Growth and EGFR Regulation in Breast Cancer Cells by Vitamin D and Retinoid Compounds

Development of a reliable cell-based reporter gene assay to measure the bioactivity of anti-HER2 therapeutic antibodies

Human epidermal growth factor receptor 2 (HER2) is a key player in the pathogenesis and progression of breast cancer and is currently a primary target for breast cancer immunotherapy. Bioactivity determination is necessary to guarantee the safety and efficacy of therapeutic antibodies targeting HER2. Nevertheless, currently available bioassays for measuring the bioactivity of anti-HER2 mAbs are either not representative or have high variability. Here, we established a reliable reporter gene assay (RGA) based on T47D-SRE-Luc cell line that expresses endogenous HER2 and luciferase controlled by serum response element (SRE) to measure the bioactivity of anti-HER2 antibodies. Neuregulin-1 (NRG-1) can lead to the heterodimerization of HER2 on the cell membrane and induce the expression of downstream SRE-controlled luciferase, while pertuzumab can dose-dependently reverse the reaction, resulting in a good dose-response curve reflecting the activity of the antibody. After optimizing the relevant assay parameters, the established RGA was fully validated based on ICH-Q2 (R1), which demonstrated that the method had excellent specificity, accuracy, precision, linearity, and stability. In summary, this robust and innovative bioactivity determination assay can be applied in the development and screening, release control, biosimilar assessment and stability studies of anti-HER2 mAbs.

EGFR Transgene Stimulates Spontaneous Formation of MCF7 Breast Cancer Cells Spheroids with Partly Loss of HER3 Receptor

Multicellular spheroids with 3D cell-cell interactions are a useful model to simulate the growth conditions of cancer. There is evidence that in tumor spheroids, the expression of various essential molecules is changed compared to the adherent form of cell cultures. These changes include growth factor receptors and ABC transporters and result in the enhanced invasiveness of the cells and drug resistance. It is known that breast adenocarcinoma MCF7 cells can spontaneously form 3D spheroids and such spheroids are characterized by high expression of EGFR/HER2, while the natural phenotype of MCF7 cells is EGFRlow/HER2low. Therefore, it was interesting to reveal if high epidermal growth factor receptor (EGFR) expression is sufficient for the conversion of adherent MCF7 to spheroids. In this study, an MCF7 cell line with high expression of EGFR was engineered using the retroviral transduction method. These MCF7-EGFR cells assembled in spheroids very quickly and grew predominantly as a 3D suspension culture with no special plates, scaffolds, growth supplements, or exogenous matrixes. These spheroids were characterized by a rounded shape with a well-defined external border and 100 µM median diameter. The sphere-forming ability of MCF7-EGFR cells was up to 5 times stronger than in MCF7wt cells. Thus, high EGFR expression was the initiation factor of conversion of adherent MCF7wt cells to spheroids. MCF7-EGFR spheroids were enriched by the cells with a cancer stem cell (CSC) phenotype CD24-/low/CD44- in comparison with parental MCF7wt cells and MCF7-EGFR adhesive cells. We suppose that these properties of MCF7-EGFR spheroids originate from the typical features of parental MCF7 cells. We showed the decreasing of HER3 receptors in MCF7-EGFR spheroids compared to that in MCFwt and in adherent MCF7-EGFR cells, and the same decrease was observed in the MCF7wt spheroids growing under the growth factors stimulation. To summarize, the expression of EGFR transgene in MCF7 cells stimulates rapid spheroids formation; these spheroids are enriched by CSC-like CD24-/CD44- cells, they partly lose HER3 receptors, and are characterized by a lower potency in drug resistance pomp activation compared to MCF7wt. These MCF7-EGFR spheroids are a useful cancer model for the development of anticancer drugs, including EGFR-targeted therapeutics.

Immunomodulatory effects of vitamin D3 on gene expression of MDGF, EGF and PDGFB in endometriosis

RESEARCH QUESTION

Endometriosis, an inflammatory disease, is assumed to be associated with an increased production of growth-related cytokines. Based on the emerging immunomodulatory role of vitamin D3 in different inflammatory conditions, this study aimed to examine its modulatory effect on the expression levels of the genes for platelet-derived growth factor-B (PDGFB), monocyte/macrophage-derived growth factor (MDGF, also known as PPBP) and epidermal growth factor (EGF) in peritoneal fluid mononuclear cells (PFMC) in women with and without endometriosis.

