Thyroid epithelial cell transformation by a retroviral vector expressing SV40 large T

From Infancy to Fancy: A Glimpse into the Evolutionary Journey of Podocytes in Culture

Podocytes are critical components of the filtration barrier and responsible for maintaining healthy kidney function. An assault on podocytes is generally associated with progression of chronic glomerular diseases. Therefore, podocyte pathophysiology is a favorite research subject for nephrologists. Despite this, podocyte research has lagged because of the unavailability of techniques for culturing such specialized cells ex vivo in quantities that are adequate for mechanistic studies. In recent years, this problem was circumvented by the efforts of researchers, who successfully developed several in vitro podocyte cell culture model systems that paved the way for incredible discoveries in the field of nephrology. This review sets us on a journey that provides a comprehensive insight into the groundbreaking breakthroughs and novel technologic advances made in the field of podocyte cell culture so far, beginning from its inception, evolution, and progression. In this study, we also describe in detail the pros and cons of different models that are being used to culture podocytes. Our extensive and exhaustive deliberation on the status of podocyte cell culture will facilitate researchers to choose wisely an appropriate model for their own research to avoid potential pitfalls in the future.

Culture methods of glomerular podocytes

Podocytes (glomerular visceral epithelial cells) cover the exterior surface of the glomerular capillaries and contribute to the glomerular filtration membrane. Failure of podocyte function is involved in the progression of chronic glomerular disease; accordingly, research interest into podocyte biology is driven by the need for better protection and perhaps recovery of these cells in renal diseases. This review aims at summarizing available techniques for podocyte cell cultures from both the past and present, with special attention to the currently used methods. The establishment of classical primary cultures is based on isolation of glomeruli by differential sieving. Plating of glomeruli onto a collagen surface is followed by an outgrowth of cobblestone-like cells that, after replating, differentiate into arborized, mature podocytes. Currently, the majority of research studies use immortalized podocytic cell lines most often derived from transgenic mice bearing a conditional immortalizing gene. The podocytes can also be collected and cultured from healthy or diseased animal or patient urine. The urinary podocytes obtained from subjects with active glomerulopathies display higher proliferation potential and viability in vitro, perhaps due to disease-induced transdifferentiation. Finally, a list of phenotypic markers useful for identification and characterization of the cultured podocytic elements is provided.

PAX8PPARgamma stimulates cell viability and modulates expression of thyroid-specific genes in a human thyroid cell line

OBJECTIVE

Paired box gene 8/peroxisome proliferator-activated receptor gamma (PAX8PPARgamma) translocation is a molecular event associated with follicular thyroid tumorigenesis and is generated by a chromosomal rearrangement between PAX8 and PPARgamma genes. In this study, we investigated the effects of PAX8PPARgamma fusion protein on cell growth and on thyroid-specific gene expression in immortalized human thyroid cells (Nthy-ori 3-1).

METHODS

PAX8PPARgamma-, PAX8-, and thyroid transcription factor-1 (TTF-1)-transfected cell culture models; count of live and dead cells; mRNA analysis by reverse transcription-polymerase chain reaction (RT-PCR) and quantitative RT-PCR; and protein analysis by western blotting and gel shift assays.

RESULTS

Cells transfected with the PAX8PPARgamma fusion gene showed higher cell viability at 24, 48, and 72 hours after transfection than cells transfected with control vectors. A PAX8 expression vector increased thyroglobulin (Tg), sodium/iodide symporter (NIS), and thyroid-stimulating hormone (thyrotropin) receptor (TSHR) mRNA levels in a dose-dependent manner. TTF-1 expression vector promoted a significant increase of Tg mRNA level, but had no effect on NIS and TSHR mRNA levels. PAX8PPARgamma transfectants presented a significant decrease in TSHR mRNA level compared to empty vector, but had no effect on Tg and NIS mRNA levels. PAX8 plus PAX8PPARgamma significantly lowered Tg and TSHR mRNA expression levels, but upregulated NIS mRNA level, compared to PAX8 plus control vector.

CONCLUSION

The results obtained with this in vitro system demonstrated that PAX8PPARgamma increases thyroid cell viability and has opposite effects on thyroid-specific gene expression, suggesting that the presence of this rearrangement may contribute to the malignant transformation of thyroid follicular cells.

