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Correspondence: Gopal C. Kundu, Laboratory of Tumor Biology, Angiogenesis and Nanomedicine Research, National Centre for Cell Science, Pune 411007, India.
Functional characterization and understanding of the intricate signaling mechanisms in stem-like cells is crucial for the development of effective therapies in melanoma. We have studied whether melanoma cells are phenotypically distinct and hierarchically organized according to their tumorigenic nature. We report that melanoma-specific CD133+ cancer stem cells exhibit increased tumor-initiating potential, tumor-endothelial cell interaction, and lung metastasis. These cells are able to transdifferentiate into an endothelial-like phenotype when cultured under endothelial differentiation-promoting conditions. Mechanistically, Notch1 upregulates mitogen-activated protein kinase activation through CD133, which ultimately controls vascular endothelial growth factor and matrix metalloproteinase expression in CD133+ stem cells leading to melanoma growth, angiogenesis, and lung metastasis. Blockade or genetic ablation of Notch1 and mitogen-activated protein kinase pathways abolishes melanoma cell migration and angiogenesis. Chromatin immunoprecipitation and reporter assays revealed that Notch1 intracellular domain regulates CD133 expression at the transcriptional level. Andrographolide inhibits Notch1 intracellular domain expression, Notch1 intracellular domain-dependent CD133-mediated mitogen-activated protein kinase and activator protein-1 activation, and epithelial to mesenchymal-specific gene expression, ultimately attenuating melanoma growth and lung metastasis. Human malignant melanoma specimen analyses revealed a strong correlation between Notch1 intracellular domain, CD133, and p-p38 mitogen-activated protein kinase expression and malignant melanoma progression. Thus, targeting Notch1 and its regulated signaling network may have potential therapeutic implications for the management of cancer stem cell-mediated melanoma progression.
Malignant melanoma exhibits a high degree of phenotypic plasticity and is highly aggressive and drug resistant in nature. The phenomenon of cancer stem cell (CSC) is an emerging area of research that provides a potential explanation for aggressiveness, drug resistance, and distant metastasis in various cancers (
). CD133, CD20, CD271, ATP-binding cassette B5, and aldehyde dehydrogenase 1A have been used as markers to identify CSCs in melanoma cell lines and/or patient biopsies (
). During cancer progression, the epithelial-to-mesenchymal transition (EMT) contributes to accelerating tumor growth and metastasis through activation of Snail, Slug, and Zeb (
Notch 1–4 and their ligands, Jagged1 and 2, DLL1, 3, and 4, and multiple effector molecules, such as Hes 1–6, and Hey 1 and 2, have been identified in various stem cells in vertebrates. Notch signaling plays a crucial role in the regulation of cell fate and differentiation of stem cells derived from brain and glioblastoma. Notch signaling is involved in CD133+ CSC-dependent glioblastoma growth (
). Injecting N-[N-(3,5-difluorophenacetyl)-1-alanyl]-S-phenylglycine t-butyl ester (DAPT) (γ-secretase inhibitor, GSI) along with cisplatin significantly depletes CD133+ cells in lung adenocarcinoma, indicating that the Notch pathway is involved in the regulation of the CD133+ subpopulation (
). However, the molecular link between CD133 expression and the activation of Notch and MAPK pathways as well as the mechanism underlying the CSC-mediated melanoma growth is not well established.
CSCs in melanoma exhibit higher resistance to conventional therapies (
Dacarbazine causes transcriptional up-regulation of interleukin 8 and vascular endothelial growth factor in melanoma cells: a possible escape mechanism from chemotherapy.
). Although chemotherapeutic agents, such as dacarbazine (DTIC), doxorubicin, dabrafenib, and trametinib, are widely used in melanoma, they are however relatively less curative and exhibit several side effects (
Dacarbazine causes transcriptional up-regulation of interleukin 8 and vascular endothelial growth factor in melanoma cells: a possible escape mechanism from chemotherapy.
). Hence, there is a need to identify novel effective and selective therapeutic agents that specifically target melanoma stem cells. Several groups have reported that andrographolide (Andro), which is derived from Andrographis paniculata, is a novel anticancer agent that attenuates tumor growth in multiple cancer models and inhibits CSC-mediated multiple myeloma growth (
In this study, we report that Notch1 transcriptionally regulates CD133 expression that preferentially activates MAPK and regulates matrix metalloproteinase (MMP)-2/-9 and vascular endothelial growth factor (VEGF) expression in CD133+ cells, leading to melanoma growth, angiogenesis, and lung metastasis. Moreover, CD133+ cells exhibit the EMT and are able to transdifferentiate into endothelial-like phenotypes. Furthermore, Andro suppresses melanoma progression through attenuation of the Notch1 signaling pathway.
