Maintenance of proximal and distal cell functions in SV40-transformed tubular cell lines derived from rabbit kidney cortex

Major influence of cell differentiation status on characteristics of proteoglycans synthesized by cultured rabbit renal proximal tubule cells: role of insulin and dexamethasone

To analyze the influence of epithelial cell differentiation and the effects of hormones on the characteristics of cell-associated and secreted proteoglycans (PGs), we studied their distribution, synthesis, and biochemical features in a model of renal proximal tubule cells in primary culture in which cell differentiation could be controlled by medium composition. In cells cultured in serum-free, hormonally defined medium supplemented with insulin and dexamethasone that exhibited a high degree of morphological and functional proximal differentiation (Ronco et al., 1990), cell-associated PGs were similar to those extracted in vivo by their size estimated by Sepharose CL-6B chromatography (Kav = 0.27, vs. 0.26), composition (heparan-sulfate), and localization in a continuous basal layer of extra-cellular matrix (ECM). In contrast, major quantitative and qualitative anomalies of cell-associated PGs were observed in poorly differentiated cells grown in 1% fetal calf serum-supplemented medium (FCS). PGs alterations included: (1) reduced and irregular expression of PGs at the cell basal pole, (2) a 2.8-fold decrease in [35S]-sulfate incorporation into cell-associated PGs, (3) a 3.1-fold increase in trypsin-releasable PGs, and (4) the emergence of a high MW PG composed exclusively of chondroitin-sulfate (CS) (Kav = 0.09 on Sepharose CL-6B) as well as of putative free CS-glycosaminoglycan (GAG) chains (Kav = 0.49 on Sepharose CL-6B). The same alterations were identified in the basal defined medium devoid of hormones but were partially or totally abolished by addition of insulin and dexamethasone, respectively. At variance with cell-associated PGs, production and biochemical features of secreted PGs were not influenced by cell differentiation status and medium composition.

Development and characterization of rabbit proximal tubular epithelial cell lines

We have isolated rabbit kidney proximal tubular epithelial cell lines. The selection was based on their ability to form confluent monolayers on porous supports and to maintain receptor-mediated signal transduction and ion transport, characteristic of the proximal tubule. The isolation method consisted of several steps: (1) superficial cortical proximal tubule segments were microdissected and cultured on a matrix-coated porous support until cells formed a confluent monolayer; (2) primary cultures showing hormone-regulated ion transport typical for the proximal tubule were selected and co-cultured with irradiated fibroblasts; and (3) the epithelial cells surviving after several passages were expanded and passaged on porous substrates. Most of the cell lines developed in this manner were obtained by co-culture with irradiated fibroblasts producing a recombinant retrovirus encoding SV40 large T antigen and G418 resistance. However, SV40 T antigen expression was not essential for immortalization, since neither T antigen nor G418 resistance was detected in the isolated cell lines and co-culture with non-producing 3T3 cells gave similar results. One cell line (vEPT) has been characterized in some detail with respect to morphological, biochemical, and ion transport properties. This line forms confluent monolayers with apical microvilli, tight junctions, and convolutions of the basolateral plasma membrane. Once confluent, monolayers maintain conductances of 25 to 32 mS/cm2 for several weeks in culture and possess phlorizin-sensitive short-circuit current (Isc) in glucose containing media, indicative of apical Na(+)-glucose co-transport. vEPT cells also retain receptor and signaling mechanisms for angiotensin II (Ang II). Apical and basal Ang II and 5,6-epoxy-eicosatrienoic acid (5,6-EET) modulate the Isc in a manner similar to primary cultures. The cell lines share with primary cultures expression of the cytokeratins K8, K10/K11, and K19 ("nomenclature" [21]). They also retain several receptor and signal transduction mechanisms. For example, Ang II, arachidonate, bradykinin, 5,6-EET, parathyroid hormone (residues 1 through 34), and purine nucleotides increase cytosolic Ca2+, PTH elevates cAMP levels, and Ang II enhances proximal tubule-specific arachidonic acid metabolism.