Human gingival epithelial cells produce chemotactic factors interleukin-8 and monocyte chemoattractant protein-1 after stimulation with Porphyromonas gingivalis via toll-like receptor 2

BACKGROUND

The mechanism of stimulation of human gingival epithelial cells (HGEC) by Porphyromonas gingivalis (Pg) has not been fully clarified yet. In order to investigate the possible activation of HGEC by Pg through Toll-like receptors (TLRs), we analyzed the production of chemotactic factors and the activated nuclear factor-kappa B (NF-kappaB).

METHODS

The mRNA expression of TLRs and the protein expression of TLR2 and TLR4 in HGEC and gingival tissue were assessed using reverse transcription-polymerase chain reaction (RT-PCR) assay and immunohistochemical staining. Primary cultured HGEC (nHGEC) and HGEC transformed by simian virus 40 T antigen (OBA-9) were activated by a sonic extract (SE) of Pg to examine cytokine production and NF-kappaB activation using enzyme-linked immunosorbant assay (ELISA). In addition, Pg mediated activation of NF-kappaB in a TLR2-transfectant was also investigated.

RESULTS

RT-PCR results revealed that HGEC expressed mRNA of TLR2, TLR4, TLR5, and TLR9, although the expression profiles of each cell line were slightly different. In addition, immunostaining revealed the prominent expression of TLR2 not only in nHGEC, but also in the gingival epithelium of the tissue specimen. Interestingly, nHGEC and OBA-9 secreted IL-8 and monocyte chemoattractant protein (MCP)-1 upon stimulation with Pg SE more efficiently than LPS and fimbriae of Pg. Furthermore, Pg SE increased the activated NF-kappaB not only in OBA-9, but also in 293T cells transfected with the human TLR2 gene.

CONCLUSION

TLR2 participates, at least partly, in the signaling pathway to induce chemokine production in gingival epithelium as a reaction against Pg component(s), probably other than lipopolysaccharide and fimbriae.

The PAX8/PPARγ fusion oncoprotein transforms immortalized human thyrocytes through a mechanism probably involving wild-type PPARγ inhibition

Follicular thyroid carcinoma (FTC) frequently harbors the PAX8/PPARgamma fusion gene (PPFP); however, its oncogenic role and mechanism(s) of action remain undefined. We investigated PPFP's effects on cell growth, apoptosis, cell-cell, and cell-matrix interactions in immortalized human thyroid cells (Nthy-ori 3-1) and NIH 3T3 cells. PPFP expression increased the growth of transient and stable Nthy-ori transfectants ( approximately threefold by 72 h). There was an 8.4% increase of cells in the S+G2/M phase, a 7.8% decrease in cells in the G0+G1 phase and a 66% decline in apoptosis at 72 h. Stable Nthy-ori PPFP transfectants grew in soft agar, and PPFP-transfected NIH 3T3 cells exhibited efficient focus formation, suggesting loss of anchorage-dependent growth and contact inhibition, respectively. Overexpression of PPARgamma in Nthy-ori cells did not recapitulate PPFP's growth effects. Treatment of Nthy-ori cells with an irreversible PPARgamma inhibitor mimicked the growth-promoting effects of PPFP and co-expression of PPFP and PPARgamma blocked PPARgamma transactivation activity. Our data provide functional evidence that PPFP acts as an oncoprotein, whose transforming properties depend in part on inhibition of PPARgamma. Our data suggest that PPFP contributes to malignant transformation during FTC oncogenesis by acting on several cellular pathways, at least some of which are normally regulated by PPARgamma.

Establishment and characterization of an immortalized Z310 choroidal epithelial cell line from murine choroid plexus

The choroid plexus plays a wide range of roles in brain development, maturation, aging process, endocrine regulation, and pathogenesis of certain neurodegenerative diseases. To facilitate in vitro study, we have used a gene transfection technique to immortalize murine choroidal epithelial cells. A viral plasmid (pSV3neo) was inserted into the host genome of primary choroidal epithelia by calcium phosphate precipitation. The transfected epithelial cells, i.e., Z310 cells, that survived from cytotoxic selection expressed SV40 large-T antigen throughout the life span, suggesting a successful gene transfection. The cells displayed the same polygonal epithelial morphology as the starting cells by light microscopy. Immunocytochemical studies demonstrate the presence of transthyretin (TTR), a thyroxine transport protein known to be exclusively produced by the choroidal epithelia in the CNS, in both transfected and starting cells. Western blot analyses further confirm the production and secretion of TTR by these cells. The mRNAs encoding transferrin receptor (TfR) were identified by Northern blot analyses. The cells grow at a steady rate, currently in the 110th passage with a population doubling time of 20-22 h in the established culture. When Z310 cells were cultured onto a Trans-well apparatus, the cells formed an epithelial monolayer similar to primary choroidal cells, possessing features such as an uneven fluid level between inner and outer chambers and an electrical resistance approximately 150-200 omega-cm(2). These results indicate that immortalized Z310 cells possess the characteristics of choroidal epithelia and may have the potential for application in blood-CSF barrier (BCB) research.

