Cancer stem cell characteristics, especially their self-renewal and clonogenic potentials, play an essential role in malignant progression and response to anticancer therapies. Currently, it remains largely unknown what pathways are involved in the regulation of cancer cell stemness and differentiation. Previously, we found that delta-like 1 homolog (Drosophila) or DLK1, a developmentally regulated gene, plays a critical role in the regulation of differentiation, self-renewal, and tumorigenic growth of neuroblastoma cells. Here, we show that DLK1 specifically interacts with the prohibitin 1 (PHB1) and PHB2, two closely related genes with pleiotropic functions, including regulation of mitochondrial function and gene transcription. DLK1 interacts with the PHB1–PHB2 complex via its cytoplasmic domain and regulates mitochondrial functions, including mitochondrial membrane potential and production of reactive oxygen species. We have further found that PHB1 and especially PHB2 regulate cancer cell self-renewal as well as their clonogenic potential. Hence, the DLK1–PHB interaction constitutes a new signaling pathway that maintains clonogenicity and self-renewal potential of cancer cells.
Implications: This study provides a new mechanistic insight into the regulation of the stem cell characteristics of cancer cells. Mol Cancer Res; 12(1); 155–64. ©2013 AACR.
Delta-like 1 homolog (Drosophila) or DLK1 is a member of the EGF-like homeotic supergene family with homologies to members of the notch/delta/serrate family (1). Also known as pref-1, fetal antigen (FA1), pG2, and ZOG, DLK1 is predominantly expressed in embryonic and other immature cells but not in differentiated cells (2–4), suggesting an important role of DLK1 in regulating maintenance and/or differentiation of stem cells and progenitors. Expression of DLK1 is elevated in a wide range of tumor types, including neuroblastoma, gliomas, breast cancer, colon cancer, pancreatic cancer, small-cell lung carcinoma, and leukemia (5–11). Studies from other authors and us have shown that DLK1 plays an important role in stem cell maintenance and cellular differentiation. DLK1 expression in vitro inhibits differentiation of mesenchymal progenitor cells (3, 12, 13) and hematopoietic stem cells (8, 9). Our studies have shown that DLK1 is primarily expressed in undifferentiated neuronal tumor cells (14). Overexpression of DLK1 enhances tumor cell stemness and tumorigenic growth in vivo (14, 15). Conversely, DLK1-specific RNAi or dominant DLK1 mutants enhance spontaneous neuronal differentiation and decrease tumorigenicity of neuroblastoma cells both in vitro and in vivo (14, 15). We also found that hypoxia strongly induces DLK1 expression and that DLK1 is implicated in the regulation of cancer cell stemness within the hypoxic tumor microenvironment (14, 16, 17). In addition, DLK1 is also implicated in regulating in vitro differentiation of glioma cells (7) and hematopoietic tumors (8). However, the mechanisms by which DLK1 regulates cancer cell stemness and/or differentiation remain largely unknown.
To gain mechanistic understanding of the involvement of DLK1 in intracellular signal transduction, we attempted to identify DLK1-interacting proteins using an affinity purification approach. As reported herein, we have found that DLK1 specifically interacts with the prohibitin (PHB) complex via the DLK1 cytoplasmic (DLK1-cyto) domain. PHB1 and the closely related PHB2 are encoded by evolutionarily conserved genes and possess diverse functions from mitochondrial structural integrity and function to gene transcription in the nucleus (18–20). We have found that DLK1 regulates mitochondrial membrane potential and production of reactive oxygen species (ROS). Our data further reveal a role of PHBs and especially PHB2 in the regulation of cancer cell self-renewal as well as their clonogenic potential. Hence, the DLK1–PHB interaction constitutes a new signaling pathway that promotes the maintenance of cancer cell stemness.
