HCT-116 Human Colon Adenocarcinoma Cells Research Articles

Design, Synthesis, Molecular Docking, and Anticancer Activity of Phthalazine Derivatives as VEGFR-2 Inhibitors.

Novel series of phthalazine derivatives 6-11 were designed, synthesized, and evaluated for their anticancer activity against two human tumor cell lines, HCT-116 human colon adenocarcinoma and MCF-7 breast cancer cells, targeting the VEGFR-2 enzyme. Compounds 7a,b and 8b,c showed the highest anticancer activities against both HCT116 human colon adenocarcinoma cells with IC50 of 6.04 ± 0.30, 13.22 ± 0.22, 18 ± 0.20, and 35 ± 0.45 μM, respectively, and MCF-7 breast cancer cells with IC50 of 8.8 ± 0.45, 17.9 ± 0.50, 25.2 ± 0.55, and 44.3 ± 0.49 μM, respectively, in comparison to sorafenib as reference drug with IC50 of 5.47 ± 0.3 and 7.26 ± 0.3 μM, respectively. Eleven compounds in this series were further evaluated for their inhibitory activity against VEGFR-2, where compounds 7a, 7b, 8c, and 8b also showed the highest VEGFR-2 inhibition with IC50 of 0.11 ± 0.01, 0.31 ± 0.03, 0.72 ± 0.08, and 0.91 ± 0.08 μM, respectively, in comparison to sorafenib as reference ligand with IC50 of 0.1 ± 0.02. Furthermore, molecular docking studies were performed for all synthesized compounds to predict their binding pattern and affinity towards the VEGFR-2 active site, in order to rationalize their anticancer activity in a qualitative way.

Endophytic fungi associated with Sudanese medicinal plants show cytotoxic and antibiotic potential.

In this study, we isolated 15 endophytic fungi from five Sudanese medicinal plants. Each fungal endophytic strain was identified by sequencing of internal transcribed spacer (ITS) regions of rDNA. Ethyl acetate extracts were prepared from each endophyte cultivated in vitro and tested for their respective antibacterial activities and antiproliferative activities against human cancer cells. Antibacterial screening was carried out against two bacterial strains: Gram-negative Escherichia coli and Gram-positive methicillin-resistant Staphylococcus aureus, by the broth dilution method. Cell viability was evaluated by the MTT procedure after exposure of MCF7 breast cancer cells and HT29 or HCT116 human colon adenocarcinoma cells to each endophytic extract. Of interest, Byssochlamys spectabilis isolated from Euphorbia prostata showed cytotoxicity (IC50 = 1.51 ± 0.2 μgmL(-1)) against MCF7 cells, but had a low effect against HT29 or HCT116 cells (IC50 > 20 μgmL(-1)). Cladosporium cladosporioides 2, isolated from Vernonia amygdalina leaves, showed antiproliferative activities against MCF7 cells (IC50 = 10.5± 1.5 μgmL(-1)) only. On the other hand, B. spectabilis and Alternaria sp. extract had antibacterial activities against the S. aureus strain. The findings of this work revealed that endophytic fungi associated with medicinal plants from Sudan could be considered as an attractive source of new therapeutic compounds.