Results
CD133+ mouse and human melanoma cells express high levels of Oct3/4, Nanog, and Sox10
To investigate CSC-specific heterogeneity in melanoma, the expression of stem cell markers in mouse (B16F10, B16F1) and human (A375, SK-MEL-2, SK-MEL-28) melanoma cell lines was analyzed by flow cytometry. The results indicated that the majority of these markers are present in a heterogeneous manner (Figure 1a). Several studies have demonstrated that CD133+ cells exhibit stem cell properties in various cancers (
). Therefore, to study the stem-like properties of CD133+ cells in B16F10, we isolated and analyzed the purity and the expression of stem cell-specific transcription factors in these cells. The data revealed that CD133+ cells exhibit increased Oct3/4, Nanog, and Sox10 expression compared with CD133− cells (Figure 1b and c, Supplementary Figure S1a online). Additionally, CD133+ cells derived from A375, SK-MEL-2, or SK-MEL-28 exhibit increased Sox10 and Oct3/4 expression (Supplementary Figure S1a). We next examined whether CD133+ cells exhibit dual positivity with CD20 or CD166 populations in B16F10 cells. The results revealed that CD133+ cells are 0.4% and 7.4% positive for CD20 and CD166, respectively (Supplementary Figure S1b). Moreover, CD133– cells did not exhibit any overlap with CD271+ cells (Supplementary Figure S1c).
Figure 1Melanoma cells constitute a heterogeneous CSC subpopulation. (a) Bar graph represents the flow cytometry analyses of a heterogeneous subpopulation of CSCs using specific markers (CD133, CD20, CD271, ABCG2, ABCG5, VEGFR1, CD166, and CD44) in mouse melanoma B16F10 and B16F1 as well as human melanoma A375, SK-MEL-2, and SK-MEL-28 cells. Data are presented as the mean ± SEM of three independent experiments. (b) FACS analysis of CD133+ and CD133− subpopulations and their purity in B16F10 cells. (c) Immunoblot analyses of stemness-specific markers (Oct3/4, Nanog, and Sox10) in unsorted, CD133+, and CD133– B16F10 cells. ABC, ATP-binding cassette; CD, cluster of differentiation; CSC, cancer stem cell; PE, phycoerythrin; SEM, standard error of the mean; VEGF, vascular endothelial growth factor.
The CD133+ subpopulation expresses a distinct gene expression profile
To determine the distinct gene expression profile of CD133+ with respect to the CD133– melanoma subpopulation, the global gene expression profile was analyzed using an Illumina Mouse WG6 whole genome microarray. The scatter plots and unsupervised hierarchical cluster analysis revealed a clear demarcation between these two groups, indicating that the CD133+ subpopulation has a distinct molecular signature (Figure 2a and b). A total of 411 genes were upregulated and 355 genes were downregulated in CD133+ compared with CD133– cells as represented by volcano plots and heat map analysis (Figure 2c, Supplementary Figure S2a online). Further, Gene Ontology data suggested that many of the upregulated genes in CD133+ cells were associated with angiogenesis, adhesion, differentiation, and metastasis. However, several downregulated genes were associated with apoptosis (Supplementary Figure S2b and Table S1 online). These data prompted us to functionally validate these findings with respect to the EMT, tumor growth, angiogenesis, and metastasis in melanoma.
Figure 2Molecular profile, tumorigenicity, and growth kinetics of CD133+ cells. (a–c) Scatter plots, cluster analysis, and volcano plot of differentially expressed genes in CD133+ versus CD133– cells. (d–f) CD133+, CD133–, or unsorted B16F10-Luc cells were injected subcutaneously (s.c.) into NOD/SCID mice and tumor images were captured using IVIS, quantified and represented in mean flux (n = 6 mice). *P < 0.007, **P < 0.0007. Tumor volumes were measured twice weekly. *P < 0.007. (g) CD133 expression was analyzed by immunoblot in tumor lysates. (h) In vivo limiting dilution analyses of indicated cells in C57BL/6J mice (n = 10 mice). *P < 0.00004, **P < 0.000006, ***P < 0.0000007. (i, j) CD133+ and CD133– cells were injected s.c. into NOD/SCID mice to generate primary isografts. Further, primary isograft-derived cultures were reimplanted for secondary isografts. *P < 0.05, **P < 0.02 (n = 6 mice). (k) Schematic representation of experimental outline for (i) and (j). Data are mean ± SEM. IVIS, in vivo imaging system; NOD, nonobese diabetic; SCID, severe combined immunodeficiency; SEM, standard error of the mean.
