu PAGE 4% to 12% Bis-Tris Gel and transferred to a PVDF membrane. Membranes were blocked and incubated with primary antibodies (HIF-1α; 1:200; Santa Cruz Biotechnology) in 5% milk TBST buffer [150 mmol/L Tris-HCl (pH 8), 150 mmol/L NaCl, 5% nonfat dry milk, and 0.1% Tween 20]. The membranes were washed thrice with wash buffer and incubated with horseradish peroxidase–conjugated secondary antibodies and washed again before being processed with chemiluminescence. Bands were scanned and quantified using a ChemiDoc XRS system (Bio-Rad Laboratories).
Real-time PCR. Total RNA was prepared using RNeasy kit (Qiagen), and cDNAs were prepared by using SuperScript First-Strand Synthesis system (Invitrogen) for reverse transcription-PCR according to the manufacturer's instructions. Real-time PCR was done same as described previously (23) with specific primers for HIF-1α (forward, 5′-CATAAAGTCTGCAACATGGAAGGT-3′; reverse, 5′-ATTTGATGGGTGAGGAATGGGTT-3′) and β-actin (forward, 5′-CTACCTCATGAAGATCCTCACCGA-3′; reverse, 5′-ACGTAGCACAGCTTCTCCTTAATG-3′). cDNA corresponding to 1 μg RNA was added to the iQ SYBER green supermix (Bio-Rad Laboratories). PCRs were carried out in a real-time PCR cycler (iCycler; Bio-Rad Laboratories) according to the manufacturer's instructions. The thermal profile was 95°C for 30 s and 56°C for 30 s. Melting curve analysis was done for each PCR reaction to confirm the specificity of amplification. At the end of each phase, fluorescence was measured and used for quantitative purposes. The HIF-1α expression data were normalized to β-actin, and relative transcript level was calculated.
3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. The cell viability assay was done using CellTiter 96 Non-Radioactive Cell Proliferation Assay kit (Promega Corp., Madison, WI) according to the manufacturer's instructions. Briefly, melanoma cells were washed with PBS and suspended in a final concentration of 1 × 105/mL in assay medium, and 50 μL of cell suspension was subsequently dispensed into 96-well plates. The plates were incubated at 37°C for 24 h in a humidified CO2 incubator. The medium was aspirated from the wells; 100 μL of serum-free medium was added into each well, and cells were incubated for 24 h at 37°C in a hypoxic chamber (1% O2). For color development, 15 μL of dye solution was added to each well, and the plates were incubated at 37°C for 4 h followed by addition of 100 μL of solubilization/stop solution to each well. Absorbance was recorded at 570 nm using a 96-well plate reader. Experiments were carried out in triplicate.
WST-1 cell proliferation assay. Cell proliferation was measured using a WST-1 Cell Proliferation Assay (Roche Diagnostics, Indianapolis, IN). Briefly, melanoma cells were washed with PBS and suspended in a final concentration of 1 × 105/mL in assay medium, and 50 μL of cell suspension was dispensed into 96-well plates. The plates were incubated at 37°C for 24 h in a humidified CO2 incubator. The medium was aspirated from the wells; 100 μL of serum-free medium was added into each well, and cells were incubated for 24 h at 37°C in a hypoxic chamber (1% O2). After the treatment period, WST-1 reagent was added to cell culture medium (10 μL in 100 μL media), mixed gently, and incubated at 37°C for 20 min. Plates were shaken vigorously on an orbital shaker for 1 min, and absorbance was measured at 450 nm using a 96-well plate reader. Experiments were carried out in triplicate. Trypan blue dye exclusion assay was done as we described previously (23).
Statistical analysis. One-way ANOVA, followed by Tukey's multiple comparison test, was used to analyze the expression data for VEGF and HIF-1α in 5 melanocyte cell lines and 30 melanoma cell lines. Student's t test or one-way ANOVA was used to analyze other gene expression, cell viability, and proliferation data. Statistical significance was determined if two-sided Ptest < 0.05.
