Construction and characterization of multiple human colon cancer cell lines for inducibly regulated gene expression

Construction and characterization of regulated cycle inhibiting factors induced upon Tet-On system in human colon cancer cell lines

A previous study has proven that cycle inhibiting factors (Cifs) inhibit Cullin E3 ubiquitin ligases, resulting in cell cycle arrest. More importantly, Cifs are also involved in cancer progression by deamidating Nedd8. Here we aimed to explore a novel insight into the treatment implications of Cifs in colon cancers by Tet-on system. The anticancer activity of Cif by doxycycline induction was investigated in the colon cell lines based upon Tet-On system. The expression of Cif in the colon cancer cells was determined by western blot. Furthermore, the cell viability and flow cytometry analysis were respectively performed to evaluate the cell proliferation and survival of colon cells. More importantly, the p21 and p27 levels were also evaluated after the induction of Cif with Tet-On system. Multiple clones of colon cancer cells for doxycycline-regulated Cif expression were constructed for maintenance purposes including HCT116 and SW480 cell lines. The result of western blot displayed good inducibility of expressing Cif in the cell lines. The clones with Cif preserved their transformed phenotype compared with the control group (clones with GFP or with Cif), in terms of the inhibition of cancer cell proliferation and survival. Furthermore, western blot analysis showed that p27 and p21 were accumulated in the clones with Cif, compared with the colon cancer cell lines with GFP or with Cif. Using the Tet-On system, we developed an efficient approach toward generation of colon cancer cells induced with Cif. These engineered colons tightly controlled Cif expression in vitro, which is a good inducible model system for cancer treatment.

Caspase-8 expression and its Src-dependent phosphorylation on Tyr380 promote cancer cell neoplastic transformation and resistance to anoikis

Caspase-8 expression is lost in a small percentage of tumors suggesting that the retention of its functionality may positively contribute to tumor progression. Consistently, several non-apoptotic functions of Caspase-8 have been identified and Caspase-8 has been shown to modulate cell adhesion, migration and to promote tumor progression. We have previously identified the Src-dependent phosphorylation of Caspase-8 on Tyr380 as a molecular mechanism to downregulate the proapoptotic function of Caspase-8; this phosphorylation occurs in colon cancer and may promote cell migration in neuroblastoma cell lines. However, the occurrence of Caspase-8 phosphorylation on Tyr380 and its significance in different carcinoma cellular models, have not been clarified yet. Here we show that Caspase-8 expression may promote cell transformation in glioblastoma and in hepatocarcinoma cell lines. In these systems Caspase-8 is phosphorylated on Tyr380 in a Src kinase dependent manner and this phosphorylation is required for transformation and it is enhanced by hypoxic conditions. Using a cancer cellular model characterized by Src constitutive activation engineered to express either Caspase-8-wt or Caspase-8-Y380F we could show that Caspase-8 expression and its phosphorylation on Tyr380, but not its enzymatic activity, promote in vitro cell transformation and resistance to anoikis. This work demonstrates a dual role for Caspase-8 in cancer, suggesting that Tyr380 phosphorylation may represent a molecular switch to hijack its activity from tumor suppressor to tumor promoter.

A caspase-3 'death-switch' in colorectal cancer cells for induced and synchronous tumor apoptosis in vitro and in vivo facilitates the development of minimally invasive cell death biomarkers

Novel anticancer drugs targeting key apoptosis regulators have been developed and are undergoing clinical trials. Pharmacodynamic biomarkers to define the optimum dose of drug that provokes tumor apoptosis are in demand; acquisition of longitudinal tumor biopsies is a significant challenge and minimally invasive biomarkers are required. Considering this, we have developed and validated a preclinical 'death-switch' model for the discovery of secreted biomarkers of tumour apoptosis using in vitro proteomics and in vivo evaluation of the novel imaging probe [(18)F]ML-10 for non-invasive detection of apoptosis using positron emission tomography (PET). The 'death-switch' is a constitutively active mutant caspase-3 that is robustly induced by doxycycline to drive synchronous apoptosis in human colorectal cancer cells in vitro or grown as tumor xenografts. Death-switch induction caused caspase-dependent apoptosis between 3 and 24 hours in vitro and regression of 'death-switched' xenografts occurred within 24 h correlating with the percentage of apoptotic cells in tumor and levels of an established cell death biomarker (cleaved cytokeratin-18) in the blood. We sought to define secreted biomarkers of tumor apoptosis from cultured cells using Discovery Isobaric Tag proteomics, which may provide candidates to validate in blood. Early after caspase-3 activation, levels of normally secreted proteins were decreased (e.g. Gelsolin and Midkine) and proteins including CD44 and High Mobility Group protein B1 (HMGB1) that were released into cell culture media in vitro were also identified in the bloodstream of mice bearing death-switched tumors. We also exemplify the utility of the death-switch model for the validation of apoptotic imaging probes using [(18)F]ML-10, a PET tracer currently in clinical trials. Results showed increased tracer uptake of [(18)F]ML-10 in tumours undergoing apoptosis, compared with matched tumour controls imaged in the same animal. Overall, the death-switch model represents a robust and versatile tool for the discovery and validation of apoptosis biomarkers.