Materials and Methods
Retroviral vectors expressing DLK1-FL (full-length DLK1), DLK1-Ecto, DLK1-Δcyto (deletion of DLK1-cyto domain), and DLK1-DM (Y339F/S355A mutations in DLK1-cyto domain) have been described in our previous study (14). Flag-tagged DLK1 (DLK1-Flag) was cloned by in-frame fusion of the full-length coding sequence of DLK1 to the 5′ of two tandem repeats of the Flag-tag sequence in pCMV-2xFlag. The Flag-tagged DLK1-cyto domain (DLK1-cyto-Flag) was cloned by in-frame fusion of the DLK1-cyto domain–coding sequence (amino acids 328-383) to the 3′ of two tandem repeats of the Flag-tag sequence in pCMV-2xFlag. The Flag-tagged PHB1 and PHB2 were from Dr. Valerie Bosch of Deutsches Krebsforschungszentrum (DKFZ; Heidelberg, Germany; ref. 21) and subcloned into a retroviral vector. The shDLK1 constructs were described in our previous study (14), with the target sequence positions in human DLK1 mRNA (nm_003836.5) being 1062-1080 for shDLK1-2, 1308-1326 for shDLK1-4, and 1426-1444 for shDLK1-6. All clones were sequence validated.
Cell culture and transfection
Neuroblastoma cell lines SK-N-BE(2)C [abbreviated as BE(2)C] and SK-N-ER (abbreviated as ER) were maintained in minimum essential medium (MEM) and F12 (1:1) with 10% FBS. Cells were transduced with retrovirus (DLK1-FL, DLK1-ΔCyto, DLK1-DM, or vector control) and then purified by flow cytometry for the expression of GFP. MCF7 cells (American Type Culture Collection, ATCC) were maintained in RPMI-1640 medium containing 10% FBS. The human hepatocellular cancer cell line HepG2 and human embryonic kidney cell line 293T (ATCC) were cultured in MEM containing 10% FBS. All culture media were supplemented with 25 mmol/L HEPES (pH7.4) to maintain pH stability under hypoxia.
For transfection with siRNA oligos, cells were grown to approximately 80% confluence and then incubated with On-Target SMARTpool siRNAs (Thermo Scientific) according to the manufacture's protocol. After incubation for 48 hours, cells were then trypsinized and plated for further experiments.
Affinity pull-down by coimmunoprecipitation
Whole-cell lysates (WCL) were prepared by incubating cells expressing different DLK1 constructs or empty vector with the modified radioimmunoprecipitation assay buffer (RIPA; 50 mmol/L Tris-HCl, pH7.5, 150 mmol/L NaCl, 0.5% sodium deoxycholate, and 1% NP40) for 30 minutes on ice. The lysates were cleared by centrifugation at 4°C for 15 minutes at 13,200 rpm. Immunoprecipitation was carried out by incubation of WCL (500 μg total protein) with ≤4 μg of monoclonal anti-N terminus DLK1 antibody (R&D Systems), rabbit anti-PHB2 antibody (Novus), or monoclonal anti-PHB1 antibody (Thermo Scientific; clone II-14-10) overnight at 4°C. Thirty μL of recombinant protein A agarose beads were then added and incubated for an hour at 4°C. Immune complexes were eluted in 1 × SDS sample buffer and fractionated by SDS–PAGE under reducing conditions. Silver staining was done using the SilverQuest Silver Staining Kit (LC6070; Invitrogen) according to the manufacturer's protocol.
Western blot analysis
Western blot analysis were analyzed as described previously (14) with the following antibodies: polyclonal rabbit anti-DLK1 C-terminus (1:2,000; Millipore); polyclonal rabbit anti-PHB2 (1:1,000; Novus), monoclonal mouse anti-PHB1 (1:400; Thermo Scientific), or rabbit anti-glyceraldehyde 3-phosphate dehydrogenase (Cell Signaling Technology). When necessary, Clean-Blot IP Detection Reagents (Thermo Scientific) were used to eliminate cross-reaction with immunoglobulin G (IgG) heavy and light chains.
The immune complex with anti-DLK1 antibody was fractioned using SDS–PAGE under reducing conditions. The protein-containing gel pieces were subjected to mass spectrometry at Taplin Biological Mass Spectrometry Facility of Harvard Medical School (Boston, MA).