Abstract 2516: SL01, an oral gemcitabine prodrug as a potent anticancer drug

Abstract SL01,dodecyl-3-((1-((2R,4R,5R)-3,3-difluoro-4-hydroxy-5-(hydroxymethyl)-tetrahydrofuran-2-yl)-2-oxo-1,2-dihydropyrimidin-4-yl)carbamoyl)pyrazine-2-carboxylate, is a gemcitabine prodrug synthesized by introducing the moiety of 3-(dodecyloxycarbonyl)pyrazine-2-carbonyl at the N4-position on the cytidine ring of gemcitabine. Our goal is to investigate the efficacy and the safety of this oral prodrug as a potent oral anticancer agent, which may supplant the use of gemcitabine in clinic. The evaluation was accomplished by determining its inhibition of the growth of several tumors with gemcitabine administrated by injection as a control, and also by exploring its pharmacokinetics and its safety. In vivo, SL01 could effectively inhibit the growth of NCI-H460 human non-small cell lung cancer, MCF-7 human breast cancer, BxPC-3 human pancreatic cancer, ES-2 human bladder cancer, HepG2 human hepatocellular carcinoma cells and HCT-116 human colon adenocarcinoma cells in xenografts nude mice without loss their body weight significantly. After three weeks oral administration at the dosage of 30 umol/kg, the inhibition rates were 58.0%, 66.5%, 41.5%, 77.1%, 41.5% and 63.1%, respectively. Molecular analysis indicated that the high efficacy of SL01 was associated with its ability in inducing apoptosis, as evidenced by increase of TUNEL staining cells, activation of caspase-9, caspase-3 and cleaved poly ADP-ribose polymerase in tumor tissues. Pharmacokinetics studies indicated that SL01 exhibited a longer mean retention time and half-life than that of gemcitabine after oral administration in rat, further analysis showed that the bioavailability of SL01 was 22.1%, which indicating that the introduction of 3-(dodecyloxycarbonyl)pyrazine-2-carbonyl moiety to gemcitabine could improve its bioavailability and reduce its first-pass metabolism. The metabolism analysis suggested that SL01 could transform into gemcitabine by hydrolyzing and then releasing into blood to keep the relatively long steady-state concentrations. Toxicity studies indicated that the LD50 was >1.4 g/kg for its acute toxicity, and the tolerated dosage was 45 mg/kg for 4 weeks evaluation in rat. Based on these results, SL01 is proposed as a potent oral anticancer agent, which may supplant the use of gemcitabine in clinic. Acknowledgment: This project was supported by the Ministry of Science and Technology of China (2010ZX09401-302-2-07and 2011ZX09102-001-03). Citation Format: Wenbao Li, Hua Yuan, Xianjun Qu. SL01, an oral gemcitabine prodrug as a potent anticancer drug. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2516. doi:10.1158/1538-7445.AM2014-2516

Cyclin-dependent kinases 4 and 6 control tumor progression and direct glucose oxidation in the pentose cycle

Cyclin-dependent kinases CDK4 and CDK6 are essential for the control of the cell cycle through the G(1) phase. Aberrant expression of CDK4 and CDK6 is a hallmark of cancer, which would suggest that CDK4 and CDK6 are attractive targets for cancer therapy. Herein, we report that calcein AM (the calcein acetoxymethyl-ester) is a potent specific inhibitor of CDK4 and CDK6 in HCT116 human colon adenocarcinoma cells, inhibiting retinoblastoma protein (pRb) phosphorylation and inducing cell cycle arrest in the G(1) phase. The metabolic effects of calcein AM on HCT116 cells were also evaluated and the flux between the oxidative and non-oxidative branches of the pentose phosphate pathway was significantly altered. To elucidate whether these metabolic changes were due to the inhibition of CDK4 and CDK6, we also characterized the metabolic profile of a CDK4, CDK6 and CDK2 triple knockout of mouse embryonic fibroblasts. The results show that the metabolic profile associated with the depletion of CDK4, CDK6 and CDK2 coincides with the metabolic changes induced by calcein AM on HCT116 cells, thus confirming that the inhibition of CDK4 and CDK6 disrupts the balance between the oxidative and non-oxidative branches of the pentose phosphate pathway. Taken together, these results indicate that low doses of calcein can halt cell division and kill tumor cells. Thus, selective inhibition of CDK4 and CDK6 may be of greater pharmacological interest, since inhibitors of these kinases affect both cell cycle progression and the robust metabolic profile of tumors.

Abstract 5697: Cellular iron depletion weakens isothiocyanate-induced HO-1 upregulation