DESIGN

PFMC from 10 women with endometriosis and 10 control participants were treated with vitamin D3.The gene expression levels of PDGFB, MDGF and EGF were measured 6, 24 and 48 h following vitamin D3 administration using real-time PCR.

RESULTS

Gene expression levels of EGF and PDGFB were higher in the PFMC of women with endometriosis than the control group (P = 0.006, P < 0.001, respectively). Although MDGF expression showed an increase in the endometriosis group compared with non-endometriotic controls, no significant difference was found. Vitamin D3 significantly decreased EGF expression at 6, 24 and 48 h (P < 0.001, P < 0.001 and P = 0.007, respectively), MDGF at 24 and 48 h (P < 0.001 and P = 0.009, respectively) and PDGFB at 6 h (P = 0.047) in the endometriosis group. Vitamin D3 treatment had no significant effect on expression of the genes in the PFMC of non-endometriotic women.

CONCLUSIONS

The study concluded that PDGFB and EGF gene expression increases in endometriosis, and vitamin D3 could markedly decrease this expression, suggesting its therapeutic potential in endometriosis.

LXRα-mediated downregulation of EGFR suppress colorectal cancer cell proliferation

Liver X receptors (LXRs) are members of the nuclear receptor family, including the LXRα (NR1H3) and LXRβ (NR1H2) subtypes, which are related to the metabolism of glucose and cholesterol and possess anti-inflammatory functions. Mounting evidence has linked LXRs to the inhibition of cell proliferation in a variety of cancers. We revealed a differential distribution for NR1H3, but not for NR1H2, in colorectal cancer and adjacent normal tissues. We found that NR1H3 enhanced the inhibitory action of GW3965, an agonist of LXRs, on the proliferation of colorectal cancer cells. Upregulation of NR1H3 enhanced the inhibition of cell proliferation by GW3965 while silencing of NR1H3 attenuated the inhibitory effect of GW3965 on cell proliferation. Bioinformatic prediction and luciferase assays showed that NR1H3 was able to inhibit the activity of the epidermal growth factor receptor (EGFR) promoter. Moreover, we demonstrated that activation of NR1H3 inhibited the growth of transplanted tumors in an animal experiment, with the inhibition accompanied by downregulation of EGFR. Our findings suggest that NR1H3 controls cell proliferation by affecting EGFR promoter activity. The high expression of EGFR was due to the downregulation of NR1H3 which is a novel molecular mechanism in the development of colorectal cancer.

Predictive blood plasma biomarkers for EGFR inhibitor-induced skin rash

Epidermal growth factor receptor overexpression in human cancer can be effectively targeted by drugs acting as specific inhibitors of the receptor, like erlotinib, gefitinib, cetuximab and panitumumab. A common adverse effect is a typical papulopustular acneiform rash, whose occurrence and severity are positively correlated with overall survival in several cancer types. We studied molecules involved in epidermal growth factor receptor signaling which are quantifiable in plasma, with the aim of identifying biomarkers for the severity of rash. With a predictive value for the rash these biomarkers may also have a prognostic value for survival and disease outcome.

The concentrations of amphiregulin, hepatocyte growth factor (HGF) and calcidiol were determined by specific enzyme-linked immunosorbent assays in plasma samples from 211 patients.

We observed a significant inverse correlation between the plasma concentration of HGF and overall survival in patients with an inhibitor-induced rash (p-value = 0.0075; mean overall survival low HGF: 299 days, high HGF: 240 days) but not in patients without rash. The concentration of HGF was also significantly inversely correlated with severity of rash (p-value = 0.00124).

High levels of HGF lead to increased signaling via its receptor MET, which can activate numerous pathways which are normally also activated by epidermal growth factor receptor. Increased HGF/MET signaling might compensate the inhibitory effect of epidermal growth factor receptor inhibitors in skin as well as tumor cells, leading to less severe skin rash and decreased efficacy of the anti-tumor therapy, rendering the plasma concentration of HGF a candidate for predictive biomarkers.