Both stromal cell and colonocyte epidermal growth factor receptors control HCT116 colon cancer cell growth in tumor xenografts

Colon cancer growth requires growth-promoting interactions between malignant colonocytes and stromal cells. Epidermal growth factor receptors (EGFR) are expressed on colonocytes and many stromal cells. Furthermore, EGFR is required for efficient tumorigenesis in experimental colon cancer models. To dissect the cell-specific role of EGFR, we manipulated receptor function on stromal cells and cancer cells. To assess the role of stromal EGFR, HCT116 human colon cancer cells were implanted into immunodeficient mice expressing dominant negative (DN) Egfr(Velvet/+) or Egfr(+/+). To assess the role of cancer cell EGFR, HCT116 transfectants expressing inducible DN-Egfr were implanted into immunodeficient mice. To dissect EGFR signals in vitro, we examined colon cancer cells in monoculture or in cocultures with fibroblasts for EGFR transactivation and prostaglandin synthase 2 (PTGS2) induction. EGFR signals were determined by blotting, immunostaining and real-time PCR. Tumor xenografts in Egfr(Velvet/+) mice were significantly smaller than tumors in Egfr(+/+) mice, with decreased proliferation (Ki67) and increased apoptosis (cleaved caspase-3) in cancer cells and decreased stromal blood vessels. Mouse stromal transforming growth factor alpha (TGFA), amphiregulin (AREG), PTGS2 and Il1b and interleukin-1 receptor 1 (Il1r1) transcripts and cancer cell beta catenin (CTNNB1) and cyclin D1 (CCND1) were significantly lower in tumors obtained from Egfr(Velvet/+) mice. DN-EGFR HCT116 transfectants also formed significantly smaller tumors with reduced mouse Areg, Ptgs2, Il1b and Il1r1 transcripts. Coculture increased Caco-2 phospho-active ERBB (pERBB2), whereas DN-EGFR in Caco-2 cells suppressed fibroblast PTGS2 and prostaglandin E2 (PGE2). In monoculture, interleukin 1 beta (IL1B) transactivated EGFR in HCT116 cells. Stromal cell and colonocyte EGFRs are required for robust EGFR signals and efficient tumor growth, which involve EGFR-interleukin-1 crosstalk.

Absent in Melanoma 2 (AIM2) is an important mediator of interferon-dependent and -independent HLA-DRA and HLA-DRB gene expression in colorectal cancers

Absent in Melanoma 2 (AIM2) is a member of the HIN-200 family of hematopoietic, IFN-inducible, nuclear proteins, associated with both, infection defense and tumor pathology. Recently, AIM2 was found to act as a DNA sensor in innate immunity. In addition, we and others have previously demonstrated a high frequency of AIM2-alterations in microsatellite unstable (MSI-H) tumors. To further elucidate AIM2 function in colorectal tumors, we here addressed AIM2-responsive target genes by microarray based gene expression profiling of 22 244 human genes. A total of 111 transcripts were significantly upregulated, whereas 80 transcripts turned out to be significantly downregulated in HCT116 cells, constitutively expressing AIM2, compared with AIM2-negative cells. Among the upregulated genes that were validated by quantitative PCR and western blotting we recognized several interferon-stimulated genes (ISGs: IFIT1, IFIT2, IFIT3, IFI6, IRF7, ISG15, HLA-DRA, HLA-DRB, TLR3 and CIITA), as well as genes involved in intercellular adhesion and matrix remodeling. Expression of ISGs correlated with expression of AIM2 in 10 different IFN-γ treated colorectal cancer cell lines. Moreover, small interfering RNA-mediated knock-down of AIM2 resulted in reduced expression of HLA-DRA, HLA-DRB and CIITA in IFN-γ-treated cells. IFN-γ independent induction of HLA-DR genes and their encoded proteins was also demonstrated upon doxycyclin-regulated transient induction of AIM2. Luciferase reporter assays revealed induction of the HLA-DR promoter upon AIM2 transfection in different cell lines. STAT-signaling was not involved in IFN-γ independent induction of ISGs, arguing against participation of cytokines released in an autostimulating manner. Our data indicate that AIM2 mediates both IFN-γ dependent and independent induction of several ISGs, including genes encoding the major histocompatibility complex (MHC) class II antigens HLA-DR-α and -β. This suggests a novel role of the IFN/AIM2/ISG cascade likewise in cancer cells.