Abstract Heme oxygenase-1 (HO-1) is an important cytoprotective gene, whose expression is known to be elevated in various carcinomas. Also, overexpression of HO-1 is known to inhibit apoptosis particularly in cancer cells. Interestingly, exposing cancer cells to certain chemopreventive agents such as isothiocyanates causes induction of HO-1, perhaps as an attempt by the cells to help them survive the stressful situation. However, it is not completely known what nutritive factors or related components in cancer cells influence HO-1 induction as caused by isothiocyanates. Thus, the aim was to test the hypothesis that adequate iron is needed to enable cancer cells to optimally express HO-1 when induced by isothiocyanates. In HCT-116 human colon adenocarcinoma cells, phenethylisothiocyanate and sulforaphane increased expression of HO-1 and also heme oxygenase activity. However, cells made iron-deficient with the iron chelator, desferrioxamine (DFO), and then exposed to isothiocyanates had much lower heme oxygenase activity. This suppressive effect of DFO on isothiocyanate-mediated induction of heme oxygenase activity was in association with repression of HO-1 protein and mRNA expression. The suppression by DFO of isothiocyanate-induced HO-1 upregulation was reversed upon replenishing the iron-deficient cells with the missing iron. Additionally, it was found that thiol antioxidants (glutathione and N-acetylcysteine) inhibited isothiocyanate-induced HO-1 upregulation, implying that the HO-1 upregulation involved an oxidant signaling pathway. Indeed, NADPH oxidase (NOX) inhibitors, namely, diphenyleneiodonium (DPI) and apocynin, and a superoxide scavenger (Tiron) inhibited isothiocyanate-induced upregulation of HO-1 as well. DPI was the most potent and also inhibited nuclear translocation of redox-sensitive Nrf-2, which is known to activate the HO-1 gene. These findings suggest the possibility that adequate amounts of iron, which at the atomic level can serve as the pivotal element of heme in NOX, must be present in the cells to permit what appears to be NOX-dependent upregulation of HO-1 by isothiocyanates. It is concluded that iron-deficient HCT-116 cells are less able to increase expression of cytoprotective HO-1 when exposed to isothiocyanates, which may make the cancer cells more susceptible to killing. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5697.

Novel Oligoamine Analogues Inhibit Lysine-Specific Demethylase 1 and Induce Reexpression of Epigenetically Silenced Genes

Abnormal DNA CpG island hypermethylation and transcriptionally repressive histone modifications are associated with the aberrant silencing of tumor suppressor genes. Lysine methylation is a dynamic, enzymatically controlled process. Lysine-specific demethylase 1 (LSD1) has recently been identified as a histone lysine demethylase. LSD1 specifically catalyzes demethylation of mono- and dimethyl-lysine 4 of histone 3 (H3K4), key positive chromatin marks associated with transcriptional activation. We hypothesized that a novel class of oligoamine analogues would effectively inhibit LSD1 and thus cause the reexpression of aberrantly silenced genes. Human colorectal cancer cells were treated with the oligoamines and changes in mono- and dimethyl-H3K4 and other chromatin marks were monitored. In addition, treated cells were evaluated for the reexpression of the aberrantly silenced secreted frizzled-related proteins (SFRP) Wnt signaling pathway antagonist genes. Finally, the effects of the LSD1 inhibitors were evaluated in an in vivo xenograft model. Treatment of HCT116 human colon adenocarcinoma cells in vitro resulted in increased H3K4 methylation and reexpression of silenced SFRP genes. This reexpression is also accompanied by a decrease in H3K9me2 repressive mark. Importantly, cotreatment with low doses of oligoamines and a DNA methyltransferase inhibitor highly induces the reexpression of the aberrantly silenced SFRP2 gene and results in significant inhibition of the growth of established tumors in a human colon tumor model in vivo. The use of LSD1-inhibiting oligoamine analogues in combination with DNA methyltransferase inhibitors represents a highly promising and novel approach for epigenetic therapy of cancer.

Upregulation of GADD153 by Butyrate: Involvement of MAPK

Butyrate inhibits the proliferation of cancer cells, but the early molecular events initiated by butyrate have not been fully identified. Herein, butyrate is shown to affect the growth arrest and DNA damage-inducible gene 153 (GADD153) in HCT-116 human colon adenocarcinoma cells. Despite absence of any detectable cellular DNA damage, the expression of GADD153 was upregulated before several features characteristic of apoptosis appeared. Butyrate-induced upregulation of GADD153 mRNA was attenuated by actinomycin D, but apparently not by cycloheximide. In investigating possible involvement of MAPK in mediating the effect of butyrate on GADD153 mRNA expression, the extracellular regulated kinase (ERK) inhibitor PD98059, but neither the JNK inhibitor SP600125 nor the p38 MAPK inhibitor SB203580, blunted the ability of butyrate to upregulate GADD153 mRNA expression. U0126, a selective inhibitor of upstream MEK, had a similar effect as PD98059 on butyrate-induced GADD153 mRNA upregulation. Collectively, these findings suggest that butyrate caused activation of the GADD153 gene at the level of transcription involving mainly the MEK/ERK branch of the MAPK signal transduction pathway. Moreover, these molecular events were not the result of any DNA damage and occurred before several features characteristic of apoptosis became evident.