CD133+ cells exhibit increased tumor-initiating and long-term tumorigenic potential
We further investigated the tumorigenic potential of CD133+ cells using an in vivo model. CD133+ cells exhibited increased tumor growth in nonobese diabetic/severe combined immunodeficiency (NOD/SCID) (Figure 2d–g) and C57BL/6J mice models (Supplementary Figure S2c–e). Moreover, B16F10 CD20+ and SK-MEL-28 CD133+ cells also exhibit increased tumorigenic properties compared with CD20– and CD133– cells, respectively (Supplementary Figure S2f and g). Furthermore, in vivo growth kinetics under limiting dilution revealed that CD133+ cells exhibit increased tumor-initiating potential and tumor growth compared with CD133– cells (Figure 2h, Supplementary Table S2 online). We next evaluated the long-term tumorigenic potential of the CD133+ subpopulation by serial implantation of CD133+ and CD133– cells into NOD/SCID mice. The results revealed that CD133+ cells exhibit increased tumorigenic potential in primary and secondary isografts (Figure 2i–k). The expression of CD133 in primary cultures derived from CD133+ primary and secondary isografts was analyzed, and the percentage of cells expressing CD133 remained similar. Moreover, CD133– cells did not revert into the CD133+ subpopulation in CD133– primary and secondary isografts (Supplementary Figure S3a online).
CD133+ cells exhibit high drug efflux capacity and survival potential
Previous reports have demonstrated that the acquisition of chemoresistance is associated with increased expression of multidrug resistance proteins, a characteristic feature of the side population. Interestingly, our data showed that CD133+ exhibits a higher percentage of side population (15.7%) compared with the CD133– subpopulation (1.6%) (Figure 3a). Furthermore, the effect of this observed drug efflux on cell viability was investigated using DTIC, doxorubicin, dabrafenib, and trametinib. Our results indicated that CD133+ cells exhibit increased cell viability compared with CD133– cells in response to these drugs (Figure 3b). Moreover, enhanced colony formation and decreased cleaved caspase 3 levels indicate that CD133+ cells exhibit increased cell survival (Supplementary Figure S3b and c). Taken together, these data indicate that the CD133+ subpopulation may have higher chemoresistance properties.
Figure 3Chemoresistance and angiogenic properties of CD133+ cells. (a) SP analysis of CD133+ and CD133– cells. (b) Effect of dacarbazine, doxorubicin, dabrafenib, and trametinib on CD133+ and CD133– cells’ viability. *P < 0.004 (n = 3). (c) Vasculogenic mimicry in CD133+ and CD133– cells. Scale bars = 30 μm. Bar graph represents the number of tube junctions/hpf; *P < 0.0001 (n = 3). (d) Migration of HUVECs toward CD133+ or CD133– cells and their quantitation in a comigration assay. Scale bars = 30 μm. *P < 0.00005 (n = 3). (e) Tube formation using HUVECs (1 × 104) in the presence of conditioned medium (CM) of CD133+ or CD133– cells and their quantitation. Scale bars = 30 μm. *P < 0.00004 (n = 3). (f, left) qRT-PCR analysis of VEGF in indicated cells (n = 3). (f, right) Immunoblot of VEGF in CD133+ and CD133– cells. (g) Flow cytometry analysis of CD31, VEGFR2, and VEGFR1 in unsorted, CD133+, and CD133– cells. (h) CD31 and VEGF expression in tumor sections derived from CD133+ and CD133– cells based on immunofluorescence and data quantitation. Scale bars = 20 μm. *P < 0.0005 (n = 3). Data are mean ± SEM. HUVECs, human umbilical vein endothelial cells; SEM, standard error of the mean; SP, side population; VEGF, vascular endothelial growth factor.
CD133+ cells exhibit enhanced tube formation, tumor-endothelial cell interaction, and potential to transdifferentiate into endothelial-like cells
To examine the involvement of CD133+ cells in angiogenesis, we performed functional tube formation assays. Our data revealed that CD133+ cells form well-defined tube-like networks compared with CD133– cells (Figure 3c). Furthermore, we examined the role of CD133+ cells in tumor-endothelial cell interactions by a comigration assay. Strikingly, our results showed that human umbilical vein endothelial cells are highly migratory toward CD133+ cells compared with CD133– cells (Figure 3d). In addition, conditioned media of CD133+ cells exhibited enhanced tube formation using human umbilical vein endothelial cells (Figure 3e). Furthermore, our data revealed that CD133+ cells exhibit enhanced expression of VEGF, VEGFR1, VEGFR2, CD34, and Cox2 (Figure 3f, Supplementary Figure S3d–f).