Trichloroacetic acid precipitation
About 4 mL of 24-hour conditioned medium was mixed with 1 mL of trichloroacetic acid [TCA; 100% (w/v)] for 1 to 2 hours at 4°C. Precipitates were collected by centrifugation at 13,2000 rpm for 10 minutes at 4°C. After cold acetone wash for three times, protein pellets were dried in a 95°C heat block for 3 minutes and then solubilized in 1× SDS sample buffer. Secreted DLK1 in conditioned media was analyzed using Western blot analysis.
Immunofluorescence and confocal microscopy
For immunofluorescence staining of DLK1 and PHBs, BE(2)C cells were fixed with ice-cold methanol for 10 minutes and then washed three times with PBS. Nonspecific binding was blocked by incubation with 5% horse serum for 30 minutes. Cells were then incubated overnight at 4°C in an antibody mixture containing mouse anti-DLK1 (1:100; R&D Systems) plus rabbit anti-PHB1 serum or mouse anti-DLK1 plus rabbit anti-PHB1 serum (1:300). The anti-PHB1 and anti-PHB2 antisera were provided by Dr. Valerie Bosch (DKFZ). The bound antibodies were visualized by incubation with Alexa 488–conjugated anti-mouse IgG (1:500) and Alexa 555–conjugated goat anti-rabbit IgG (1:500), all obtained from Invitrogen. Nuclei were stained with TO-PRO3 (1:5,000; Invitrogen). Images were acquired on a Zeiss LSM 510 Meta confocal microscope. Colocalization between DLK1 and PHB was analyzed using Zeiss ZEN2010 software.
BE(2)C cells were stained in the serum-free medium by JC-1 dye (Invitrogen; M-34152, 2 μmol/L) for 30 minutes at 37°C according to the manufacture's protocol. Nuclei were stained with Hoechst 33342 (2 μg/mL). Microscopic examination was done within an hour of staining.
CMH2XROS staining and flow cytometry
Be(2)C cells were incubated in the serum-free medium with CMH2XROS dye (400 nmol/L) for 45 minutes at 37°C, and the reaction was stopped by washing the cells in ice-cold PBS. The cells were trypsinized and then fixed is 4% paraformaldehyde for 10 minutes at room temperature. After washing in cold PBS, fixed cells were then subjected to fluorescence-activated cell sorting (FACS; FACS DIVA; BD Biosciences). All samples were analyzed under the same gain and amplifier settings.
Tumor sphere formation assay
Tumor cells were trypsinized into single-cell suspension in tumor sphere medium and plated into tissue culture dishes precoated with polyhydroxyethylmethacrylate (polyHEMA; Sigma-Aldrich) as described in our previous publication (14). After incubation for 4 to 6 days, tumor spheres were counted under the microscope.
Tumor cells were plated at a clonal density (<1 cell/mm2) in 6-well plates and incubated undisturbed for 10 to 14 days. Colonies were stained with Crystal Violet. Plating efficiency = number of colonies (≥50 cells/colony) per input cells × 100%.
Real-time reverse transcription PCR
Total cellular RNA was isolated with the TRizol reagent (Invitrogen) and treated with DNase I for 10 minutes before first-strand cDNA was synthesized using Superscript II (Invitrogen). Real-time PCR was performed on StepOne Plus (Applied Biosystems) using Power SYBR Green PCR Master Mix (Applied Biosystems) under the following conditions: initiation at 95°C × 10 minutes, 40 cycles at 95°C × 15 seconds, and 60°C × 60 seconds. Of note, 18S rRNA was used as a control for normalization. Specificity of the primers (Table 1) was confirmed by a single peak on the dissociation curve.
|Gene .||Primer sequence .||Amplicon size .|
|DLK1||Forward: 5′-CTGAAGGTGTCCATGAAAGAG-3′||273 bp|
|PHB1||Forward: 5′-TGTCATCTTTGACCGATTCCG-3′||125 bp|
|PHB2||Forward: 5′-GTGCGCGAATCTGTGTTCAC-3′||135 bp|
|18S rRNA||Forward: 5′-CGGACAGGATTGACAGATTG-3′||83 bp|
|Gene .||Primer sequence .||Amplicon size .|
|DLK1||Forward: 5′-CTGAAGGTGTCCATGAAAGAG-3′||273 bp|
|PHB1||Forward: 5′-TGTCATCTTTGACCGATTCCG-3′||125 bp|
|PHB2||Forward: 5′-GTGCGCGAATCTGTGTTCAC-3′||135 bp|
|18S rRNA||Forward: 5′-CGGACAGGATTGACAGATTG-3′||83 bp|
Statistical differences between two groups were analyzed using the two-tailed, unpaired Student t test. Comparison among ≥3 groups was done using one-way ANOVA (Prism Software; GraphPad Software, Inc.)