Tetracycline-inducible protein expression in pancreatic cancer cells: Effects of CapG overexpression

AIM: To establish stable tetracycline-inducible pancreatic cancer cell lines.

METHODS: Suit-2, MiaPaca-2, and Panc-1 cells were transfected with a second generation reverse tetracycline-controlled transactivator protein (rtTA2S-M2), under the control of either a cytomegalovirus (CMV) or a chicken β-actin promoter, and the resulting clones were characterised.

RESULTS: Use of the chicken (β-actin) promoter proved superior for both the production and maintenance of doxycycline-inducible cell lines. The system proved versatile, enabling transient inducible expression of a variety of genes, including GST-P, CYP2E1, S100A6, and the actin capping protein, CapG. To determine the physiological utility of this system in pancreatic cancer cells, stable inducible CapG expressors were established. Overexpressed CapG was localised to the cytoplasm and the nuclear membrane, but was not observed in the nucleus. High CapG levels were associated with enhanced motility, but not with changes to the cell cycle, or cellular proliferation. In CapG-overexpressing cells, the levels and phosphorylation status of other actin-moduating proteins (Cofilin and Ezrin/Radixin) were not altered. However, preliminary analyses suggest that the levels of other cellular proteins, such as ornithine aminotransferase and enolase, are altered upon CapG induction.

CONCLUSION: We have generated pancreatic-cancer derived cell lines in which gene expression is fully controllable.

Blocking phosphoinositide 3-kinase activity in colorectal cancer cells reduces proliferation but does not increase apoptosis alone or in combination with cytotoxic drugs

In response to growth factors, class IA phosphoinositide 3-kinases (PI3K) phosphorylate phosphatidylinositol-4,5-bisphosphate, converting it to phosphatidylinositol-3,4,5-trisphosphate to activate protein kinase B/Akt. This is widely reported to promote tumorigenesis via increased cell survival, proliferation, migration, and invasion, and many tumor types, including colorectal cancer, exhibit increased PI3K signaling. To investigate the effect of inhibiting PI3K and as an alternative to the use of small molecular inhibitors of PI3K with varying degrees of selectivity, HT29 and HCT116 colorectal cancer cells bearing mutant PIK3CA were generated that could be induced with doxycycline to express synchronously a dominant negative subunit of PI3K, Deltap85alpha. On induction, decreased levels of phosphorylated protein kinase B were detected, confirming PI3K signaling impairment. Induction of Deltap85alpha in vitro reduced cell number via accumulation in G(0)-G(1) phase of the cell cycle in the absence of increased apoptosis. These effects were recapitulated in vivo. HT29 cells expressing Deltap85alpha and grown as tumor xenografts had a significantly slower growth rate on administration of doxycycline with reduced Ki67 staining without increased levels of apoptotic tissue biomarkers. Furthermore, in vitro Deltap85alpha expression did not sensitize HT29 cells to oxaliplatin- or etoposide-induced apoptosis, irrespective of drug treatment schedule. Further analysis comparing isogenic HCT116 cells with and without mutation in PIK3CA showed no effect of the mutation in either proliferative or apoptotic response to PI3K inhibition. These data show in colorectal cancer cells that PI3K inhibition does not provoke apoptosis per se nor enhance oxaliplatin- or etoposide-induced cell death.