Analysis and isolation of renal tubular cells by flow cytometry

The cells of the renal cortex have rich heterogeneity of structure and function. Flow cytometry, the technique of rapid laser-based single cell analysis, can give information about cellular mixtures not obtainable by any other means. We examined a variety of fluorescent markers to identify populations of renal cells by flow cytometry. Cellular digests of rat cortex were fluorescently stained with either enzymatic activity probes, or polyclonal antibodies. Fluorescent staining for the proximal marker gamma-glutamyl-transpeptidase (tau-GT) was an order of magnitude brighter than autofluorescence, and stained 71 +/- 11% of the cells. Second, we colocalized enzymatic and antibody markers. There was tight colocalization of tau-GT enzyme activity, detected with fluorogenic substrates, with specific surface binding of tau-GT antibodies. Third, populations of fluorescently labelled cells can be rapidly isolated by flow cytometry sorting. Flow cytometry sorting isolated 10(7) cells positive for the proximal tubular marker tau-GT in a little under one hour. The sorted cells were viable with 99 +/- 2% trypan blue exclusion (N = 8). Sodium-dependent phloridzin-inhibitable glucose uptake was present in sorted cells, with greater uptake/mg protein than in unsorted controls. The sorted cells grew in culture as a monolayer of tightly adherent cuboidal cells. Hence, flow cytometry allows us to quantitate the heterogeneity in mixed renal cellular digests. Flow cytometry allows us to rapidly isolate millions of cells according to fluorescently tagged markers. The isolated cells are viable, retain sodium-dependent transport properties, and grow in culture.

Characterization of a simian virus 40-transformed human podocyte cell line producing type IV collagen and exhibiting polarized response to atrial natriuretic peptide

Biology of glomerular visceral epithelial cells ("podocytes") and their role in inflammatory process remain obscure, partly because of the lack of well-differentiated podocyte cultures. We have established a human cell line by transfecting with a replication-defective SV40 plasmid (pSVHB1), a primary culture of podocytes derived from an enriched preparation of unencapsulated glomeruli free of tubule and Bowman's capsule contaminants. Podocyte specificity of the primary culture was assessed by a dual immunomorphological and functional approach. The resulting cell line (HGVEC.SV1) was cloned and the clonal cells were adapted to hormonally defined medium supplemented with only 2% newborn bovine serum. Clone A4 has been exhibiting over 35 passages, a combination of markers unique to podocytes, including expression of vimentin, podocalyxin, ectoenzymes (CALLA antigen and mRNA), heparan-sulfate proteoglycans (molecular mass of core protein = 75 kDa), and production of type IV collagen (alpha 1 and alpha 5 chains) established by immunoprecipitation and Northern blot analysis. Cytokeratin was detected in rare cellular foci and the search of Von Willebrand factor was negative. This clonal cell line has been used to demonstrate: (1) that human podocytes are highly sensitive to atrial natriuretic peptide (ANP) which induced a dose-dependent increase in cGMP production (x20 at 0.5 microM ANP), and (2) that secretion of ANP-stimulated cGMP is dramatically polarized as 93% of extracellular cGMP were released in the apical medium when filter-grown HGVEC. SV1A4 cells were stimulated at their basal pole.

Regulation by calcitonin of Na(+)-K(+)-Cl- cotransport in a rabbit thick ascending limb cell line

The hormonal regulation of a Na(+)-K(+)-Cl- cotransport was investigated in a renal tubule cell line (RC.SV2 cells) transformed by the simian virus 40. This cell line has the main characteristics of cells from the thick ascending limb of Henle, including the presence of Tamm-Horsfall protein and stimulation of adenosine 3',5'-cyclic monophosphate (cAMP) production by calcitonin (CT). Kinetic studies with 22Na+, 36Cl-, and 86Rb+ indicated the existence of a Na(+)-K(+)-Cl- cotransport with a stoichiometry of 1Na+:1K+: 2Cl-. All compounds stimulating cAMP production enhanced the ouabain-resistant bumetanide-sensitive (Or-Bs) Rb+ influx mediated by Na(+)-K(+)-Cl- cotransport. CT (100 ng/ml) increased the Or-Bs influx twofold by enhancing maximum velocity without changing the apparent Michaelis constant. The K(+)-channel blocker barium blunted the CT-stimulated Or-Bs influx by 64-74%, whereas the Cl(-)-channel blocker 5-nitro-2-(3-phenylpropylamino)benzoate reduced the CT-stimulated influx by 28-40%. These results suggest that CT stimulates the Na(+)-K(+)-Cl- cotransport by a cAMP-dependent mechanism and that K+ recycling through K+ membrane channels is an important modulator of cotransporter-mediated ion fluxes.