Reduced host cell reactivation of oxidative DNA damage in human cells deficient in the mismatch repair gene hMSH2

Germ line mutations in the mismatch repair (MMR) genes hMSH2 and hMLH1 account for approximately 98% of hereditary non-polyposis colorectal cancers. In addition, there is increasing evidence for an involvement of MMR gene expression in the response of cells to UV-induced skin cancer. The link between MMR and skin cancer suggests an involvement of MMR gene expression in the response of skin cells to UV-induced DNA damage. In this report, we have used two reporter gene assays to examine the role of hMSH2 and hMLH1 in the repair of oxidative DNA damage induced by UVA light and DNA damage caused by methylene blue plus visible light (MB+VL). UVA and MB+VL produce 8-hydroxyguanines in DNA that are repaired by base excision repair (BER). AdHCMVlacZ is a replication-deficient recombinant adenovirus that expresses the beta-galactosidase (beta-gal) reporter gene under the control of the human cytomegalovirus (CMV) immediate-early promoter. We show a reduced host cell reactivation for beta-gal expression of UVA-treated and MB+VL-treated AdHCMVlacZ in hMSH2-deficient LoVo human colon adenocarcinoma cells compared to their hMSH2-proficient counterpart SW480 cells, but not in hMLH1-deficient HCT116 human colon adenocarcinoma cells compared to hMLH1-proficient HCT116-chr3 cells. We have also reported previously that enhanced expression of the undamaged AdHCMVlacZ reporter gene is induced by the pre-treatment of cells with lower levels of the DNA-damaging agent and to higher expression levels in transcription-coupled repair (TCR)-deficient compared to TCR-proficient cells. Here we show that pre-treatment of cells with UVA or MB+VL enhanced expression of the undamaged reporter gene to a higher level in LoVo compared to SW480 cells but there was little or no difference in HCT116 compared to HCT116-chr3 cells. These results suggest a substantial involvement of hMSH2 but little or no involvement of hMLH1 in the repair of UVA- and MB+VL-induced oxidative DNA damage by BER.

Folate Deficiency Induces Cell-Specific Changes in the Steady-State Transcript Levels of Genes Involved in Folate Metabolism and 1-Carbon Transfer Reactions in Human Colonic Epithelial Cells1

Intracellular folate homeostasis is essential for the 1-carbon transfer reactions necessary for DNA synthesis and biological methylation reactions in colonic epithelial cells. Perturbed 1-carbon transfer reactions resulting from folate depletion predispose normal colonic epithelial cells to neoplastic transformation while inhibiting the growth of colon cancer cells. Using an in vitro model of folate deficiency, we determined the effects of folate deficiency on the steady-state transcript levels of genes involved in intracellular folate metabolism and 1-carbon transfer reactions in HCT116 and Caco2 human colon adenocarcinoma cells. In HCT116 cells, folate depletion was associated with changes in transcript levels of genes favoring increased folate uptake and intracellular folate retention, the provision of metabolically more effective substrates for folate-dependent enzymes, and reduced folate hydrolysis and efflux. In HCT116 cells, folate depletion was associated with changes in transcript levels of genes favoring the preferential shuttling of the flux of 1-carbon units to the methionine cycle over the nucleotide synthesis pathway. In Caco2 cells, some adaptive responses in response to folate depletion were not as apparent as in HCT116 cells, and in some cases, the direction of change was counterintuitive. In Caco2 cells, the metabolic priority in response to folate depletion was to shuttle the available folate pools to the nucleotide biosynthesis pathway at the expense of biological methylation reactions. In both HCT116 and Caco2 cells, folate depletion was associated with the conservation of the existing pattern and extent of DNA methylation.