Phosphatidic Acid (PA) can Displace PPARα/LXRα Binding to The EGFR Promoter Causing its Transrepression in Luminal Cancer Cells

The expression of the epidermal growth factor receptor (EGFR) is highly regulated in normal cells, whereas some cancer cells have high constitutive levels. Understanding naturally-occurring ways of downregulating EGFR in cancer cells was investigated. Phosphatidic acid (PA) or Nuclear Receptors (NR) PPARα/RXRα/LXRα, enhance EGFR expression, mediated by the promoter region -856(A) to -226(T). Unexpectedly, the combination of NRs and PA caused repression. PA induces a conformational change in the nuclear receptor PPARα (increase of alpha-helices at the expense of decreasing beta-sheets), as evidenced by circular dichroism. This represses the naturally-enhancing capability of PPARα on EGFR transcription. PPARα-overexpressing cells in the presence of PA > 300 nM or the enzyme that produces it, phospholipase D (PLD), downregulate EGFR expression. The reasons are two-fold. First, PA displaces PPARα binding to the EGFR promoter at those concentrations. Second, NR heterodimer-dependent promoter activity is weakened in the presence of PA in vivo. Since other genes considered (β-catenin, cyclin D3, PLD2 and ACOX-1) are also downregulated with a PA + PPARα combination, the transrepression appears to be a global phenomenon. Lastly, the reported effect is greater in MCF-7 than in MDA-MB-231 breast cancer cells, which could provide a novel basis for regulating excessive expression of EGFR in luminal cancer cells.

Synthesis and biological activity assays of some new N1-(flavon-7-yl)amidrazone derivatives and related congeners

A series of new N1-(flavon-7-yl)amidrazones incorporating N-piperazines and related congeners were synthesized by reacting the hydrazonoyl chloride derived from 7-aminoflavone and 7-amino-2-methylchromen-4-one with the appropriate piperazine. The chemical structures of the newly prepared compounds were confirmed by elemental analyses, (1)H NMR, (13)C NMR, and ESI-HRMS spectral data. The antitumor activity of these compounds was evaluated on breast cancer (MCF-7 and T47D) and Leukemic (K562) cell lines by a cell viability assay utilizing the tetrazolium dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Although with varying degrees, a significant growth inhibitory and cytotoxic effect was observed on all three cancer cell lines. Among the compounds tested compounds, 5a, 15a, and 18b, were the most active against T47D cell line with IC(50) values of 1.42, 1.92, and 2.92 μM, respectively. By using other cancer cell lines and with further characterization of their biological mechanism of action, these compounds could prove to be useful candidates as anticancer drugs.

Thiolated chitosan nanoparticles as a delivery system for antisense therapy: evaluation against EGFR in T47D breast cancer cells

Thiolated chitosan has high transfection and mucoadhesive properties. We investigated the potential of two recently synthesized polymers: NAC-C (N-acetyl cysteine-chitosan) and NAP-C (N-acetyl penicillamine-chitosan) in anticancer drug delivery targeting epidermal growth factor receptor (EGFR). Doxorubicin (DOX) and antisense oligonucleotide (ASOND)-loaded polymer nanoparticles were prepared in water by a gelation process. Particle characterization, drug loading, and drug release were evaluated. To verify drug delivery efficiency in vitro experiments on a breast cancer cell line (T47D) were performed. EGFR gene and protein expression was analyzed by real time quantitative polymerase chain reaction and Western blotting, respectively. A loading percentage of 63% ± 5% for ASOND and 70% ± 5% for DOX was achieved. Drug release data after 15 hours showed that ASOND and DOX were completely released from chitosan-based particles while a lower and more sustained release of only 22% ± 8% was measured for thiolated particles. In a cytosol simulated release medium/reducing environment, such as found intracellularly, polymer-based nanoparticles dissociated, liberating approximately 50% of both active substances within 7 hours. ASOND-loaded polymer nanoparticles had higher stability and high mucoadhesive properties. The ASOND-loaded thiolated particles significantly suppressed EGFR gene expression in T47D cells compared with ASOND-loaded chitosan particles and downregulated EGFR protein expression in cells. This study could facilitate future investigations into the functionality of NAP-C and NAC-C polymers as an efficient ASOND delivery system in vitro and in vivo.