To evaluate the ability of CD133+ cells to transdifferentiate into an endothelial-like phenotype, CD133+, CD133–, or unsorted cells were cultured in endothelial differentiation conditions. The flow cytometry results revealed that the expression of endothelial-specific markers, such as CD31, VEGFR2, and VEGFR1, was augmented in CD133+ cells at day 14, suggesting that these cells acquired an endothelial-like phenotype (Figure 3g). Additionally, CD133+ tumors also exhibit increased CD31 and VEGF expression (Figure 3h). Taken together, these data demonstrate that CD133+ cells promote angiogenesis in melanoma.
CD133+ cells undergo the EMT and exhibit enhanced metastatic potential
To further examine whether CD133+ cells undergo the EMT, we analyzed the expression of EMT-specific genes. The results revealed significant upregulation of mesenchymal-specific markers, such as N-cadherin, Slug, Snail, and Twist, whereas expression of the epithelial-specific marker E-cadherin was downregulated in CD133+ cells derived from B16F10, A375, SK-MEL-2, or SK-MEL-28 (Supplementary Figure S4a–d, Tables S3, and S4 online). Our EMT data prompted us to examine the metastatic potential of B16F10-derived CD133+ cells. Interestingly, CD133+ cells exhibit increased migratory potential compared with CD133– cells, and this effect was abrogated on silencing CD133 expression (Figure 4a and b, Supplementary Figure S4e and f). Furthermore, our data revealed that CD133+ cells express high levels of MMP-2/-9 and exhibit enhanced cell attachment (Figure 4c and d, Supplementary Figure S4g). Moreover, when injected through the tail vein, CD133+ cells exhibited enhanced seeding and metastatic potential toward lungs but not liver (Figure 4e and f, Supplementary Figure S4h).
Figure 4Role of CD133+ cells in metastasis and transcriptional regulation of CD133 by Notch1. (a) Wound migration at 0 and 12 hours in indicated cells. *P < 0.0006 (n = 3). (b) Wound migration in CD133 silenced CD133+ cells. *P < 0.00008 (n = 3). (c) qRT-PCR of MMP-2/-9 (n = 3). (d) Zymography in indicated cells. (e) CD133+ and CD133– B16F10-Luc cells were injected through the tail vein. Metastatic sites were captured by IVIS. (f) Immunostaining of antimelanoma antigens. Scale bars = 100 μm. (g, h) Immunoblot and immunofluorescence of Notch1 pathway-associated proteins. Scale bars = 20 μm. (i, j) Effect of GSI-IX (γ-secretase inhibitor) on CD133 expression as shown by flow cytometry and immunoblot. Blue indicates CD133+ and red represents CD133– subpopulations. (k) Flow cytometry analysis of CD133 in NICD1-overexpressing B16F10 cells. (l, m) Schematic representation of NICD1-binding sites on a CD133 promoter. Chromatin immunoprecipitation was performed using CD133+ cells with anti-NICD1 antibody and PCR amplified with CD133 promoter-specific primers. (n) CD133 promoter activity by luciferase reporter assay in cells and conditions as indicated. *P < 0.009; #P < 0.003 (n = 3). Data are mean ± SEM. IVIS, in vivo imaging system; MMP, matrix metalloproteinase; NICD1, Notch1 intracellular domain; SEM, standard error of the mean; siRNA, small interfering RNA.
Notch1 signaling regulates CD133 expression at both transcriptional and posttranscriptional levels
Notch signaling plays a pivotal role in the regulation of differentiation and self-renewal of normal and CSCs. Interestingly, we found that Notch1 pathway-specific genes, such as Notch1, Notch1 intracellular domain (NICD1), Hes1, and Jagged1, are highly upregulated in CD133+ compared with CD133– cells (Figure 4g and h, Supplementary Figure S5a online). Furthermore, CD133+ tumors exhibit increased Notch1 expression (Supplementary Figure S5b). Notch2, Notch3, and Notch4 expression remained unchanged, indicating that Notch1 signaling is predominant in CD133+ cells (Supplementary Figure S5c). Similarly, Notch1 signaling was also activated in CD133+ cells derived from A375, SK-MEL-2, and SK-MEL-28 (Supplementary Figure S5d).