Comparison between 5,10,15,20-Tetraaryl- and 5,15-Diarylporphyrins as Photosensitizers: Synthesis, Photodynamic Activity, and Quantitative Structure−Activity Relationship Modeling

The synthesis of a panel of seven nonsymmetric 5,10,15,20-tetraarylporphyrins, 13 symmetric and nonsymmetric 5,15-diarylporphyrins, and one 5,15-diarylchlorin is described. In vitro photodynamic activities on HCT116 human colon adenocarcinoma cells were evaluated by standard cytotoxicity assays. A predictive quantitative structure-activity relationship (QSAR) regression model, based on theoretical holistic molecular descriptors, of a series of 34 tetrapyrrolic photosensitizers (PSs), including the 24 compounds synthesized in this work, was developed to describe the relationship between structural features and photodynamic activity. The present study demonstrates that structural features significantly influence the photodynamic activity of tetrapyrrolic derivatives: diaryl compounds were more active with respect to the tetraarylporphyrins, and among the diaryl derivatives, hydroxy-substituted compounds were more effective than the corresponding methoxy-substituted ones. Furthermore, three monoarylporphyrins, isolated as byproducts during diarylporphyrin synthesis, were considered for both photodynamic and QSAR studies; surprisingly they were found to be particularly active photosensitizers.

Detection of an involvement of the human mismatch repair genes hMLH1 and hMSH2 in nucleotide excision repair is dependent on UVC fluence to cells.

There is conflicting evidence for the role of the mismatch repair (MMR) genes hMLH1 and hMSH2 in the transcription-coupled repair (TCR) pathway of nucleotide excision repair. In the present work, we have examined the role of these MMR genes in nucleotide excision repair using two reporter gene assays. AdHCMVlacZ is a replication-deficient recombinant adenovirus that expresses the beta-galactosidase reporter gene under the control of the human cytomegalovirus immediate early promoter. We have reported previously a reduced host cell reactivation (HCR) for beta-galactosidase expression of UVC-irradiated AdHCMVlacZ in TCR-deficient Cockayne syndrome (CS) fibroblasts compared with normal fibroblasts, indicating that HCR depends, at least in part, on TCR. In addition, we have reported that UVC-enhanced expression of the undamaged reporter gene is induced at lower UVC fluences to cells and at higher levels after low UVC fluences in TCR-deficient compared with normal human fibroblasts, suggesting that persistent damage in active genes triggers increased activity from the human cytomegalovirus-driven reporter construct. We have examined HCR and UV-enhanced expression of the reporter gene in hMLH1-deficient HCT116 human colon adenocarcinoma cells and HCT116-chr3 cells (the MMR-proficient counterpart of HCT116) as well as hMSH2-deficient LoVo human colon adenocarcinoma cells and their hMSH2-proficient counterpart SW480 cells. We show a greater UV-enhanced expression of the undamaged reporter gene after low UVC exposure in HCT116 compared with HCT116-chr3 cells and in LoVo compared with SW480 cells. We show also a reduced HCR in HCT116 compared with HCT116-chr3 cells and in LoVo compared with SW480 cells. However, the reduction in HCR was less or absent when cells were pretreated with UVC. These results suggest that detection of an involvement of hMLH1 and hMSH2 in TCR is dependent on UVC (254 nm) fluence to cells.

Inhibition of macromolecular synthesis by cryptophycin-52.

Cryptophycin (CP)-52, a synthetic analog of CP-1, possesses potent and selective antiproliferative activity against human solid tumors both and. Based on an algorithm developed in this laboratory using HCT-116 human colon adenocarcinoma cells, CP-52 exhibited a time- and concentration-dependent antiproliferative effect in the clonogenic assay. Inhibition of both DNA and RNA synthesis was observed in the absence of any effect on protein synthesis following a 24-h exposure to CP-52, at a time when proliferating cells were arrested in the G2/M phase of the cell cycle. In summary, we interpret these data to indicate that the selective inhibition of DNA synthesis may be a major causative factor responsible for the antiproliferative activity of CP-52 and subsequent G2/M arrest.