DLK1 interacts with PHB1 and PHB2
Previously, we reported that the cytoplasmic domain of DLK1 is necessary for promoting self-renewal and clonogenicity of neuroblastoma cells (14). To delineate the mechanisms of DLK1-mediated signal transduction, we performed coimmunoprecipitation to identify proteins that interact with DLK1 (Fig. 1A). Upon electrophoresis separation and mass spectrometry, we identified PHB2 as a potential interacting protein with DLK1. PHB2 has been implicated in a wide range of cellular processes, including proliferation, apoptosis, transcription, mitochondrial protein folding, and cell surface receptor signaling. Its pleiotropic functions are mirrored by its broad subcellular distribution in plasma membrane, nucleus, and cytoplasm, in addition to its predominant localization in the mitochondria (18, 20). PHB2 primarily forms a complex with PHB1 that shares approximately 50% amino acid sequence identity with PHB2. Using a series of coimmunoprecipitation, we found that DLK1 can interact with both PHB1 and PHB2 under either normoxic or hypoxic (1% O2) conditions (Fig. 1B). It is worth noting that only a small portion of DLK1 is coimmunoprecipitated with PHB1 or PHB2, compared with the levels of DLK1 in whole-cell extract. Consistent with this observation, confocal microscopy analysis showed a low level (<1%) of colocalization between DLK1 and PHBs (Fig. 1C). A similarly low level of colocalization was also found under hypoxic conditions (data not shown). The interaction between DLK1 and PHBs is likely regulated spatially and temporally, especially at the plasma membrane. In addition to neuroblastoma cells, DLK1–PHB interaction also occurs in other cell types, including the murine nontumor cell line 3T3-L1 (Fig. 3), HepG2 human hepatocellular carcinoma cells and primary human brain tumor cells (Supplementary Fig. S1), suggesting an important role of DLK1–PHB interaction in a variety of cell types.
Because DLK1 is a transmenbrane protein, we examined whether the DLK1-cyto domain is involved in interaction with PHBs. Toward this end, we ectopically expressed (Fig. 2A) DLK1-FL, DLK1 extracellular domain (DLK1-EC), DLK1 without its intracellular domain (DLK1-ΔIC), and DLK1 with mutations of two putative phosphorylation sites (tyrosine 339-to-phenoalanine and serine 355-to-alanine) in its cytoplasmic domain (DLK1-DM), respectively, in SK-N-ER (ER) cells with low levels of endogenous DLK1. As shown in Fig. 2B, anti-DLK1 antibodies were able to pull-down both PHB1 and PHB2 in ER cells transfected with DLK1-FL, DLK1-EC, and vector control, respectively. However, the interaction between DLK1 and PHBs was strongly reduced in ER cells transfected with DLK1-DM and DLK1-ΔIC. Consistent with this observation, anti-PHB2 antibodies failed to pull-down DLK1 when DLK1-DM or DLK1-ΔIC was overexpressed (Fig. 2C). To determine whether the DLK1-cyto domain directly interacts with PHBs, we ectopically expressed Flag-tagged DLK1 full-length (Flag-DLK1-FL) or DLK1-cyto domain in 293T cells and found that the Flag-tagged DLK1-Cyto, in addition to Flag-DLK1-FL, coimmunoprecipitates with PHB2 (Fig. 2D). Collectively, these data strongly indicate that DLK1-cyto domain is directly involved in the interaction with the PHB complex. Our data further demonstrate an involvement of tyrosine 339 and serine 355 of the cytoplasmic domain in the interaction between DLK1 and PHBs.