To elucidate the dynamic interrelationship between Notch1 signaling and CD133 expression, we treated CD133+ cells independently with two Notch inhibitors, namely GSI-IX and GSI-X. Strikingly, these inhibitors suppress CD133 expression in CD133+ cells and increased CD133– population (Figure 4i and j, Supplementary Figure S5e and f). However, GSI-IX does not restore the expression of CD133 in the CD133– subpopulation (Supplementary Figure S5e). Overexpression of Notch1IC increases the expression of CD133 in B16F10, A375, or SK-MEL-2 cells (Figure 4k, Supplementary Figure S5g and h). Moreover, siRNA-mediated knockdown of Notch1 downregulates CD133 expression in CD133+ cells, indicating that Notch1 regulates CD133 expression (Supplementary Figure S5i). To further confirm the recruitment of NICD1 at the promoter of CD133, chromatin immunoprecipitation and luciferase reporter assays were performed, and the results showed that the NICD1 is recruited within –700 to –451 bp upstream of the transcription start site in the CD133 promoter (Figure 4l–n).
Notch1 regulates CD133-dependent MAPK and activator protein-1 (AP-1) activation
To further explore whether MAPK signaling is involved in CD133-dependent melanoma progression, the expression of p-MAPK/extracellular signal-regulated kinase 3/6, p-p38, c-Fos, and c-Jun was analyzed in CD133+ and CD133– cells. The data revealed that the expression of these molecules is upregulated in CD133+ cells or tumors (Figure 5a, Supplementary Figure S6a and b online). Further, similar results were also observed in A375, SK-MEL-2, and SK-MEL-28 cells (Supplementary Figure S6a). Electrophoretic mobility shift assay (EMSA) data revealed enhanced AP-1-DNA binding in CD133+ cells (Figure 5b, Supplementary Figure S6c). These data suggest that MAPK signaling is predominantly active in CD133+ cells. To elucidate the dynamic interplay between Notch1 and MAPK signaling, CD133+ cells were treated with SB203580 (p38 MAPK inhibitor), and CD133 expression was analyzed. The results showed that SB203580 has no effect on CD133 but attenuates the expression of p-p38, c-Fos, and c-Jun, suggesting that CD133 acts upstream of the MAPK pathway (Figure 5c, Supplementary Figure S6d). SB203580 also abrogates the expression of p-p38, c-Fos, and c-Jun in CD133– cells; however, these cells express very low levels of p38 MAPK and its associated molecules (Supplementary Figure S6e).
Figure 5Role of Notch1 signaling on CD133-dependent MAPK activation. (a) Immunoblots of p-MEK3/6, p-p38, c-Fos, and c-Jun in unsorted, CD133+, or CD133– cells. (b) AP-1-DNA binding in cells as indicated in (a) by EMSA. (c) Effect of SB203580 (p38 MAPK inhibitor) on p-p38, c-Jun, c-Fos, and CD133 expression in CD133+ cells as shown by immunoblots. (d) Effect of GSI-IX on NICD1, p-p38, c-Jun, and c-Fos levels in CD133+ cells. (e, f) Immunoblot of p-p38, c-Jun, Notch1, and CD133 in Notch1- or CD133-silenced CD133+ cells. (g) Effect of GSI-IX and SB203580 on migration of HUVECs toward CD133+ cells. The data are mean ± SEM (n = 3). *P < 0.005; **P < 0.0005. (h) Wound migration assay at indicated conditions. The data are mean ± SEM (n = 3). *P < 0.002; **P < 0.0006. (i) Notch1, CD133, or c-Jun was silenced by their specific siRNA in CD133+ cells, and the expression of metastasis- and angiogenesis-specific genes was analyzed by qRT-PCR. Bar graph represents mean ± SEM (n = 2). AP-1, activator protein-1; EMSA, electrophoretic mobility shift assay; GSI, γ-secretase inhibitor; HUVECs, human umbilical vein endothelial cells; MAPK, mitogen-activated protein kinase; MEK, MAPK/ERK kinase; MMP, matrix metalloproteinase; NICD1, Notch1 intracellular domain; SEM, standard error of the mean; siRNA, small interfering RNA; VEGF, vascular endothelial growth factor.
We further studied whether Notch1 signaling regulates CD133-dependent MAPK activation. CD133+ cells were treated with GSI-IX, and expression of NICD1, p-p38, c-Fos, and c-Jun was analyzed by western blot. The data showed that GSI-IX inhibits the expression of these specific molecules (Figure 5d). Moreover, GSI-IX downregulates CD133 expression in CD133+ cells (Figure 4i and j). EMSA results revealed that GSI-IX and SB203580 inhibit AP-1-DNA binding in CD133+ cells (Supplementary Figure S6f). Furthermore, silencing Notch1 or CD133 downregulates p-p38 and c-Jun expression, demonstrating that Notch1 and CD133 regulate MAPK signaling (Figure 5e and f).