The coupling of epidermal growth factor receptor down regulation by 1alpha,25-dihydroxyvitamin D3 to the hormone-induced cell cycle arrest at the G1-S checkpoint in ovarian cancer cells

1alpha,25-dihydroxyvitamin D3, 1,25(OH)(2)D(3), regulates gene expression through the vitamin D receptor. The present studies identify the epidermal growth factor receptor, EGFR, as a target gene suppressed by 1,25(OH)(2)D(3) in human ovarian cancer cells. The suppression was detected at both mRNA and protein levels in vitamin D-sensitive human ovarian cancer cells. A novel vitamin D response element was identified in intron 1 of the EGFR genome, a known hotspot for its transcriptional regulation. Chromatin immunoprecipitations and reporter gene analyses showed that the intronic DNA element bound to vitamin D receptor and a co-repressor and was functional in mediating transcriptional suppression of EGFR promoter by 1,25(OH)(2)D(3) under stable transfection conditions. Consistent with the EGFR down regulation, 1,25(OH)(2)D(3) suppressed activation of the external signal regulated kinase by epidermal growth factors. Over expression of an active EGFR in vitamin D sensitive ovarian cancer cells caused resistance to 1,25(OH)(2)D(3)-induced growth suppression and diminished the hormonal regulation of cyclin D1, cyclin E, Skp2 and p27, a group of cell cycle regulators that mediate 1,25(OH)(2)D(3)-induced cell cycle arrest at G1-S checkpoint. Taken together, our studies demonstrate that 1,25(OH)(2)D(3) suppresses the response of human ovarian cancer cells to mitogenic growth factors and couple the suppression to the cell cycle arrest at G1-S checkpoint by the hormone.

Vitamin D and differentiation in cancer

This paper reviews the current understanding of the vitamin D-induced differentiation of neoplastic cells, which results in the generation of cells that acquire near-normal, mature phenotype. Examples of the criteria by which differentiation is recognized in each cell type are provided, and only those effects of 1alpha,25-dihydroxyvitamin D(3) (1,25D) on cell proliferation and survival that are associated with the differentiation process are emphasized. The existing knowledge, often fragmentary, of the signaling pathways that lead to vitamin D-induced differentiation of colon, breast, prostate, squamous cell carcinoma, osteosarcoma, and myeloid leukemia cancer cells is outlined. The important distinctions between the different mechanisms of 1,25D-induced differentiation that are cell-type and cell-context specific are pointed out where known. There is a considerable body of evidence that the principal human cancer cells can be suitable candidates for chemoprevention or differentiation therapy with vitamin D. However, further studies are needed to fully understand the underlying mechanisms in order to improve the therapeutic approaches.

MicroRNA regulates human vitamin D receptor

Most of the biological effects of 1alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) are elicited by the binding to vitamin D receptor (VDR), which regulates gene expression. Earlier studies reported no correlation between the VDR protein and mRNA levels, suggesting the involvement of posttranscriptional regulation. MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression through translational repression or mRNA degradation. A potential miR-125b recognition element (MRE125b) was identified in the 3'-untranslated region of human VDR mRNA. We investigated whether VDR is regulated by miR-125b. In luciferase assays using a plasmid containing the MRE125b, the antisense oligonucleotide for miR-125b significantly increased (130% of control) the reporter activity in KGN cells, whereas the precursor for miR-125b significantly decreased (40% of control) the reporter activity in MCF-7 cells, suggesting that miR-125b functionally recognized the MRE125b. By electrophoretic mobility shift assays, it was demonstrated that the overexpression of miR-125b significantly decreased the endogenous VDR protein level in MCF-7 cells to 40% of control. 1,25(OH)(2)D(3) drastically induced the CYP24 mRNA level in MCF-7 cells, but the induction was markedly attenuated by the overexpression of miR-125b. In addition, the antiproliferative effects of 1,25(OH)(2)D(3) in MCF-7 cells were significantly abolished by the overexpression of miR-125b. These results suggest that the endogenous VDR level was repressed by miR-125b. In conclusion, we found that miR-125b posttranscriptionally regulated human VDR. Since the miR-125b level is known to be downregulated in cancer, such a decrease may result in the upregulation of VDR in cancer and augmentation of the antitumor effects of 1,25(OH)(2)D(3).