HIF-1α gene expression is increased in melanomas with mutant V600E BRAF. We did a microarray profiling study on 30 melanoma cell lines derived from various stages of human melanoma progression and 5 foreskin-derived melanocyte cell lines. One of three RGP, 7 of 10 VGP, and 12 of 15 metastatic melanomas carry the BRAFV600E mutation; BRAF mutation status is unknown in two metastatic melanoma cell lines. We found that HIF-1α expression was significantly higher in melanomas harboring the BRAFV600E mutation in comparison with melanomas without the mutation, or melanocytes (P < 0.01; Fig. 1A). The up-regulation of HIF-1α in the melanoma cells was not associated with a global up-regulation of genes. Many genes remained stable, such as protein phosphatase 2 and heat shock protein 90 kDa beta etc. When we compared HIF-1α gene expression during melanoma progression regardless of V600E mutation status, we found that there was a significant increase of HIF-1α gene expression in melanoma cells compared with melanocytes (P < 0.05; Fig. 1B), suggesting that HIF-1α expression may be regulated by other factors as well. Because a majority of melanoma cell lines carry the BRAFV600E mutation in this study, it is not reliable to perform a statistical analysis on the effect of BRAFV600E mutation on HIF-1α expression after adjusting for tumor stage, despite cell lines with V600E BRAF mutation seemed to have higher HIF-1α expression than cell lines with wild-type BRAF within the same stage (data not shown). Additional studies with more BRAF wild-type cell lines are necessary to show the effects of BRAF mutations on HIF-1α expression after adjusting for tumor stage in the future. Because VEGF expression is directly regulated by HIF-1α, we analyzed VEGF expression during melanoma progression and showed that VEGF expression pattern was similar to HIF-1α (Fig. 1C).
BRAF knockdown resulted in decreased HIF-1α expression. To further investigate the effect of V600E BRAF mutation on HIF-1α expression, we used two well-characterized RNA interference constructs to stably knockdown mutant V600E BRAF (BRAFmutsiRNA) or both wild-type and mutant V600E BRAF (BRAFsiRNA) in WM793 melanoma cells. Similar to the previous results in 1205Lu melanoma cells (9), BRAFsiRNA and BRAFmutsiRNA stable transfection significantly reduced BRAF expression in transfected melanoma cells (Fig. 2A). Although BRAFsiRNA targets both wild-type and V600E mutant BRAF, the level of BRAF expression in BRAFsiRNA stably transfected cells was similar to that in BRAFmutsiRNA-transfected cells, suggesting different efficacy of the siRNA systems, or that it is still possible that siRNA for mutant BRAF may also down-regulate to some extent the expression of wild-type BRAF. BRAF knockdown cells seemed to have more dendritic processes than wild-type cells (Fig. 2D).
We examined HIF-1α expression in multiple clones of BRAF knockdown cells, and all the clones showed decreased HIF-1α expression (Fig. 2A). The controls were cells transfected with the pSUPER vector. Knockdown of both wild-type and mutated V600E BRAF decreased HIF-1α gene expression by 15%, whereas knockdown of mutated BRAF resulted in 33% decrease of HIF-1α gene expression (P < 0.05), compared with vector-transfected control cells (Fig. 2B). Similarly, VEGF expression was also significantly decreased in BRAF knockdown cells (Fig. 2C). However, BRAF knockdown did not result in a general down-regulation of protein expression. There was no change of erythropoietin receptor expression in these cells (data not shown). These data further suggest that HIF-1α expression is regulated by BRAF in melanoma cells.
