Characterization of chemically and virally transformed variants of Madin-Darby canine kidney (MDCK) epithelial cells

Galectin-8 induces partial epithelial-mesenchymal transition with invasive tumorigenic capabilities involving a FAK/EGFR/proteasome pathway in Madin-Darby canine kidney cells

Epithelial cells can acquire invasive and tumorigenic capabilities through epithelial–mesenchymal-transition (EMT). The glycan-binding protein galectin-8 (Gal-8) activates selective β1-integrins involved in EMT and is overexpressed by certain carcinomas. Here we show that Gal-8 overexpression or exogenous addition promotes proliferation, migration, and invasion in nontumoral Madin–Darby canine kidney (MDCK) cells, involving focal-adhesion kinase (FAK)-mediated transactivation of the epidermal growth factor receptor (EGFR), likely triggered by α5β1integrin binding. Under subconfluent conditions, Gal-8–overexpressing MDCK cells (MDCK-Gal-8^H) display hallmarks of EMT, including decreased E-cadherin and up-regulated expression of vimentin, fibronectin, and Snail, as well as increased β-catenin activity. Changes related to migration/invasion included higher expression of α5β1 integrin, extracellular matrix-degrading MMP13 and urokinase plasminogen activator/urokinase plasminogen activator receptor (uPA/uPAR) protease systems. Gal-8–stimulated FAK/EGFR pathway leads to proteasome overactivity characteristic of cancer cells. Yet MDCK-Gal-8^H cells still develop apical/basolateral polarity reverting EMT markers and proteasome activity under confluence. This is due to the opposite segregation of Gal-8 secretion (apical) and β1-integrins distribution (basolateral). Strikingly, MDCK-Gal-8^H cells acquired tumorigenic potential, as reflected in anchorage-independent growth in soft agar and tumor generation in immunodeficient NSG mice. Therefore, Gal-8 can promote oncogenic-like transformation of epithelial cells through partial and reversible EMT, accompanied by higher proliferation, migration/invasion, and tumorigenic properties.

Evaluation of tumorigenic potential of high yielding cloned MDCK cells for live-attenuated influenza vaccine using in vitro growth characteristics, metastatic gene expression and in vivo nude mice model

Several mammalian cell lines, including Madin-Darby canine kidney (MDCK) cells have been approved by regulators for manufacturing of human vaccines. A new MDCK 9B9-1E4 cloned cell line has been created which is capable of producing live attenuated influenza vaccine (LAIV) with high yield. This cell line was shown to be non tumorigenic in eight week old adult athymic nude mouse model. This property is desirable for vaccine production and is unique to this cell line and is not known to be shared by other MDCK cell lines that are currently used for vaccine production. This significant difference in tumorigenic phenotype required further characterization of this cell line to ensure its safety for use in vaccine production. This is particularly important for LAIV production where it is not possible to incorporate a virus inactivation and/or removal step during manufacturing. Characterization of this cell line included extensive adventitious agent testing, tumorigenicity and oncogenicity assessment studies. Here, we describe the development of tumorigenic MDCK cell lines for use as positive controls and in vitro methods to aid in the evaluation of the tumorigenicity of MDCK 9B9-1E4 cloned cells. Tumorigenic MDCK cells were successfully generated following Hras and cMyc oncogene transfection of MDCK 9B9-1E4 cloned cells. In this study we demonstrate the lack of tumorigenic potential of the MDCK 9B9-1E4 cloned cell line in adult athymic nude mice model.

Validation of the safety of MDCK cells as a substrate for the production of a cell-derived influenza vaccine

Cell culture-based production methods may assist in meeting increasing demand for seasonal influenza vaccines and developing production flexibility required for addressing influenza pandemics. MDCK-33016PF cells are used in propagation of a cell-based seasonal influenza vaccine (Optaflu^®); but, like most continuous cell lines, can grow in immunocompromised mice to produce tumors. It is, therefore, essential that no residual cells remain within the vaccine, that cell lysates or DNA are not oncogenic, and that the cell substrate does not contain oncogenic viruses or oncogenic DNA. Multiple, redundant processes ensure the safety of influenza vaccines produced in MDCK-33016PF cells. The probability of a residual cell being present in a dose of vaccine is approximately 1 in 10³⁴. Residual MDCK-DNA is ≤10 ng per dose and the ß-propiolactone used to inactivate influenza virus results in reduction of detectable DNA to less than 200 base pairs (bp). Degenerate PCR and specific PCR confirm exclusion of oncogenic viruses. The manufacturing process has been validated for its capacity to remove and inactivate viruses. We conclude that the theoretical risks arising from manufacturing seasonal influenza vaccine using MDCK-33016PF cells are reduced to levels that are effectively zero by the multiple, orthogonal processes used during production.