DLK1-EC domain increases DLK1–PHB2 interaction in a paracrine manner
It has been reported that DLK1-FL can be enzymatically cleaved in its juxtamembrane region by TNF-α–converting enzyme (TACE or ADAM-17) to release the extracellular domain as a soluble form of DLK1 (22). Functionally, the large soluble DLK1 containing the entire extracellular domain potentiates inhibition of adipogenic differentiation (23, 24). We found that DLK1 was cleaved from BE(2)C cells and released into the culture medium (Fig. 3A, lane 2). We hypothesized that the soluble DLK1 might exert autocrine/paracrine effects on neuroblastoma cells to regulate their cancer cell stemness. To test this hypothesis, we treated cells with conditioned medium from ER cells that ectopically expressed the secreted DLK1-EC domain (DLK1-EC, Fig. 3A, lane 4). Interestingly, the conditioned medium containing soluble DLK1-EC strongly enhanced the interaction between DLK1 and PHB2 in BE(2)C cells, as shown by coimmunoprecipitation (Fig. 3B, lane 3 vs. lane 5). The same result was also obtained in 3T3-L1 cells (Fig. 3B). Furthermore, the DLK1-EC–conditioned medium was able to increase the clonogenic potential of neuroblastoma cells (Fig. 3C and D), which is consistent with the role of soluble DLK1 in inhibition of cellular differentiation. Our data suggest that the soluble extracellular domain of DLK1 may facilitate stemness maintenance in part by promoting DLK1–PHB interaction in an autocrine/paracrine manner.
DLK1 modulates mitochondrial functions
Because the PHB complex is predominantly localized in mitochondria and plays a critical role mitochondrial biogenesis, assembly, and functions (18, 19), we set out to investigate whether interaction between DLK1 and PHBs would have a strong impact on mitochondrial functions. First, we examined the role of DLK1 in the regulation of mitochondrial membrane potential using JC-1, a mitochondrion-specific dye that forms the fluorescent (emission = 590 nm) J-aggregates in mitochondria with high membrane potential (25, 26). Here, we observed that the high DLK1-expressing BE(2)C cells had much fewer J-aggregates–positive (JC-1+) cells than did the low DLK1-expressing ER cells (Supplementary Fig. S2). Interestingly, when the endogenous DLK1 expression was knocked down in BE(2)C cells using short hairpin RNA (shRNA), the numbers of JC-1+ BE(2)C cells were significantly increased (Fig. 4A). Consistent with this observation, overexpression of DLK1-DM or DLK1-ΔIC that lacks interaction with PHBs also significantly increased the population of JC-1+ BE(2)C cells (Fig. 4B). Conversely, overexpression of DLK1-FL or DLK1-EC in ER cells resulted in decreases of JC-1 fluorescence intensity, indicating reduced mitochondrial membrane potential (Fig. 4C). Collectively, these data suggest that DLK1 is capable of regulating mitochondrial membrane potential via interaction with PHBs.
It has been shown that high mitochondrial membrane potential may lead to increased production of ROS (27). We therefore investigated the effects of DLK1 on ROS production using the fluorescent dye CMH2XROS as an indicator. Knocking down DLK1 expression in BE(2)C cells led to increases in CMH2XROS fluorescence (Fig. 4D and E), indicating increased mitochondrial ROS production. This result is consistent with increased numbers of JC-1+ cells with high mitochondrial membrane potential upon DLK1 knockdown (Fig. 4A). However, overexpression of DLK1 in the DLK1-low ER cells did not significantly change CMH2XROS fluorescence (data not shown). It is likely that overexpression of DLK1 is not sufficient to reduce mitochondrial membrane potential in ER cells. Nonetheless, these data collectively demonstrate that DLK1 has the potential to regulate the mitochondrial function.
PHB1 and PHB2 are required for maintaining tumor cell clonogenicity and self-renewal.