BRAF is essential for melanoma cell survival under hypoxic conditions. To show biological function of decreased HIF-1α expression in BRAF knockdown cells, we cultured these cells in moderate hypoxic conditions for 24 h. We assessed cell survival and proliferation using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), trypan blue dye exclusion, and WST-1 cell proliferation assay. There was a 43% decrease of survival in the cells with mutant V600E BRAF knockdown in comparison with control cells under hypoxic conditions (P < 0.01; Fig. 3A). There was an additional 50% decrease of cell survival when both wild-type and mutant V600E BRAF were knocked down in comparison with cells with only mutant V600E BRAF knockdown using MTT assay (P < 0.05; Fig. 3A). Trypan blue dye exclusion assay showed same results as MTT assay (data not shown). Cell proliferation showed similar changes as cell survival after BRAF knockdown; however, the additional decrease of cell proliferation in cells with both wild-type and mutant BRAF knockdown was not as prominent comparing to cells with only mutant BRAF knockdown, despite the decease was still statistically significant (Fig. 3A). Although the efficiency of the different siRNAs may account for the observations, it is likely that both wild-type and mutant V600E BRAF may contribute to melanoma cell adaptation to hypoxic conditions.
To eliminate the off-target effects of siRNA experiment, we did a rescue experiment using myc-tagged BRAFV600E plasmids. We transfected the plasmids into both BRAF stable knockdown cell lines. Forty-eight hours after transfection, BRAF protein expression showed a dose-dependent increase in these cells (Fig. 3B). These BRAF-rescued cells were subjected to hypoxic condition for 24 h, and we measured the cell proliferation using WST-1 assay. Our data clearly showed that the addition of BRAFV600E in BRAF knockdown cells rescued their hypoxic intolerance (Fig. 3C). The rescuing effects of BRAFV600E was less prominent in cells with both wild-type and mutant V600E BRAF knockdown, further suggesting that both wild-type and mutant V600E BRAF may contribute to melanoma cell adaptation to hypoxic conditions.
Overexpression of BRAFV600E increases melanoma cell survival under hypoxic condition. To confirm that BRAFV600E mutation plays a role in hypoxic adaptation for melanoma cells, we transiently transfected BRAF wild-type melanoma cells (Sbcl2 and WM3211) with myc-tagged BRAFV600E plasmid. After transfection, tumor cells showed significantly increased BRAF and HIF-1α mRNA and protein expression than the control cells (Fig. 4A and B). We then exposed these cells under hypoxic condition for 24 h and did cell survival and proliferation assays using MTT and WST-1 assays. There was a significant increase in cell survival in both Sbcl2 and WM3211 cells transfected with BRAFV600E compared with the control cells (P < 0.01, Fig. 4C), whereas cell proliferation was significantly increased only in Sbcl2 cells but not in WM3211 cells. These data support that V600E BRAF mutation increases cell survival under hypoxic condition.
Mutant V600E BRAF increases cell survival in hypoxic condition through HIF-1α. We next studied whether the effect of mutant V600E BRAF is mediated through HIF-1α. We transiently knocked down HIF-1α using commercially available HIF-1α siRNA in Sbcl2 and WM793 melanoma cells (Fig. 4D). HIF-1α knockdown decreased HIF-1α expression in these cell lines and significantly decreased cell survival and proliferation of these cells in hypoxic condition (Fig. 4E). These data suggest that HIF-1α mediates the effect of wild-type and mutant V600E BRAF in melanoma under hypoxic conditions.
BRAF inhibitor (BAY 43-9006) inhibits HIF-1α expression. We studied whether pharmacologic inhibition of BRAF would affect HIF-1α expression. BAY 43-9006 was engineered to inhibit the BRAF kinase; however, it is not a specific BRAF kinase inhibitor as it is known to inhibit other proteins. The ability of BAY 43-9006 to block activation of the MAPK pathway was confirmed by measuring ERK1/2 phosphorylation in Sbcl2 and WM793 cells. After cells were preincubated with BAY 43-9006 at different concentration (1, 2.5, 5, and 10 μmol/L) for 2 h, there was a dose-dependent inhibition of phosphorylated MEK1/2 and phosphorylated ERK, whereas there were no significant changes in the expression of total ERK and MEK in melanoma cells (Fig. 5A). The hypoxia mimic CoCl2 is known to stabilize HIF-1α in cells. BAY 43-9006 (10 μmol/L) significantly inhibited HIF-1α expression in melanoma cells in the presence of CoCl2 (Fig. 5B). BAY 43-9006 also inhibited HIF-1α mRNA level in melanoma cells (Fig. 5C), suggesting that BAY 43-9006 can inhibit HIF-1α at transcriptional level. To further address whether the observed effects of V600E BRAF mutation on HIF-1α expression is a result of ERK pathway activation, we treated WM793 cells with a MEK inhibitor UO126. Indeed, UO126 significantly decreased HIF-1α gene and protein expression (Fig. 5B and C). Our findings suggest that pharmacologic inhibition of BRAF may result in decreased HIF-1α expression, and the therapeutic effects of BAY 43-9006 may be mediated in part through the HIF-1α pathway.