Evidence for a potential tumor suppressor role for the Na,K-ATPase beta1-subunit

The Na,K-ATPase, consisting of two essential subunits (alpha, beta), plays a critical role in the regulation of ion homeostasis in mammalian cells. Recent studies indicate that reduced expression of the beta1 isoform (NaK-beta1) is commonly observed in carcinoma and is associated with events involved in cancer progression. In this study, we present evidence that repletion of NaK-beta1 in Moloney sarcoma virus-transformed Madin-Darby canine kidney cells (MSV-MDCK), a highly tumorigenic cell line, inhibits anchorage independent growth and suppresses tumor formation in immunocompromised mice. Additionally, using an in vitro cell-cell aggregation assay, we showed that cell aggregates of NaK-beta1 subunit expressing MSV-MDCK cells have reduced extracellular regulated kinase (ERK) 1/2 activity compared with parental MSV-MDCK cells. Finally, using immunohistochemistry and fully quantitative image analysis approaches, we showed that the levels of phosphorylated ERK 1/2 are inversely correlated to the NaK-beta1 levels in the tumors. These findings reveal for the first time that NaK-beta1 has a potential tumor-suppressor function in epithelial cells.

Membrane-type-1 matrix metalloproteinase confers tumorigenicity on nonmalignant epithelial cells

Overexpression of membrane-type-1 matrix metalloproteinase (MT1-MMP) in tumor cells has previously been shown to enhance tumor growth and metastasis. To establish if MT1-MMP is also able to confer tumorigenicity on nonmalignant epithelial cells, we transfected human MT1-MMP cDNA into Madin-Darby canine kidney (MDCK) cells expressing a tetracycline-repressible transactivator. Induction of MT1-MMP in the absence of doxycycline (Dox) was associated with activation of exogenous MMP-2 as well as with formation of large cysts and increased invasiveness in collagen matrices. Transfected cells were inoculated subcutaneously into two groups of nude mice, one of which received Dox to inhibit expression of MT1-MMP. Formation of tumor xenografts was observed in 11 of 17 mice maintained without Dox, but only in two of nine mice that received Dox (P<0.05). The xenografts were composed of tubular structures interspersed within a highly cellular stroma. The epithelial cells delimiting the lumen were polarized, as indicated by the basolateral distribution of Na,K-ATPase. Despite their differentiated appearance, the tumors lacked a well-defined boundary, and epithelial tubules invaded adjacent muscular layers. These results demonstrate that conditional expression of MT1-MMP in nonmalignant MDCK epithelial cells is by itself sufficient to drive formation of invasive tumors.

Effects of 5-azacytidine, sodium butyrate, and phorbol esters on amino acid transport system A in a kidney epithelial cell line, MDCK: evidence for multiple mechanisms of regulation

Neutral amino acid transport by system A was investigated in the epithelial cell lines MDCK and MDCK-T1. The latter line is a chemically induced, oncogenically transformed line derived from MDCK. Inducers of differentiation, sodium butyrate and 5-azacytidine, and a tumor promoter, TPA, were used as probes to delineate pathways of regulation involved in system A response to a variety of physiological conditions and agents. Azacytidine, an inhibitor of DNA methylation, and butyrate, an enhancer of histone acetylation, inhibited expression of system A, had little effect on system ASC, and slightly stimulated system L. Inhibition of system A expression by butyrate and azacytidine occurred under different conditions. Increases in system A activity due to amino acid starvation or transformation were inhibited by butyrate but not by azacytidine. Repressed system A activity, normally observed in the presence of high levels of amino acids, was more sensitive to azacytidine than to butyrate. The tumor promoter, TPA, stimulated system A activity in MDCK cells under normal growth conditions but did not stimulate activity in amino acid-starved MDCK cells or in MDCK-T1 cells. Stimulation of system A activity by TPA was prevented by prior exposure to butyrate but not to azacytidine. These results suggest 1) that system A expression observed in growing amino-acid-repressed MDCK cells is modulated by an azacytidine-sensitive mechanism and 2) that the elevated expression of system A activity induced by amino acid starvation, by chemical transformation to MDCK-T1, and by TPA is modulated by a butyrate-sensitive mechanism.