Previously, we demonstrated that DLK1 played a necessary and sufficient role in maintaining neuroblastoma cell stemness, self-renewal, and clonogenic potential (14, 15). The interaction between DLK1 and PHBs suggests that the PHB complex may also play a role in regulation of cancer cell stemness. To test this hypothesis, we investigated the role of PHB1 and PHB2 in the regulation of self-renewal using the tumor sphere formation assay and clonogenic potential using the clonogenic assay, as described in our previous study (14). When treated with siRNAs against the PHB1 or PHB2 gene, BE(2)C cells formed significantly fewer tumor spheres in the serum-free suspension culture (Fig. 5A and 5C), indicating reduced self-renewal potential. Although the inhibition by siPHB1 was similar to that by siDLK1, siPHB2 resulted in the strongest inhibition of tumor sphere formation. Consistent with these findings, siPHB1 and siPHB2 significantly decreased the clonogenic growth of BE(2)C cells, again with siPHB2 eliciting the strongest inhibition (Fig. 5D). Similar observations were also found in the hepatocellular carcinoma HepG2 cells and human breast cancer MCF7 cells (Supplementary Figs. S3 and S4). It is worth noting that all siRNA treatments did not reduce cell viability. These data strongly indicate an important role of PHB1 and especially PHB2 in the regulation of stem cell self-renewal and clonogenic growth.
We further found (Fig. 5C and Supplementary Fig. S5) that ectopic expression of either PHB1 or PHB2 was able to rescue tumor sphere–forming potential of siDLK1-treated BE(2)C cells, although PHB1 seems to negatively affect tumor sphere formation. The clonogenic potential of the siDLK1-treated cells was also partially improved by ectopically expressed PHB1 or PHB2 (Fig. 5D and Supplementary Fig. S5). In contrast, DLK1 ectopic expression did not seem to have a strong impact on clonogenic potential of siPHB1- or siPHB2-treated cells. These findings suggest that PHBs likely function downstream of DLK1 to regulate different aspects of stemness.
It is interesting to note that PHB1 protein levels were strongly reduced in siPHB2-treated cells without affecting PHB1 mRNA levels; and conversely, PHB2 levels were strongly decreased in siPHB1-treated cells with no decrease of PHB2 mRNA (Fig. 5A and B). This observation suggests that formation of the PHB1–PHB2 heterodimeric complex is critical for the stability of each subunit. Even more interestingly, knockdown of PHB1 or PHB2 also resulted in significant decrease of DLK1 (Fig. 5A, lanes 4 and 5). In contrast, siDLK1 did not decrease the levels of PHB1 or PHB2 (Fig. 5A, lanes 1–3). Furthermore, the DLK1-cyto domain, despite its interaction with PHBs, did not affect PHB1 and PHB2 protein expression (Supplementary Fig. S6). Because neither siPHB1 nor siPHB2 negatively affected DLK1 mRNA levels (Fig. 5B), the interaction between DLK1 and the PHB complex may exert a strong impact on the stability of DLK1 protein. It is also probable that other PHB-dependent pathways may regulate DLK1 stability via yet unknown posttranslational mechanisms.
DLK1 is overexpressed in several types of cancers (7–9, 14). However, it remains largely unknown whether and how DLK1, as a transmembrane protein, can influence intracellular signal transduction. In this work, we have discovered that DLK1 interacts with the PHB1–PHB2 complex via its cytoplasmic domain, a previously unknown molecular interaction that is involved in the regulation of cancer cell stemness.
The PHB complex, primarily localized in the inner membrane of mitochondria, plays a critical role in the maintenance of mitochondrial morphology and normal functions (18, 19). Knocking down PHB1 in endothelial cells (28) or knocking down PHB2 in mouse embryonic fibroblasts (29) results in depolarization of mitochondrial membranes. On the other hand, ectopic PHB1 expression facilitates the maintenance of mitochondrial membrane potential in cardiomyocytes (30). In this study, we have found that overexpression of DLK1 reduces mitochondrial membrane potential in neuroblastoma cells, as indicated by decreased formation and, hence, fluorescence of J-aggregates. Conversely, knocking down DLK1 results in increased formation of the J-aggregates and, hence, elevated mitochondrial membrane potential. Consistent with the observations that DLK1-cyto domain is required for interaction with PHBs, the two dominant-negative mutants, DLK1-ΔIC and DLK1-DM, also increase the J-aggregate formation. Because excessively high mitochondrial membrane potential (>150 mV) can lead to increased formation of free radicals and ROS (27), our data suggest that DLK1, via interaction with PHBs, helps to prevent the development of detrimentally high mitochondrial membrane potential. Consistent with this notion, our data further demonstrate that knocking down DLK1 leads to increased ROS production, as indicated by elevated fluorescence of CMH2XROS.