Human thyroid cancer cells as a source of iso-genic, iso-phenotypic cell lines with or without functional p53

Differentiated thyroid carcinomas (in contrast to the rarer anaplastic form) are unusual among human cancers in displaying a remarkably low frequency of p53 mutation and appear to retain wild-type (wt) p53 function as assessed by the response of derived cell lines to DNA damage. Using one such cell line, K1, we have tested the effect of experimental abrogation of p53 function by generating matched sub-clones stably expressing either a neo control gene, a dominant-negative mutant p53 (143ala) or human papilloma virus protein HPV16 E6. Loss of p53 function in the latter two groups was confirmed by abolition of p53-dependent 'stress' responses including induction of the cyclin/CDK inhibitor p21WAF1 and G1/S arrest following DNA-damage. In contrast, no change was detected in the phenotype of 'unstressed' clones, with respect to any of the following parameters: proliferation rate in monolayer, serum-dependence for proliferation or survival, tumorigenicity, cellular morphology, or tissue-specific differentiation markers. The K1 line therefore represents a 'neutral' background with respect to p53 function, permitting the derivation of functionally p53 + or - clones which are not only iso-genic but also iso-phenotypic. Such a panel should be an ideal tool with which to test the p53-dependence of cellular stress responses, particularly the sensitivity to potential therapeutic agents, free from the confounding additional phenotypic differences which usually accompany loss of p53 function. The results also further support the hypothesis that p53 mutation alone is not sufficient to drive progression of thyroid cancer to the aggressive anaplastic form.

Direct growth stimulation of normal human epithelial cells by mutant p53

We developed a high-titer amphotropic retroviral vector that expresses mutant (Ala143) human p53 to test directly the response of genetically normal human epithelial cells to p53 mutation. Contrary to our prediction, we found that in pancreatic epithelium (whose tumors display a high frequency of p53 mutation) but not in thyroid (whose tumors show an exceptionally low mutation frequency), expression of mutant p53 induced a dramatic, though self-limiting, proliferative response. This result questions the assumption that p53 mutation is relevant only to the later stages of tumorigenesis.

Stepwise transformation of primary thyroid epithelial cells by a mutant Ha-ras oncogene: an in vitro model of tumor progression

Activating mutations of the ras oncogene family occur at high frequency in all stages of thyroid tumorigenesis, both human and experimental. To test the causal nature of this association, and to investigate the biological role of ras mutation, we introduced a mutant c-Ha-ras gene into normal rat thyroid follicular cells using an ecotropic retroviral vector. The major immediate effect was to greatly extend the proliferative lifespan of these cells in culture from less than 3 to more than 15 doublings, without any observable loss of growth-factor dependence or differentiated functions. This in vitro phenotype strongly supports an initiating role for ras mutation in the genesis of benign thyroid tumors (adenomas) in vivo. Spontaneous transformation was observed at low frequency on continuous culture of mutant ras-expressing cells, giving rise to fully immortalized, growth factor-independent, highly tumorigenic lines. Transformation was associated with (i) loss of responsiveness to the growth inhibitor TGF-beta 1, and (ii) greatly increased nuclear levels of p53 protein, which unexpectedly was not due to point mutation in the conserved regions of the p53-coding sequence. We postulate that these two phenomena are causally related to each other and to the transformed phenotype.