Transformation of renal tubule epithelial cells by simian virus-40 is associated with emergence of Ca(2+)-insensitive K+ channels and altered mitogenic sensitivity to K+ channel blockers

We compared the pattern of K+ channels and the mitogenic sensitivity to K+ channel blocking agents in primary cultures of rabbit proximal tubule cells (PC.RC) (Ronco et al., 1990) and two derived SV-40-transformed cell lines exhibiting specific functions of proximal (RC.SV1) and more distal (RC.SV2) tubule cells (Vandewalle et al., 1989). First, K+ channel equipment surveyed by the patch-clamp technique was modified after SV-40 transformation in both cell lines; although a high conductance Ca(2+)-activated K+ channel [K+200 (Ca2+)] remained the most frequently recorded K+ channel, the transformed state was characterized by emergence of three Ca(2+)-insensitive K+ channels (150, 50, and 30 pS), virtually absent from primary culture, contrasting with reduced frequency of two Ca(2+)-sensitive K+ channels (80 and 40 pS). Second, quinine (Q), tetraethylammonium ion (TEA) and charybdotoxin (CTX), at concentrations not affecting cell viability, all decreased 3H-TdR incorporation and cell growth in PC.RC cultures, but only TEA had similar effects in transformed cells. The latter were further characterized by paradoxical effects of Q that induced a marked increase in thymidine incorporation. Q also exerted contrasting effects on channel activity: it inhibited the [K+200 (Ca2+)] when the channel was highly active, with a Ki (0.2 mM) similar to that measured for 3H-TdR incorporation in PC.RC cells (0.3 mM), but increased the mean current through poorly active channels. TEA blocked all K+ channels with conductance greater than or equal to 50 pS, including the [K+200 (Ca2+)], in a range of concentrations that substantially affected cell proliferation. The unique effect of TEA on SV-40-transformed cells might be related to broad inhibition of K+ channels.

Selective inhibition of the synthesis of phosphoenolpyruvate carboxykinase in freshly isolated proximal tubule cells

The behaviour of renal phosphoenolpyruvate carboxykinase (PEPCK) in non-growing kidney cell suspension culture was investigated. In heterogeneous kidney cells, glomeruli-proximal tubule mixtures and purified proximal tubule fragments, the enzyme activity fell with a t1/2 of 3.3 h. Hormonal additions did not modify the process. The fall was also observed in cells prepared from animals preinduced with dexamethasone. The in vitro response of PEPCK is tissue specific. The decline was not the result of a decrease in cell viability, enzyme leakage nor due to the mitochondrial translocation of cytosolic PEPCK. The response observed is due to the selective inhibition of the synthesis of the enzyme and its destruction by non-lysosomal mechanisms. Acute and chronic acidification do not affect PEPCK activity. However, the degradation of PEPCK is temperature dependent and is greatly accelerated by glutathione.

SV40-immortalization of rabbit articular chondrocytes: alteration of differentiated functions

Cell lines were established from rabbit articular chondrocytes following transfection with a plasmid encoding SV40 early function genes. This resulted in cell immortalization (130 passages have been completed for the oldest cell line) with acquisition of characteristics of partial transformation such as reduced serum requirements for normal and clonal growth. The immortalized chondrocytes, called SVRAC, did not form multilayer foci when maintained in postconfluent culture. Their ability to form colonies in soft agar was not increased in comparison with normal chondrocytes, but they were weakly tumorigenic in nude mice. SVRAC lost the ability to synthesize type II collagen and Alcian blue-stainable matrix, which are markers of the differentiated chondrocyte phenotype, and synthesized predominantly type I collagen. Studies of collagen gene expression showed that pro alpha 1 (II) mRNA was undetectable, whereas pro alpha 1 (I) collagen mRNA was expressed even in late passage cultures. Unlike normal dedifferentiated chondrocytes, SVRAC were unable to re-express the differentiated phenotype in response to tridimensional culture or microfilament depolymerization. Cell lines obtained from chondrocytes transfected either in primary culture or just after release of cells from cartilage displayed the same behaviour. Thus SV40 early genes were able to immortalize rabbit articular chondrocytes, but the resulting cell lines displayed an apparently irreversibly dedifferentiated phenotype. These cell lines can be used as models to identify regulatory pathways that are required for the maintenance or reexpression of differentiated function in chondrocytes.