Notch1 and MAPK regulate the EMT, migration, and angiogenesis in CD133+ cells
To examine the effects of GSI-IX or SB203580 on tumor-endothelial cell interactions and CD133+ cell migration, comigration and wound migration assays were performed. The results revealed that blocking the Notch1 or MAPK pathway inhibits the CD133+-endothelial cell interaction and CD133+ cell migration (Figure 5g, Supplementary Figure S6g and h). Silencing Notch1, CD133, or c-Jun attenuates MMP-2/-9 and VEGF expression and inhibits the migratory potential of CD133+ cells (Figure 5h and i). Furthermore, Snail and Slug expression was downregulated, and E-cadherin was upregulated in Notch1 or CD133 silenced CD133+ cells (Supplementary Figure S6i and j). These results suggest that the Notch1 pathway is involved in the regulation of the CD133+ tumor-endothelial cell interaction and migration in CD133+ cells.
Andro inhibits CD133+ cell viability and migration through the downregulation of Notch1-dependent CD133 expression and MAPK activation
Melanoma-specific CSCs are resistant to conventional therapies. However, DTIC, dabrafenib, and trametinib are widely used chemotherapeutic agents for melanoma, but these agents are associated with drug resistance and several side effects (
Dacarbazine causes transcriptional up-regulation of interleukin 8 and vascular endothelial growth factor in melanoma cells: a possible escape mechanism from chemotherapy.
). Therefore, it is important to identify novel effective and selective therapeutic agents that specifically target melanoma-specific CSCs. Our data showed that Andro reduces cell viability in unsorted, CD133+, and CD133– cells (Figure 6a). Furthermore, unlike DTIC or dabrafenib in combination with trametinib, Andro selectively downregulates CD133 expression in CD133+ cells (Supplementary Figure S7a and c online). Recent studies have demonstrated that differentiated tumor cells (non-CSCs) transform into CSCs on long-term exposure to therapeutic agents (
). Our data suggest that Andro, DTIC, or the combination of dabrafenib and trametinib treatment does not restore CD133 expression in CD133– cells (Supplementary Figure S7b and c). In addition, Andro inhibits the expression of CD133 even at a dose of 25 μM in SK-MEL-2 and SK-MEL-28 cells (Supplementary Figure S7d).
Figure 6Andrographolide (Andro) abrogates tumorigenic and metastatic potential of CD133+ cells by targeting Notch1-dependent MAPK pathway. (a) A cell viability assay was performed in CD133+, CD133–, and unsorted B16F10 cells treated with Andro at indicated doses. Bar graph denotes mean ± SEM (n = 3). *P < 0.008 and **P < 0.0008 compared with untreated cells. (b) Immunoblots of Notch1 regulated signaling molecules in Andro-treated CD133+ cells. (c) Effect of Andro on CD133+ cell-derived tumors in C57BL/6J mice (n = 6). Data are mean ± SEM. *P < 0.005; **P < 0.0007; ***P < 0.0006. (d) Immunoblots of NICD1, p-p38, c-Jun, and c-Fos from mice tumors lysate treated with Andro. (e) CD133+ B16F10-Luc (1 × 103) cells were injected through the tail vein of NOD/SCID mice, and mice were treated with Andro intraperitoneally. Metastatic sites were captured by IVIS. (f) H&E staining of lung metastases. Scale bars = 100 μm. (g) Immunofluorescence of VEGF in tumor sections as indicated. Scale bars = 20 μm. (h) Schematic representation of Notch1 signaling that regulates CD133-mediated MAPK activation. MAPK activation leads to AP-1-dependent VEGF and MMP-2/-9 expression, which contributes to melanoma growth, angiogenesis, and metastasis. AP-1, activator protein-1; H&E, hematoxylin and eosin; IVIS, in vivo imaging system; MAPK, mitogen-activated protein kinase; MMP, matrix metalloproteinase; NICD1, Notch1 intracellular domain; NOD, nonobese diabetic; SCID, severe combined immunodeficiency; SEM, standard error of the mean; VEGF, vascular endothelial growth factor.
Our previous data demonstrated that Notch1 and MAPK pathways are highly upregulated in CD133+ cells. Therefore, to examine whether Andro plays a role in Notch1 and MAPK pathways, CD133+ cells were treated with Andro, and the expression and activation of NICD1, Hes1, Hey1, p-p38, p-MAPK/extracellular signal-regulated kinase 3/6, and c-Fos were analyzed by western blot. Our data showed that Andro downregulates the expression and activation of these signaling molecules (Figure 6b). The attenuation of NICD1 and CD133 expression was also observed even at lower dose (0–25 μM) of Andro without affecting cell viability in these cells (Figure 6a, Supplementary Figure S7e–g). Thus, our data suggest that the observed inhibition of Notch1 and CD133 signaling is not due to toxicity rather than its inhibitory effect. Moreover, compared with dabrafenib or trametinib, Andro significantly inhibits the migratory potential of CD133+ cells through the downregulation of MAPK activation (Supplementary Figure S7h and i).