BRAF knockdown increases HIF-1α degradation. To investigate the mechanism underlying the effects of BRAF on HIF-1α expression, we studied whether BRAF knockdown in melanoma cells decreases the translational rate of HIF-1α. MG 132 has been shown to inhibit the degradation of HIF-1α in vitro (24). BRAFsiRNA or control vector-transfected WM793 cells were treated with a proteosome inhibitor (10 μmol/L MG 132) for 1, 2, and 6 h. Lower dose of MG132 did not completely block HIF-1α degradation in these cells (data not shown). We collected cell lysates immediately after treatment and subjected the cell lysates to Western blot against HIF-1α. The result showed that HIF-1α protein accumulated at a similar rate in BRAF knockdown and control cells (Fig. 6A), suggesting that HIF-1α translational rate is not changed. To test whether BRAF stabilizes the HIF-1α transcripts, we measured HIF-1α protein stability in the presence of cycloheximide, which blocks de novo protein synthesis. As shown in Fig. 6B, in the absence of HIF-1α stabilizer CoCl2, HIF-1α was stably expressed at a low level in pSuper-transfected cells, whereas it was degraded in the BRAF knockdown cells. In the presence of CoCl2 and cycloheximide, HIF-1α level persisted in these cells at a similar level. These data suggest that BRAF knockdown may destabilize HIF-1α protein in melanoma cells.
Decreased expression of HIF-1α in BRAF knockdown cells is associated with up-regulation of VHL. Stability of HIF-1α protein depends upon its interaction with VHL. Under normoxic condition, HIF-1α protein is constantly turned over due to hydroxylation, recognition by VHL, and subsequent destruction by the proteasome. To investigate whether the decreased HIF-1α expression in BRAF knockdown cells is related to VHL expression, we did Western blot against VHL in BRAFsiRNA- or BRAFmutsiRNA-transfected melanoma cells and found that there was indeed an increase in VHL protein expression in these cells compared with controls (Fig. 6C), suggesting that the increased degradation of HIF-1α protein in BRAF knockdown cells may be due to increased expression of VHL protein.
BRAF mutation is the most common genetic alteration in melanoma and thyroid cancer, occurring in about 70% of melanoma and 45% of sporadic papillary thyroid cancers (25, 26). This mutation is mutually exclusive with other common genetic alterations in both cancers such as NRAS, suggesting its independent oncogenic role (27). BRAF mutation is therefore likely involved in the pathogenesis and progression of these cancers, and it may represent a novel therapeutic target (28). Mutant V600E is the most common activating BRAF mutation in melanoma (29). Despite recently shown clinical efficacy of BRAF inhibitors, such as BAY 43-9006 in phase I and II clinical trials, the role of BRAF mutations during melanoma initiation or progression is only started to be understood. In this study, we showed that BRAFV600E mutation in melanoma is associated with increased HIF-1α expression and cell survival under hypoxic conditions. Knockdown or pharmacologic inhibition of BRAF results in significant reduction of HIF-1α expression in tumor cells. Decreased HIF-1α expression in BRAF knockdown melanoma cells is likely resulted from both decreased transcription and increased degradation of HIF-1α protein.