Vitamin D3 signalling in the brain enhances the function of phosphoprotein enriched in astrocytes--15 kD (PEA-15)

In spite of growing evidence linking vitamin D₃ levels to mental health disorders, little is known about its direct targets in the brain. This study set out to investigate targets of vitamin D₃ in a human brain stem cell line. We employed arrays with antibodies directed against more than 600 structural and signalling proteins, including phospho-variants. Over 180 proteins responded to vitamin D₃, such as cyclin-dependent protein-serine kinase 1/2, epidermal growth factor receptor-tyrosine kinase, protein kinase A, protein-serine kinase Bγ and protein-serine kinase Cα. PEA-15 (phosphoprotein enriched in astrocytes-15 kD, also known as PED), known to be involved in various anti-proliferative and anti-apoptotic effects, was strongly up-regulated. In silico promoter analysis revealed conserved binding sites for vitamin D₃ receptor, suggesting a strong vitamin D₃ dependency of the PEA-15 promoter. PEA-15 up-regulation by vitamin D₃ could be confirmed by Western blot in two different cell lines. Analysis of mRNA and protein phosphorylation status of PEA-15 suggests that increased PEA-15 promoter activity and increased protein stabilization contribute to the overall rise of PEA-15 protein. In a functional test of this novel pathway, we demonstrated that vitamin D₃ was able to rescue cells from TRAIL-induced apoptosis through regulation of the PEA-15 expression and function. Summarized, our study presents novel targets of vitamin D₃ relevant for apoptosis and cell proliferation, and thus strongly supports a function of vitamin D₃ in the brain that impacts on processes highly relevant for major neurological disorders.

The transcription factor FOXO3a is a crucial cellular target of gefitinib (Iressa) in breast cancer cells

Gefitinib is a specific inhibitor of the epidermal growth factor receptor (EGFR) that causes growth delay in cancer cell lines and human tumor xenografts expressing high levels of EGFR. An understanding of the downstream cellular targets of gefitinib will allow the discovery of biomarkers for predicting outcomes and monitoring anti-EGFR therapies and provide information for key targets for therapeutic intervention. In this study, we investigated the role of FOXO3a in gefitinib action and resistance. Using two gefitinib-sensitive (i.e., BT474 and SKBR3) as well as three other resistant breast carcinoma cell lines (i.e., MCF-7, MDA-MB-231, and MDA-MB-453), we showed that gefitinib targets the transcription factor FOXO3a to mediate cell cycle arrest and cell death in sensitive breast cancer cells. In the sensitive cells, gefitinib treatment causes cell cycle arrest predominantly at the G(0)-G(1) phase and apoptosis, which is associated with FOXO3a dephosphorylation at Akt sites and nuclear translocation, whereas in the resistant cells, FOXO3a stays phosphorylated and remains in the cytoplasm. The nuclear accumulation of FOXO3a in response to gefitinib was confirmed in tumor tissue sections from breast cancer patients presurgically treated with gefitinib as monotherapy. We also showed that knockdown of FOXO3a expression using small interfering RNA (siRNA) can rescue sensitive BT474 cells from gefitinib-induced cell-proliferative arrest, whereas reintroduction of active FOXO3a in resistant MDA-MB-231 cells can at least partially restore cell-proliferative arrest and sensitivity to gefitinib. These results suggest that the FOXO3a dephosphorylation and nuclear localization have a direct role in mediating the gefitinib-induced proliferative arrest and in determining sensitivity to gefitinib.

The complexity of targeting EGFR signalling in cancer: from expression to turnover

The epidermal growth factor receptor (ErbB1 or EGFR) has been found to be altered in a variety of human cancers. A number of agents targeting these receptors, including specific antibodies directed against the ligand-binding domain of the receptor and small molecules that inhibit kinase activity are either in clinical trials or are already approved for clinical treatment. However, identifying patients that are likely to respond to such treatments has been challenging. As a consequence, it still remains important to identify additional alterations of the tumor cell that contribute to the response to EGFR-targeted agents. While EGFR-mediated signalling pathways have been well established, there is still a rather limited understanding of how intracellular protein-protein interactions, ubiquitination, endocytosis and subsequent degradation of EGFR contribute to the determination of sensitivity to EGFR targeting agents and are emerging areas of investigation. This review primarily focuses on the basic signal transduction pathways mediated through activated membrane bound and/or endosomal EGFR and emphasizes the need to co-target additional proteins that function either upstream or downstream of EGFR to improve cancer therapy.