Andro suppresses EMT, angiogenesis, and metastatic potential of CD133+ cells through abrogation of the MAPK pathway
To further investigate the role of Andro in tumorigenicity, CD133+ cells were injected into C57BL/6J mice subcutaneously, and Andro was administered intraperitoneally. The results revealed that Andro suppresses tumor growth and attenuates the expression of NICD1, CD133, p-p38, c-Jun, and c-Fos in tumors (Figure 6c and d, Supplementary Figure S7j and k). We further examined the role of Andro in the EMT, angiogenesis, and metastatic potential of CD133+ cells under in vitro and in vivo conditions. The results revealed that Andro downregulates mesenchymal to epithelial transition (MET)-specific gene expression and lung metastasis in CD133+ cells (Figure 6e and f, Supplementary Figure S8a and b online). Moreover, Andro also suppresses VEGF expression in these tumors (Figure 6g). Overall, our data revealed that Andro attenuates CD133+ cell-mediated EMT, angiogenesis, and metastasis.
Notch1, CD133, and MAPK expression are highly correlated with malignant melanoma progression
We further correlated the in vitro and in vivo findings by examining the expression and colocalization of NICD1, p-MAPK/extracellular signal-regulated kinase 3/6, p-p38, c-Fos, and CD133 in human malignant melanoma specimens by immunofluorescence. The increased expression of NICD1 and CD133 colocalized with p-p38 and p-MAPK/extracellular signal-regulated kinase 3/6 in human malignant melanoma compared with peripheral normal tissues (Supplementary Figure S9a and b online). Taken together, these results suggest that Notch1 and its regulated MAPK signaling networks may act as potential therapeutic targets for the management of CSC-mediated malignant melanoma (Figure 6h).
Discussion
In this study, we report that melanoma-derived CD133+ cells exhibit stem cell-like phenotypic characteristics that play a vital role in the EMT and melanoma progression. The Notch1 pathway regulates the expression of CD133, which preferentially activates MAPK, leading to AP-1-mediated MMP-2/-9 and VEGF expression and contributing to melanoma growth, angiogenesis, and metastasis. As a proof of principle, pharmacological inhibitors or genetic ablation of Notch1 inhibits the expression of CD133 that ultimately abrogates MAPK signaling and the tumorigenic potential of CD133+ cells.
Our results indicate that melanoma is hierarchically organized into phenotypically distinct subpopulations of tumorigenic and nontumorigenic cells. We report that CD133+ melanoma-specific CSCs are highly tumorigenic in nature. Several studies on melanoma and breast cancer suggest that functional CSCs are present in these cancer cells with higher metastatic potential and a characteristic molecular signature (
). Our results demonstrate that CD133+ cells derived from melanoma exhibit a distinct gene expression profile compared with CD133– cells. CD133+ cells exhibit increased drug efflux as indicated by the side population phenotype, which is further supported by enhanced cell viability in response to DTIC, doxorubicin, dabrafenib, and trametinib. In addition to increased cell survival, these results provide a possible explanation for enhanced chemoresistance in CD133+ cells.
During tumor development, angiogenesis plays an important role, and various proangiogenic genes are expressed in CSCs derived from solid tumors (
). Our data revealed that CD133+ cells are associated with increased angiogenic activity in vitro and in vivo. The tumor-endothelial cell interaction plays a crucial role in neovascularization (
). Indeed, our results suggest that endothelial cells exhibit enhanced migratory and tube formation ability toward CD133+ compared with CD133– cells. Several studies have reported that tumor neovascularization is regulated by many angiogenic factors either from pre-existing or recruited endothelial cells (
). Our present findings demonstrate that CD133+ but not CD133– cells in melanoma have potential to transdifferentiate into endothelial-like cells, suggesting that neovascularization could be a functional property of these cells.
Previous studies have identified that CSCs switch between two distinct phenotypes: proliferative and migratory (
). Our data indicate that CD133+ cells are highly migratory in nature as shown by the wound migration assay. The inhibition of the cell cycle on mitomycin C treatment followed by a cell migration assay suggests that the enhanced migration of CD133+ cells is not due to proliferation. The EMT programming is initially required for invasion and dissemination of tumor cells (
). During tumor metastasis, the activation of prometastatic genes, such as urokinase-type plasminogen activator and MMP-2/-9, plays an important role in the degradation of extracellular matrix proteins that facilitate tumor invasion (
). Our results revealed that CD133+ cells express high levels of MMP-2/-9, leading to enhanced metastasis toward lungs.