HIF-1α is an essential mediator of O2 homeostasis, it is instrumental in the oxygen-dependent regulation of a variety of genes, such as erythropoietin, VEGF, nitric oxide synthase, heme oxygenase 1, and glucose transporters (30, 31). HIF-1α is also involved in tumor progression and metastasis, and its expression is correlated with early relapse and metastatic disease. Recently, it has been shown that HIF-1α induces genetic instability by transcriptionally down-regulating mismatch repair system, which is crucial for maintaining cellular genetic integrity (32), and HIF-1α–induced mutations may occur in the very early stages during tumor development. Because BRAF mutation occurs in precursor melanocytic lesions such as dysplastic nevi, mutant V600E BRAF induced HIF-1α expression may potentially involved in the early transformation of the precursor lesions.
HIF-1α activation has been shown to be regulated by the PI3K and MAPK pathways (33, 34). Our microarray results showed that melanomas with mutant V600E BRAF have significantly increased HIF-1α gene expression. Papillary thyroid cancers (PTC) are also known to harbor V600E BRAF mutation. It has been shown recently that VEGF, one of the direct targets of HIF-1α, is significantly up-regulated in V600E BRAF–positive PTCs, compared with V600E BRAF–negative PTCs, by immunohistochemistry (35). Therefore, it seems that the effect of V600E BRAF mutation on HIF-1α is not confined to melanoma cells. Overexpression of mutant V600E BRAF in melanoma cells with wild-type BRAF increased hypoxic tolerance of these cells. Suppression of wild-type and/or mutant V600E BRAF resulted in marked reduction of HIF-1α at both gene expression and protein level. Decreased HIF-1α expression rendered melanoma cells more susceptible to hypoxia-induced cell death. The BRAF knockdown is specific, and it can be rescued by reintroducing a BRAFV600E plasmid into BRAF stable knockdown cells. Our data suggest that mutant V600E BRAF promotes cell survival under hypoxic conditions through up-regulation of HIF-1α expression in melanoma cells.
HIF-1α expression is primarily regulated at the posttranslational level, and its degradation is regulated by O2-dependent prolyl hydroxylation, which targets the protein for ubiquitylation and degradation by the proteasome (36). The degradation depends on VHL tumor suppressor protein, which binds specifically to hydroxylated HIF-1α (37). We showed that BRAF knockdown does not change the translational rate of HIF-1α but increases degradation of this protein with an associated up-regulation of VHL, suggesting that mutant V600E BRAF may regulate HIF-1α posttranslational modification.
Recently, BAY 43-9006 has been used in several phase I and II clinical trials in patients with a variety of cancers, including melanoma, and showed promising clinical efficacy when combined with other chemotherapeutic agents (38). This compound was initially thought to specifically inhibit BRAF; it was recently shown that it also inhibits other molecules such as VEGF and platelet-derived growth factor receptors (39, 40). Similar to a prior report using 1205Lu melanoma cells (10), we observed that BAY 43-9006 inhibited ERK and MEK phosphorylation in WM793 melanoma cells. BAY 43-9006 also significantly reduced HIF-1α expression in melanoma cells, suggesting that some of the effects of this compound may be mediated through HIF-1α inhibition.
In conclusion, our results show for the first time that mutant V600E BRAF increases HIF-1α expression and melanoma cell survival under hypoxic conditions. These data suggest that the oncogenic effects of activating BRAF mutations are mediated in part through the HIF-1α pathway. Involvement of activating mutations of BRAF in hypoxic signaling might be critical for tumor initiation and growth because BRAF gene mutations occur early during tumor progression.
Grant support: Specialized Program of Research Excellence on Skin Cancer grant CA-093372.
We thank Dr. David Tuveson for providing BRAF siRNA constructs, Dr. Jeffrey Knauf for providing BRAFV600E plasmid construct, Dr. Qingdu Liu for setting up hypoxia chamber, and Dr. Frank Lee for suggestion and discussion.