Stable cell line of T-SV40 immortalized human glomerular visceral epithelial cells

Human subcultures (third passage) of glomerular visceral epithelial cells (VEC) isolated from one month old kidney were successfully transfected by two recombinant plasmids containing the cloned oncogenes from the simian virus 40 large T antigen and H-ras gene. One postcrisis cell clone (56/10 A1) was selected, propagated and characterized. One hundred percent of the 56/10 A1 cells (current passage greater than 100th; doubling time 30 hrs) expressed the nuclear T-SV40 antigen assayed by IF; the cells failed to express H-ras (RNA blot analysis). Immortalized cells were morphologically and phenotypically compared to parental cell type (third passage). Phenotypic characterization of the 56/10 A1 cells was achieved using indirect immunofluorescence (IF) and immunogold silver staining coupled to bright field and epipolarization microscopy. Both parental and 56/10 A1 cells displayed positivity for cytokeratin, CALLA and PHM5, whereas von Willebrand factor was not detected in the two cell types. Since we have previously shown that human glomerular epithelial cells in culture synthetize plaminogen activator (PA) related compounds, we investigated the secretion pattern of these products in parental and transfected cells. Zymographic analysis of secreted PA related compounds revealed production of free urokinase (u-PA) and type 1 plasminogen activator inhibitor (PAI-1) complexed to tissular plasminogen activator (t-PA). Finally, in the transfected cells, increased cGMP generation under atrial natriuretic factor (ANF) stimulation agreed with previous work performed on nontransfected human VEC. In conclusion, the establishment of a human permanent cell line which retains most of the phenotypic features of parental glomerular visceral epithelial cells should represent a new tool to study human glomerular cell functions.

Kidney-specific enzyme expression by human kidney cell lines generated through oncogene transfection

The human kidney cell line 293 was generated by transfection of adenovirus DNA into normal human embryonic kidney (HEK) cells (Graham et al., 1977), whereas the human kidney cell lines ST-1i and STt-4i were generated by transfection of HEK cells with plasmids encoding SV40 viral oncogenes (Abcouwer et al., 1989). In this study, we examined kidney-specific enzyme activity levels in 293, ST-1i, and STt-4i cells to determine their ability to exhibit kidney-specific gene expression. Enzymes examined were leucine aminopeptidase (LAP), gamma-glutamyl transpeptidase (gamma-GTP), and the disaccharidases trehalase and maltase. Enzymatic activity levels were compared to three other kidney cell lines (MDCK, OK, and LLC-PK1) as well as to normal human embryonic kidney (HEK) cells and the human hepatoma cell line, Hep G2. Modulation of kidney-specific enzyme activities was assessed in response to several differentiation-inducing agents (adenosine, n-butyric acid, hexamethylene bisacetamide (HMBA), dimethyl sulfoxide (DMSO), N,N'-dimethylformamide (DMF), isobutyl methyl xanthine (IBMX), di butyryl cAMP, and retinoic acid). ST-1i and STt-4i exhibit elevated levels of LAP, gamma-GTP, trehalase, and maltase, consistent with their kidney cell origin, whereas 293 cells exhibit elevated levels of just gamma-GTP and maltase. Maltase and gamma-GTP enzyme activities in ST-1i and STt-4i cells were very responsive to the various inducing agents; 293 cells were less responsive at the inducer concentrations examined. None of the three human cell lines formed domes under any of the experimental conditions. In summary, ST-1i and STt-4i are comparable to normal HEK cells in expression of kidney-specific enzymes and in responsiveness to differentiation-inducing agents, in spite of continued expression of SV40 oncogenes.