Difluoro analogue of UCS15A triggers activation of exogenously expressed c-Src in HCT 116 human colorectal carcinoma cells

UCS 15A, an antibiotic produced by Streptomyces sp., has been reported to specifically disrupt SH3 domain-mediated interactions in eukaryotic cells. Interestingly, in the case of the non-receptor tyrosine kinase Src, UCS15A was effective in suppressing the SH3 domain-mediated intermolecular rather than intramolecular interactions, and thus prevented Src interactions with certain downstream effectors without affecting Src kinase activity. Here the synthesis of a novel difluoro analogue of UCS15A is described. The effects of this compound (8) on Src activity were tested in HCT 116 colorectal carcinoma cells engineered for inducible expression of c-Src. The presence of compound (8) resulted in the increased activity of the induced c-Src implicating that (8) acts as a c-Src activator in vivo. These observations are supported by computer modelling studies which suggest that the aldehyde group of (8) may covalently bind to a lysine residue in the SH2-kinase linker region situated in the proximity of the SH3 domain, which could promote a conformational change resulting in increased Src activity.

Development of an inducible gene expression system for primary murine keratinocytes

BACKGROUND

The tetracycline (Tet) responsive system is a valuable tool that is routinely used in a wide variety of mammalian cells for regulatable expression of gene products. However, technical difficulties such as harsh selection conditions and extensive screening processes to identify suitably responsive clones limit the generation of stable cell lines. Hence, application of this system in mammalian cells with relatively slow growth rates and/or the capacity to undergo terminal differentiation such as primary mouse keratinocytes is particularly challenging.

OBJECTIVE

To our knowledge, no Tet-responsive stable cell lines have been generated from mouse keratinocytes, presumably due to their sensitivity to selection conditions. Our goal was to utilize a modified and robust Tet-expression system to generate a stable primary mouse keratinocyte cell line. These cells could be then utilized for conditional expression of potentially toxic proteins in an inducible fashion.

METHODS

We utilized a eukaryotic promoter instead of a viral promoter to express a modified reverse tetracycline transactivator in mouse keratinocytes and optimized the selection process for generating stable cell lines.

RESULTS

Here, we report the generation of a stable mouse keratinocyte cell line for Tet-regulated gene expression with minimal leakiness and high degree of Tet responsivity. This mouse keratinocyte cell line was further engineered for generation of a double stable cell line, which expresses the transcription factor AP-2alpha in an inducible manner. Importantly, the selected cells retain their inherent keratinocyte morphology, respond to differentiation signals and exhibit a persistent and highly tunable Tet-inducibility upon continuous culturing.

CONCLUSION

We have generated a tetracycline inducible gene expression model system in mouse epidermal keratinocytes. Such inducible cell lines will serve as valuable in vitro models for future gain-of-function and loss-of-function studies.

Increases in c-Yes expression level and activity promote motility but not proliferation of human colorectal carcinoma cells

Increases in the levels and/or activity of nonreceptor tyrosine kinases c-Src and c-Yes are often associated with colorectal carcinogenesis. The physiological consequences of increased c-Yes activity during the early and late stages of tumorigenesis, in addition to the degree of redundancy between c-Yes and c-Src in colorectal cancer cells, remain elusive. To study the consequences of increases in c-Yes levels and activity in later stages of colorectal carcinogenesis, we developed human colorectal cancer cell lines in which c-Yes levels and activity can be inducibly increased by a tightly controlled expression of wild-type c-Yes or by constitutively active mutants of c-Yes, c-YesY537F, and c-Yes Delta t6aa. c-Yes induction resulted in increased cell motility but did not promote proliferation either in vitro or in vivo. These results suggest that in later stages of colorectal carcinogenesis, elevations in c-Yes levels/activity may promote cancer spread and metastasis rather than tumor growth.

Protein kinase C delta is phosphorylated on five novel Ser/Thr sites following inducible overexpression in human colorectal cancer cells

Phosphorylation plays an important role in regulation of protein kinase C delta (PKCdelta). To date, three Ser/Thr residues (Thr 505, Ser 643, and Ser 662) and nine tyrosine residues (Tyr 52, Tyr 64, Tyr 155, Tyr 187, Tyr 311, Tyr 332, Tyr 512, Tyr 523, and Tyr 565) have been defined as regulatory phosphorylation sites for this protein (rat PKCdelta numbering). We combined doxycycline-regulated inducible gene expression technology with a hypothesis-driven mass spectrometry approach to study PKCdelta phosphorylation pattern in colorectal cancer cells. We report identification of five novel Ser/Thr phosphorylation sites: Thr 50, Thr 141, Ser 304, Thr 451, and Ser 506 (human PKCdelta numbering) following overexpression of PKCdelta in HCT116 human colon carcinoma cells grown in standard tissue culture conditions. Identification of potential novel phosphorylation sites will affect further functional studies of this protein, and may introduce additional complexity to PKCdelta signaling.