Interestingly, we have found that both PHB proteins, especially, PHB2, are required for maintaining cancer stem cell characteristics, including self-renewal and clonogenic potential. This novel observation is consistent with the findings that both PHB1 and PHB2 are essential for embryonic development (31, 32). Other reports have shown that PHB2 represses myogenic differentiation (33) and PHB1 prevents ROS-induced endothelial cell senescence (28). A very recent report has shown that knocking down PHB1 and/or PHB2 reduces cell growth and colony formation in hepatocellular carcinoma cells (34) and the cervical cancer HeLa cells (35). Collectively, these data suggest an important role of PHBs in the maintenance of the stem cell state. Our data further demonstrate that the interaction between PHB and DLK1 facilitates self-renewal and enhances clonogenic growth of cancer cells. It is worth noting that the siPHB-induced downregulation of sphere formation and clonogenicity could potentially be due in part to the decrease of DLK1 protein in siPHB-treated cells. On the other hand, PHBs seem to function downstream of DLK1 because ectopic expression of PHB1 or PHB2 can, to different extents, rescue the siDLK1-dependent decreases in sphere-forming potential and clonogenicity. These observations suggest that interaction between DLK1 and PHB proteins is likely to be complex and each protein may also have independent functions in the regulation of cancer cell stemness.
Although elevated PHB levels have been found in many types of human cancers, it remains controversial whether PHBs promote or suppress tumor growth and/or malignant progression. To a large extent, these controversies may be due to the highly pleotropic functions of PHB1 and PHB2 in the regulation of proliferation, cell survival, and gene transcription. It is likely that their exact functions are determined by protein–protein interactions in different subcellular locations because PHBs are also found in plasma membrane, cytoplasm, and nucleus (29, 32, 36, 37). Both PHB1 and PHB2 are capable of repressing gene transcription when located in nucleus (32, 36, 38, 39), which may partly explain the tumor suppressive function of PHB. On the other hand, the PHB1–PHB2 complex interacts with c-Raf at the plasma membrane and facilitates the Ras-induced c-Raf activation (37, 40). Our data presented herein suggest that the DLK1-cyto domain interacts with the PHB1–PHB2 complex at the plasma membrane. Our previous studies have shown that downregulation of DLK1 expression or its function results in sustained activation of the ERK pathway (14, 15). It will be interesting to determine whether DLK1 interferes with Ras–Raf–PHB interaction. Nonetheless, the DLK1–PHB interaction elicits a broad impact on cellular functions, including mitochondrial function, self-renewal, and clonogenic growth of cancer cells. Our study has thus provided a new mechanism underlying the protumorigenic role of DLK1 and PHBs.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Conception and design: Z. Yun
Development of methodology: A. Begum, Q. Lin, Y. Kim, Z. Yun
Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): A. Begum, Q. Lin, C. Yu, Z. Yun
Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): A. Begum, Q. Lin, Z. Yun
Writing, review, and/or revision of the manuscript: A. Begum, Q. Lin, Z. Yun
Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): Z. Yun
Study supervision: Q. Lin, Z. Yun
The authors thank Mingyeah Hu for technical assistance, Dr. Jiangbing Zhou of Yale University for providing primary human brain tumor cells, Dr. Nai-Kong V. Cheung of Memorial Sloan-Kettering Cancer Center, for SK-N-ER cells, Dr. Robert Ross of Fordham University for BE(2)C cells, Dr. Ravi Bhatia of City of Hope National Medical Center for retroviral constructs of DLK1-FL, DLK1-EC, and DLK1-ΔCyto, as well as Dr. Valerie Bosch of DKFZ, for Flag-PHB1, Flag-PHB2, anti-PHB1, and anti-PHB2 antisera. The authors also thank Lisa Cabral for her excellent assistance with the article.
This work was supported by a grant from the NIH (to Z. Yun; R01CA125021). Y. Kim was supported in part by an institutional postdoctoral training grant (T32) from the NIH and the Anna Fuller Fund Fellowship from Yale School of Medicine.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.