Selective cytotoxicity of L-canavanine in tumorigenic Madin-Darby canine kidney T1 cells

L-canavanine, an analog of L-arginine, was examined for toxicity in a normal canine kidney epithelial cell line, Madin-Darby canine kidney (MDCK), and in its chemically transformed derivative MDCK-T1. Under conditions where the analog reversibly arrested growth of MDCK cells, more than 90% of the tumorigenic cells were killed. This selective cytotoxicity was not due to any difference in growth rate (both lines doubled every 24 h). Nor was it caused by the inhibition of protein synthesis or DNA replication, since amino acid deprivation and drugs which inhibited replication directly and indirectly did not kill the transformed cells preferentially. To the contrary, cycloheximide killed MDCK cells preferentially. Although the mechanism for the selective cytotoxicity of canavanine in the tumorigenic cells remains obscure, one clue was afforded by the observation that the analog caused a loss in plating efficiency of the T1 cells prior to their detachment from plates. Selective sensitivity to canavanine may therefore reside in cell surface proteins, which are known to differ both qualitatively and quantitatively between normal and transformed cells. The present results support our previous findings suggesting the possible value of using canavanine as an agent for cancer chemotherapy.

Revertants of Ha-MuSV-transformed MDCK cells express reduced levels of p21 and possess a more normal phenotype

Four subclones of the originally cloned Harvey murine sarcoma virus-transformed Madin Darby canine kidney (MDCK) cells have been isolated. These subclones fall into two general classes. Two subclones have a fibroblastic morphology, have lost the growth requirement for prostaglandin E1 (PGE1), do not respond to glucagon or vasopressin, and, in general, appear transformed. Two other subclones have epithelioid morphologies, are growth-stimulated by PGE1, respond to vasopressin with an increase in intracellular cAMP. We propose that these cells represent revertants to a more non-transformed phenotype. Unlike normal cells, however, these revertants grow under anchorage-independent conditions, express detectable but reduced amounts of the transforming gene product, p21, and grow in nude mice. The appearance of such revertants may be one cause of the observed heterogeneity of tumor cells.

Methionine-sensitive glycolysis in transformed cells

Glycolysis in several tumor cell lines grown in tissue culture was inhibited by methionine. Kirsten murine sarcoma virus-transformed rat kidney cells (K-NRK) were inhibited 60-75% by 10 mM methionine, whereas normal rat kidney (NRK-49F) cells showed little or no inhibition. Inhibition of glycolysis in K-NRK cells was manifest 2-4 hr after exposure to the amino acid. Glycolysis in a chemically transformed cell line of Madin-Darby canine kidney cells was also sensitive to methionine, but maximal inhibition (75%) required 18-24 hr of incubation with the amino acid. Under the same conditions glycolysis in the nontransformed canine cells was less than 20% inhibited by methionine. In Ehrlich ascites tumor cells grown in tissue culture, 10 mM methionine inhibited glycolysis by about 50%. Inhibition of glycolysis, even by 50 mM methionine, was rapidly reversible. Within 2 hr after removal of methionine the rate of glycolytic activity was restored to that observed in control cells. Furthermore, inhibition by methionine required a minimum level (7%) of serum in the growth medium and inhibition was not sensitive to cycloheximide. Only amino acids that are transported by system A (including the nonmetabolized analogue methylaminoisobutyric acid) specifically inhibited glycolysis in tumor cells. The only exception was phenylalanine, which was toxic to both transformed and normal cell lines.

Adaptive regulatory control of System A transport activity in a kidney epithelial cell line (MDCK) and in a transformed variant (MDCK-T1)

Adaptive regulatory control of System A activity was investigated using MDCK cells and a chemically induced, oncogenic transformant of MDCK cells, MDCK-T1. Within 7 hours after transfer to an amino-acid-deficient medium, A activity of subconfluent MDCK cells had maximally derepressed, but this activity in confluent MDCK cells and in subconfluent transformed cells showed little capacity for derepression. Amino-acid-starved, subconfluent MDCK cells were used to study trans-inhibition and repression of A activity by individual amino acids. Trans-inhibition and repression were defined as the cycloheximide-insensitive and cycloheximide-sensitive components, respectively, of the total inhibition. Trans-inhibition correlated well with substrate affinity, but repression did not. Trans-inhibition and repression were further characterized using alpha-(methylamino) isobutyric acid (mAIB), a trans-inhibitor, and glutamate, an effective repressor. The apparent initial T 1/2 for inhibition by mAIB in the presence of cycloheximide was 0.5 hours, while that for repression by glutamate was 4.7 hours. Half-maximal inhibition by mAIB and repression by glutamate occurred at approximately 0.02 mM and 0.07 mM, respectively. Reversal of trans-inhibition by methionine occurred in the presence of cycloheximide within 1-4 hours after removal of methionine. The A system of the transformed MDCK-T1 cells showed elevated activity, little capacity for derepression, resistance to repression by amino acids, but retention of sensitivity to trans-inhibition. Kinetic analysis of mAIB uptake indicated that the A system of MDCK-T1 cells has become kinetically more complex in a manner which resembled amino-acid-starved rather than amino-acid-fed MDCK cells. These results suggest that the A system of MDCK-T1 cells has become resistant to adaptive regulatory control.