Direct derivation of conditionally immortal cell lines from an H-2Kb-tsA58 transgenic mouse

Studies on cell lines have greatly improved our understanding of many important biological questions. Generation of cell lines is facilitated by the introduction of immortalizing oncogenes into cell types of interest. One gene known to immortalize many different cell types in vitro encodes the simian virus 40 (SV40) large tumor (T) antigen (TAg). To circumvent the need for gene insertion in vitro to generate cell lines, we created transgenic mice harboring the SV40 TAg gene. Since previous studies have shown that TAg expression in transgenic mice is associated with tumorigenesis and aberrant development, we utilized a thermolabile TAg [from a SV40 strain, tsA58, temperature sensitive (ts) for transformation] to reduce the levels of functional TAg present in vivo. To direct expression to a broad range of tissues, we used the mouse major histocompatibility complex H-2Kb promoter, which is both widely active and can be further induced by interferons. tsA58 TAg mRNA was expressed in tissues of all animals harboring the hybrid construct. Development of all tissues was macroscopically normal except for thymus, which consistently showed hyperplasia. Fibroblast and cytokeratin+ thymic epithelial cultures from these mice were readily established without undergoing crisis and were conditionally immortal in their growth; the degree of conditionality was correlated with the levels of tsA58 TAg detected. One strain of H-2Kb-tsA58 mice has been bred through several generations to homozygosity and transmits a functional copy of the transgene.

Conditional immortalization of human thyroid epithelial cells: a tool for analysis of oncogene action

To overcome the difficulty of assessing oncogene action in human epithelial cell types, such as thyroid, which have limited proliferative potential in culture, we have explored the use of temperature-sensitive (ts) mutants of simian virus 40 (SV40) early region to create conditionally immortalized epithelial cell lines. Normal primary cultures of human thyroid follicular cells were transfected with a plasmid containing the SV40 early region from mutant tsA58. Expanding epithelial colonies were observed after 2 to 3 months, all of which grew to greater than 200 population doublings without crisis. All showed tight temperature dependence for growth. After switch-up to the restrictive temperature (40.5 degrees C), no further increase in cell number was seen after 1 to 2 days. However, DNA synthesis declined much more slowly; the dissociation from cell division led to marked polyploidy. Viability was maintained for up to 2 weeks. Introduction of an inducible mutant ras gene into ts thyroid cells led, as expected, to morphological transformation at the permissive temperature when ras was induced. Interestingly, this was associated with a marked reduction in net growth rate. At the restrictive temperature, induction of mutant ras caused rapid cell death. These results demonstrate the utility of a ts SV40 mutant to permit the study of oncogene action in an otherwise nonproliferative target cell and reveal important differences in the interaction between ras and SV40 T in these epithelial cells compared with previously studied cell types.

Conditional immortalization of human thyroid epithelial cells: a tool for analysis of oncogene action

To overcome the difficulty of assessing oncogene action in human epithelial cell types, such as thyroid, which have limited proliferative potential in culture, we have explored the use of temperature-sensitive (ts) mutants of simian virus 40 (SV40) early region to create conditionally immortalized epithelial cell lines. Normal primary cultures of human thyroid follicular cells were transfected with a plasmid containing the SV40 early region from mutant tsA58. Expanding epithelial colonies were observed after 2 to 3 months, all of which grew to greater than 200 population doublings without crisis. All showed tight temperature dependence for growth. After switch-up to the restrictive temperature (40.5 degrees C), no further increase in cell number was seen after 1 to 2 days. However, DNA synthesis declined much more slowly; the dissociation from cell division led to marked polyploidy. Viability was maintained for up to 2 weeks. Introduction of an inducible mutant ras gene into ts thyroid cells led, as expected, to morphological transformation at the permissive temperature when ras was induced. Interestingly, this was associated with a marked reduction in net growth rate. At the restrictive temperature, induction of mutant ras caused rapid cell death. These results demonstrate the utility of a ts SV40 mutant to permit the study of oncogene action in an otherwise nonproliferative target cell and reveal important differences in the interaction between ras and SV40 T in these epithelial cells compared with previously studied cell types.

Characterisation of human thyroid epithelial cells immortalised in vitro by simian virus 40 DNA transfection

Human primary thyroid follicular epithelial cells were transfected with a plasmid containing an origin-defective SV40 genome (SVori-) to produce several immortal cell lines. Two of the 10 cell lines analysed expressed specific features of thyroid epithelial function (iodide-trapping and thyroglobulin production). These two lines were characterised in detail and found to be growth factor-independent, capable of anchorage-independent growth at low frequency but non-tumorigenic in nude mice. These differentiated, These differentiated, partially transformed cell lines were shown to be suitable for gene transfer at high frequency using simple coprecipitation techniques.