We further studied the molecular mechanism by which CD133+ cells regulate melanoma progression in depth. The Notch1 pathway plays a crucial role in glioma cell survival and proliferation (
). However, the mechanism by which Notch1 regulates CD133 expression and controls CSCs-mediated melanoma growth has not been previously studied. Our data show that Notch1 augments CD133 expression, whereas the inhibition of its function substantially attenuates CD133 expression. Furthermore, CD133 promoter analysis revealed the presence of an NICD1-binding site that has been reported by
. Our chromatin immunoprecipitation and luciferase reporter assay data indicate that NICD1 binds within –700 to –451 region of the CD133 promoter and regulates CD133 expression. Taken together, our findings suggest that Notch1 regulates CD133 expression both at transcriptional and posttranscriptional levels in melanoma.
The Notch pathway is activated by MAPK signaling and promotes tumor growth in thyroid follicular cells (
). However, our findings suggest that Notch1 stimulates the p38 MAPK pathway that preferentially activates AP-1 and upregulates MMP-2/-9 and VEGF expression in CD133+ cells. We also observed that the inhibition of p38 MAPK did not alter the expression of CD133, suggesting that CD133 is upstream of the p38 MAPK pathway. However, blocking Notch1 attenuates migration and the tumor-endothelial cell interaction. Blockade or silencing of the Notch1 pathway inhibits CD133-dependent MAPK signaling and ultimately attenuates CD133+ cell migration and the tumor-endothelial cell interaction, suppressing melanoma growth, angiogenesis, and metastasis.
Failure to respond to conventional chemotherapy is attributed to chemoresistance of CSCs, and this phenomenon is one of the causes of cancer recurrence (
). Accordingly, we sought to identify potential therapeutic agent(s) that could specifically and selectively target CSCs in melanoma. Earlier results suggest that Andro acts as an anticancer agent targeting CSCs in multiple myeloma (
). Our data revealed that Andro blocks the Notch1-dependent CD133 expression and MAPK activation in CD133+ cells leading to suppression of tumor growth, angiogenesis, and metastasis in the mice melanoma model. We have also shown that Andro inhibits the expression of CD133 in human melanoma, SK-MEL-2 and SK-MEL-28 cells. Moreover, additional studies are required to confirm these effects in the human melanoma model. All these data suggested that Andro may improve the recurrence-free survival of patients with melanoma. In summary, our study demonstrates the crucial role of the Notch1 pathway that drives CD133 expression to promote MAPK activation and ultimately control melanoma growth, angiogenesis, and metastasis. Thus, targeting Notch1 and its regulated signaling network may have potential therapeutic implications in the management of CSC-mediated melanoma progression.
Materials and Methods
In vivo tumorigenicity and immunohistochemistry
All mice experiments were performed according to the guidelines of Institutional Animal Care and Use Committee of National Centre for Cell Science, Pune, India. CD133+, CD133–, or unsorted B16F10-Luc cells were mixed with Matrigel (1:1) (BD Biosciences, San Jose, CA) and injected subcutaneously into the dorsal right flank of NOD/SCID or C57BL/6J mice (6–8 weeks old). Tumor length and width were measured twice a week with Vernier Calipers. In vivo bioluminescence imaging was conducted using in vivo imaging system (Xenogen) as previously described (
). B16F10-derived CD133+ or CD133– cells (1 × 103, 5 × 102, and 1 × 102 cells) were subcutaneously injected with Matrigel into C57BL/6J mice. The melanoma-initiating cell frequency was analyzed as previously described (
In separate experiments, sorted CD133+ cells were injected into C57BL/6J mice subcutaneously and randomly divided into three groups. Once the tumor appeared, two doses of Andro (50 and 150 mg/kg body weight) were injected intraperitoneally, and tumor volumes were measured. Mice were killed, and tumors were excised, photographed, and weighed. Tumor volumes were measured using V = π/6 [(l × b)3/2]. Tumor sections were analyzed by immunohistochemistry using anti-CD31, anti-VEGF, or antimelanoma antibody.
Immunohistochemistry of human melanoma specimens
The formalin-fixed specimens of human malignant melanoma and peripheral tissues were collected with the help of a histopathologist from the local hospital with written, informed patient consent as per institutional and hospital ethical committee approvals and guidelines. The specimens were analyzed by immunohistochemistry using a confocal microscope (Zeiss).
Conflict of Interest
The authors state no conflict of interest.
Acknowledgments
This work was primarily supported by National Centre for Cell Science, Pune, India (to GCK). DK was supported by UGC. DT and HSP are supported by CSIR, Government of India. We thank Anuradha Bulbule and Priyanka Ghorpade for reading this article and Poonam R. Pandey for technical help.
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