Increases in c-Src expression level and activity do not promote the growth of human colorectal carcinoma cells in vitro and in vivo

The levels and activity of c-Src in colorectal cancer cells increase steadily during the course of colorectal carcinogenesis and are most highly elevated in advanced metastatic disease. However, the effects of increases in c-Src activity on the proliferation of colorectal cancer cells during early and late stages of tumorigenesis remain elusive. To study the consequences of increases in c-Src levels and activity on the growth of colorectal cancer cells in later stages of colorectal carcinogenesis, we developed human colorectal cancer cell lines in which c-Src levels and activity could be inducibly increased by a tightly controlled expression of wild-type c-Src or of the constitutively active mutant of c-Src, c-SrcY527F. Src induction activated multiple signaling pathways (often associated with a proliferative response) but promoted neither cell proliferation in vitro nor tumor growth in a xenograft model in vivo. These results indicate that, in more advanced stages of colorectal carcinogenesis, increases in c-Src levels and activity are likely to have functions other than the direct promotion of tumor growth.

Activated c-SRC in ductal carcinoma in situ correlates with high tumour grade, high proliferation and HER2 positivity

Overexpression and/or activity of c-Src non-receptor tyrosine kinase is associated with progression of several human epithelial cancers including breast cancer. c-Src activity in ‘pure’ ductal carcinoma in situ (DCIS) was measured to assess whether this predicts recurrence and/or correlates with HER2 expression and other clinical parameters. Activated c-Src levels were evaluated in DCIS biopsies from 129 women, with median follow-up at 60 months. High levels of activated c-Src correlated with HER2 positivity, high tumour grade, comedo necrosis and elevated epithelial proliferation. In univariate analysis, high activated c-Src level associated with lower recurrence-free survival at 5 years (P=0.011). Thus, high c-Src activity may identify a subset of DCIS with high risk of recurrence or progression to invasive cancer where therapeutics targeting c-Src may benefit this patient subset.

Generation of cells expressing improved doxycycline-regulated reverse transcriptional transactivator rtTA2S-M2

Tet-on cell lines engineered to stably express doxycycline (Dox)-regulated reverse transcriptional transactivator (rtTA) have many applications in biomedical research and biotechnology. Unfortunately, construction and maintenance of such cells often proves to be costly, labor intensive and ineffective. Moreover, the Tet-on clones generated using standard methodology were often unstable and frequently displayed significantly changed physiological properties compared with their parental cells. Here we describe an optimized protocol for generation of Tet-on cells. The protocol is based on the use of a recently developed pN1p beta actin-rtTA2S-M2-IRES-EGFP vector (where IRES is an internal ribosome entry site) and permits relatively inexpensive construction of many Tet-on clones with essentially 100% efficiency. The method is well suited for 'difficult' cell lines displaying genetic instability and high levels of epigenetic silencing. The constructed Tet-on cells remain stable with time in the absence of any selection agents, are easy to monitor and preserve the characteristics of parental cells. The protocol can be completed in 2 months.

Comparison of phosphatidylinositol-3-kinase signalling within a panel of human colorectal cancer cell lines with mutant or wild-type PIK3CA

Recent studies have identified conserved missense mutations in PIK3CA, the gene encoding the catalytic phosphatidylinositol-3-kinase subunit p110alpha, in a variety of human cancers. Further investigation demonstrated that PIK3CA mutations lead to increased basal phosphatidylinositol-3-kinase activity, promoting cell growth and invasion [Samuels, Y., Diaz, L.A., Jr., Schmidt-Kittler, O., Cummins, J.M., Delong, L., Cheong, I., Rago, C., Huso, D.L., Lengauer, C., Kinzler, K.W., Vogelstein, B. and Velculescu, V.E. (2005) Mutant PIK3CA promotes cell growth and invasion of human cancer cells. Cancer Cell 7, 561-573]. A panel of commonly used colorectal cancer cell lines was screened for these PIK3CA mutations. Constitutive and IGF-1-stimulated phosphatidylinositol-3-kinase activity, signal response and duration were assessed. In the assays used no differences distinguished cells carrying PIK3CA mutations indicating that these mutations did not significantly alter growth factor stimulated or steady state phosphatidylinositol-3-kinase activity in normal cell culture conditions.