Role of matrix metalloproteinases in HaCaT keratinocytes apoptosis induced by loxosceles venom sphingomyelinase D

Envenomation by the spider Loxosceles (brown spider) can result in dermonecrosis and severe ulceration. We have previously shown that Loxosceles sphingomyelinase D (SMaseD), the enzyme responsible for these pathological effects, induced expression of matrix metalloproteinase-9 (MMP-9), which is possibly one of the main factors involved in the pathogenesis of the cutaneous loxoscelism. The aim of this study was to further investigate the molecular mechanisms triggered by Loxosceles SMaseD involved in the initiation of the dermonecrotic lesion, using HaCaT cultures, a human keratinocyte cell line, as an in vitro model for cutaneous loxoscelism. We show here that SMaseD from Loxosceles spider venom induces apoptosis in human keratinocytes, which is associated with an increased expression of metalloproteinase-2 and -9, and that the use of metalloproteinase inhibitors, such as tetracycline, may prevent cell death and potentially may prevent tissue destruction after envenomation.

Vitamin D receptor agonists induce prostatic acid phosphatase to reduce cell growth and HER-2 signaling in LNCaP-derived human prostate cancer cells

We have previously shown that concentrations of 1alpha,25-dihydroxyvitamin D(3) (1,25D) that induce G(0)/G(1) cell cycle arrest in androgen-dependent LNCaP prostate cancer cells also decrease expression of c-Myc, a proto-oncogene that stimulates progression from G(1) to S phase of the cell cycle. Since both c-Myc expression and cell cycle progression are regulated by tyrosine kinase activation, we examined the ability of 1,25D to alter tyrosine kinase signaling in LNCaP cells and the androgen-independent LNCaP C81 (C81 LN) cell line. 1,25D selectively reduced protein tyrosine phosphorylation within both the LNCaP and C81 LN cells. This reduction in tyrosine kinase signaling appears to result from elevated levels of cellular prostatic acid phosphatase (PAcP). Western blots and biochemical assays revealed 1,25D increases the level of active PAcP in both cell lines. In addition, 1,25D decreased tyrosine phosphorylation of HER-2, an EGFR family member inactivated by PAcP, and the HER-2 downstream adaptor protein p52 Shc in C81 LN cells. Inhibition of HER-2 signaling by AG825 reduces growth of C81 LN cells and the parental LNCaP cells. These data therefore suggest that 1,25D-mediated decreases in LNCaP and C81 LN cell growth are in part due to decreases in tyrosine kinase signaling that result from up-regulation of PAcP.

Modulation of tyrosine phosphorylation signalling pathways by 1alpha,25(OH)2-vitamin D3

Hormonally active vitamin D(3), 1alpha,25(OH)(2)D(3), interacts with the classic vitamin D nuclear receptor that regulates gene transcription and with a putative cell membrane receptor that mediates rapid biological responses. 1alpha,25(OH)(2)D(3) actions on target tissues regulate: mineral metabolism and intracellular Ca(2+); protein kinase cascades leading to cell proliferation, differentiation and apoptosis; muscle growth and contractility; and the immune system. There is evidence for underlying 1alpha,25(OH)(2)D(3)-mediated protein tyrosine phosphorylation signalling in bone, intestine, muscle, epidermal and cancer cells. Extracellular-signal-regulated kinases-1/2, p38 and/or c-jun N-terminal kinase pathways play important roles in mediating 1alpha,25(OH)(2)D(3) actions. Studies to elucidate key regulatory metabolic steps and crosstalk sites in these pathways would enhance our understanding of the significance of tyrosine phosphorylation cascades in normal 1alpha,25(OH)(2)D(3) physiology, pathophysiology and pharmacology.

All-trans retinoic acid inhibits proliferation of intestinal epithelial cells by inhibiting expression of the gene encoding Kruppel-like factor 5

Retinoids are known inhibitors of epithelial cell proliferation. Previous studies indicate that Kruppel-like factor 5 (KLF5) is a pro-proliferative transcription factor. Here, we examined the effect of all-trans retinoid acid (ATRA) on proliferation of the intestinal epithelial cell line, IEC6. Treatment of IEC6 cells with ATRA inhibited their proliferation due to G1 cell cycle arrest. This inhibition was correlated with a decrease in the levels of KLF5 mRNA and promoter activity. In contrast, constitutive expression of KLF5 in stably transfected IEC6 cells with a KLF5-expressing plasmid driven by a viral promoter abrogated the growth inhibitory effect of ATRA. Moreover, ATRA inhibited proliferation of several human colon cancer cell lines with high levels of KLF5 expression but not those with low levels of KLF5 expression. Our results indicate that KLF5 is a potential mediator for the inhibitory effect of ATRA on intestinal epithelial cell proliferation.