Painting with a molecular brush: Genomic/proteomic interfacing to define the drug action profile of novel solid-tumor selective anticancer agents

XK469 is an investigational anticancer agent that exhibits antiproliferative activity in tumor-bearing animal models. We examined the drug-action profile of this agent at the molecular level regarding alterations induced in gene expression and proteins in HCT-116 human colon adenocarcinoma cells. We used a unique cDNA microarray (GeneMapTM Cancerarray) comprising 1152 human tumor-related genes and 2-D gel electrophoresis, respectively, following a 24-hour exposure to a drug concentration that killed a two-log fraction of HCT-116 clonogenic cells. Functional gene cluster profile (FGCP) analysis of the 71 out of 1152 genes that displayed a >2-fold increase or decrease in expression (over untreated control) identified a drug-specific involvement of the MAPK signal transduction pathway. MAPK signaling together with the involvement of ubiquitin proteins from 2-D gel electrophoresis suggest a novel drug-action profile at the molecular level for the in vitro antiproliferative activity of XK469. Cytometry 47:72–79, 2002. © 2001 Wiley-Liss, Inc.

Painting with a molecular brush: Genomic/proteomic interfacing to define the drug action profile of novel solid‐tumor selective anticancer agents

XK469 is an investigational anticancer agent that exhibits antiproliferative activity in tumor-bearing animal models. We examined the drug-action profile of this agent at the molecular level regarding alterations induced in gene expression and proteins in HCT-116 human colon adenocarcinoma cells. We used a unique cDNA microarray (GeneMap(TM) Cancerarray) comprising 1152 human tumor-related genes and 2-D gel electrophoresis, respectively, following a 24-hour exposure to a drug concentration that killed a two-log fraction of HCT-116 clonogenic cells. Functional gene cluster profile (FGCP) analysis of the 71 out of 1152 genes that displayed a >2-fold increase or decrease in expression (over untreated control) identified a drug-specific involvement of the MAPK signal transduction pathway. MAPK signaling together with the involvement of ubiquitin proteins from 2-D gel electrophoresis suggest a novel drug-action profile at the molecular level for the in vitro antiproliferative activity of XK469.

Increased cell surface EGF receptor expression during the butyrate-induced differentiation of human HCT-116 colon tumor cell clones

Several clonal sublines of HCT-116 human colon adenocarcinoma cells were isolated and characterized on the basis of their growth characteristics, intrinsic enterocyte-like differentiation (as assessed by alkaline phosphatase and lactase activities), and responses to butyrate, an inducer of colon tumor cell maturation. The HCT-116 sublines were found to be heterogeneous and several phenotypically distinct clones were identified. Further characterization of these clones indicated that the effects of butyrate on cell growth, alkaline phosphatase activity, and lactase activity were distinct and separable. The growth of all of the clones was inhibited by butyrate (IC 50 values varied from 0.44 to 1.5 m M), but the effects of this agent on alkaline phosphatase and lactase activities varied widely. In several sublines butyrate had no effect on either enzyme while in others one or both activities were induced. Additionally, the binding of 125I-epidermal growth factor (EGF) to cell surface receptors was found to be proportional to the expression of lactase activity in the cell. The D3 clone and other sublines with intrinsic lactase activities greater than 100 nmol/mg/min expressed a class of high-affinity EGF receptors (e.g., D3 cells had 3.48 × 10 4 EGF receptors/cell with a k d of 0.61 n M). Other clones with less lactase activity had undetectable levels of 125I-EGF binding. In clones which exhibited greater than twofold increases in lactase activity in response to butyrate, the expression of a large number of low-affinity EGF receptors was also induced. In one such clone, the P1 subline, lactase activity was increased from 70 nmol/mg/min to 230 nmol/mg/min after 96 h in 2 m M butyrate, and the expression of EGF receptors was increased from undetectable levels to 1.18 × 10 5 EGF receptors/cell ( k d of 3.2 n M). Northern blot analysis indicated that the increased 125I-EGF binding after butyrate treatment may have been due, in part, to a greater than twofold accumulation of EGF receptor mRNA. In addition, the expression of the messages for transforming growth factor alpha (TGF-α) and transforming growth factor beta (TGF-β) was examined in butyrate-treated cells. While TGF-α mRNA levels were found to correlate with EGF receptor message levels in the HCT-116 clones, TGF-β mRNA expression was not found to correlate with the butyrate-induced growth inhibition or with increases in EGF receptor expression, alkaline phosphatase activity, or lactase activity in these cells.