Establishment of permanent renal tubule cell lines by infection with the wild-type and a thermosensitive mutant of the simian virus 40

This study summarizes the properties of rabbit renal tubule cell lines transformed by the simian virus 40 (SV40). By infection with the wild type SV40, it was possible to establish three different cell lines exhibiting the main functions of proximal (RC.SV1), thick ascending limb-distal (RC.SV2), and collecting (RC.SV3) tubules. Another cell line infected with a thermosensitive mutant of SV40 (RC.SVtsA58) was highly sensitive to arginine vasopressin when cells were cultured at the restrictive temperature of 39.5 degrees C, suggesting they derived from principal cells of the collecting tubule.

Activation of the simian virus 40 (SV40) genome abrogates sensitivity to AVP in a rabbit collecting tubule cell line by repressing membrane expression of AVP receptors

To analyze the role of SV40 genome in the phenotypic alterations previously observed in SV40-transformed cell lines, we infected rabbit renal cortical cells with a temperature-sensitive SV40 mutant strain (tsA58) and compared the cell phenotypes at temperatures permissive (33 degrees C) and restrictive (39.5 degrees C) for SV40 genome expression. At both temperatures, the resulting cell line (RC.SVtsA58) expresses cytokeratin and uvomorulin, but epithelial differentiation is more elaborate at 39.5 degrees C as shown by the formation of a well-organized cuboidal monolayer with numerous tight junctions and desmosomes. Functional characteristics are also markedly influenced by the culture temperature: cells grown at 33 degrees C respond only to isoproterenol (ISO, 10(-6) M) by a sevenfold increase in cAMP cell content above basal values; in contrast, when transferred to 39.5 degrees C, they exhibit increased sensitivity to ISO (ISO/basal: 19.1) and a dramatic response to 10(-7) M dDarginine vasopressin (dDAVP/basal: 18.2, apparent Ka: 5 X 10(-9) M) which peaks 48 h after the temperature shift. The latter is associated with membrane expression of V2-type AVP receptors (approximately 50 fmol/10(6) cells) which are undetectable when SV40 genome is activated (33 degrees C). Clonal analysis, additivity studies, and desensitization experiments argue for the presence of a single cell type responsive to both AVP and ISO. The characteristics of the RC. SVtsA58 cell line at 39.5 degrees C (effector-stimulated cAMP profile, lack of expression of brush-border hydrolases and Tamm-Horsfall protein) suggest that it originates from the cortical collecting tubule, and probably from principal cells.

Altered growth regulation of rat kidney proximal tubule epithelial cells transformed in vitro by SV40 viral DNA: fibroblast growth factors (heparin-binding growth factors) are potent inducers of anchorage-independent growth

The majority of renal cancers are thought to arise from the proximal tubule epithelium, but little is known about their etiology. In this investigation, we have established an in vitro model to study the transformation of these target cells using rat kidney proximal tubule epithelial cells (RPTE) transformed in defined medium with SV40-viral DNA. Selection by passaging cells onto plastic surfaces yielded a population of cells (SV-RPTE) that expressed keratin and vimentin along with SV40 large-T antigen. The cells were morphologically transformed and lost their differentiated character as determined by several RPTE markers. SV-RPTE cells grew in soft agar in serum-supplemented medium containing insulin, epidermal growth factor, and cholera toxin, but were unable to grow when serum and growth factors were not combined. Acidic and basic fibroblast growth factors (aFGF and bFGF) were unique since they were the only single factor that induced anchorage-independent growth in the presence of serum alone. Transforming growth factor-beta 1 (TGF-beta 1) was a potent inhibitor of anchorage-independent growth, but the inhibition was partially overcome by a combination of growth factors. The growth factor responses of SV-RPTE in monolayer cultures differed from those in soft agar; the cells were more sensitive to growth stimulation by insulin and insulin-like growth factor, neither of which stimulated anchorage-independent growth. SV-RPTE cells in monolayer cultures had also lost the sensitivity to growth inhibition by TGF-beta 1 characteristic of normal RPTE. The RPTE transformation model described here will be very useful for investigating the molecular basis and etiology of renal cancers. Furthermore, the data suggest that maintenance of the transformed phenotype by aFGF and bFGF and loss of negative growth regulation by TGF-beta 1 could play a role in renal carcinogenesis.