The correlation of plasma membrane microvilli and intracellular cyclic AMP content in a rat epitheloid kidney cell line

Modulation of the intracellular concentration of cyclic AMP has been associated with a regulatory role in cell division, cell morphology, and physical properties of the plasma membrane. Untransformed rat kidney cells in culture exhibit epitheloid morphology, high intracellular cyclic AMP levels, and contact inhibition of growth. Untransformed rat kidney cells transformed with the Kirsten murine sarcoma virus exhibit a low cyclic AMP content, rapid growth rate, and a loss of contact inhibition. Scanning electron microscopy reveals a distinctive difference in the surface structure of the two cell types during G1 of the cell cycle. The surface of the transformed cell is covered with microvilli while its untransformed counterpart is devoid of microvilli. The presence of microvilli can be controlled as a function of temperature by two temperature-sensitive mutants of the Kirsten sarcoma virus (ts6t6 and ts371 cl 5). In the ts6t6 mutant, growth at 32°C results in a low cyclic AMP content and the presence of microvilli, while growth at 39°C results in a high cyclic AMP content and a decrease in microvilli. The oposite effect is seen with the ts371 cl 5 mutant. Correlation of cyclic AMP content with the presence of microvilli suggests that this surface phenomenon is a function of cyclic AMP concentration.     

The antiproliferative effect of type beta transforming growth factor occurs at a level distal from receptors for growth-activating factors

Transforming growth factor-beta (TGF beta) from human platelets blocks the ability of Mv1Lu mink lung epithelial cells to grow in response to serum mitogens, epidermal growth factor (EGF), or insulin. The phenotypic response of Mv1Lu cells to TGF beta is characterized by a flat, very enlarged cell morphology and a markedly increased production and accumulation of extracellular matrix fibronectin. The ability of TGF beta to alter the ligand binding or signal transducing activity of mitogen receptors in Mv1Lu cells has been examined. In contrast to NRK-49F rat fibroblasts, Mv1Lu cells do not respond to TGF beta with a decrease in the affinity or a change in the number of cell surface receptors for EGF. Soluble extracts from Mv1Lu cells contain a protein kinase activity which selectively phosphorylates ribosomal protein S6; this S6 kinase activity is elevated severalfold minutes after exposure of cells to mitogens. This kinase activity has been used as the parameter to measure the signaling ability of EGF receptors and insulin receptors in cells treated with TGF beta. We find that TGF beta does not alter the basal level of S6 kinase activity or its elevation by EGF or insulin. In TGF beta-treated cells rendered insensitive to the growth-promoting action of EGF, the parameters of elevation of S6 kinase activity by EGF are similar to those of control, growth-competent cells. The results suggest that TGF beta inhibits cell proliferation by acting at a level distal from the receptors for growth-activating factors.     

Characteristics of the upper cell surface in cultures of normal and SV-40 virus-transformed epithelium of mouse kidney. Study with the aid of scanning electron microscopy]

Central cells of the normal epithelial sheet are sparsely covered by microvilli. Numerous microvilli were seen in the regions of intercellular contacts. Marginal cells of sheets had a finely developed lamellar cytoplasm (lameloplasm) with smooth upper surface at their free margins. A transformed cell line (MPTR) resembled normal parent cells by its ability to form monolayered sheets in cultures. More microvilli of increased length appeared on the upper surface of central MPTR cells. The normal structure of lamelloplasm was changed at the free edge of the MPTR sheets. It is suggested that abnormal cell attachment to the substratum may be responsible for the altered cell surface morphology (increased length of microvilli, defective, structure of lamelloplasm) in the MPTR cultures.     

SPARC inhibits epithelial cell proliferation in part through stimulation of the transforming growth factor-beta-signaling system

Secreted protein, acidic and rich in cysteine (SPARC) is a multifunctional secreted protein that regulates cell-cell and cell-matrix interactions, leading to alterations in cell adhesion, motility, and proliferation. Although SPARC is expressed in epithelial cells, its ability to regulate epithelial cell growth remains largely unknown. We show herein that SPARC strongly inhibited DNA synthesis in transforming growth factor (TGF)-beta-sensitive Mv1Lu cells, whereas moderately inhibiting that in TGF-beta-insensitive Mv1Lu cells (i.e., R1B cells). Overexpression of dominant-negative Smad3 in Mv1Lu cells, which abrogated growth arrest by TGF-beta, also attenuated growth arrest stimulated by SPARC. Moreover, the extracellular calcium-binding domain of SPARC (i.e., SPARC-EC) was sufficient to inhibit Mv1Lu cell proliferation but not that of R1B cells. Similar to TGF-beta and thrombospondin-1, treatment of Mv1Lu cells with SPARC or SPARC-EC stimulated Smad2 phosphorylation and Smad2/3 nuclear translocation: the latter response to all agonists was abrogated in R1B cells or by pretreatment of Mv1Lu cells with neutralizing TGF-beta antibodies. SPARC also stimulated Smad2 phosphorylation in MB114 endothelial cells but had no effect on bone morphogenetic protein-regulated Smad1 phosphorylation in either Mv1Lu or MB114 cells. Finally, SPARC and SPARC-EC stimulated TGF-beta-responsive reporter gene expression through a TGF-beta receptor- and Smad2/3-dependent pathway in Mv1Lu cells. Collectively, our findings identify a novel mechanism whereby SPARC inhibits epithelial cell proliferation by selectively commandeering the TGF-beta signaling system, doing so through coupling of SPARC-EC to a TGF-beta receptor- and Smad2/3-dependent pathway.     

Down-regulation of transforming growth factor-beta receptors: cooperativity between the types I, II, and III receptors and modulation at the cell surface.

The types I, II, and III receptors (RI, RII, RIII) for transforming growth factor-beta (TGF-beta) become down-regulated in response to ligand, presumably via their internalization from the cell surface. This report examines the down-regulation of full-length RI, RII, and RIII in cells endogenously or transiently expressing these receptors. Down-regulation occurred rapidly (within 2 h after TGF-beta1 treatment at 37 degrees C) and showed a dose response, between 10 and 200 pM TGF-beta1, in cells expressing RI, RII, and RIII (Mv1lu and A549 cells). A comparison between Mv1Lu and mutant cell derivatives R-1B (lacking RI) or DR-26 (lacking RII) indicated that all three receptors were necessary for efficient down-regulation. Down-regulation experiments, utilizing TGF-beta-treated 293 cells transiently expressing different combinations of these receptors indicated that neither RII or RIII were down-regulated when expressed alone and that RI was required for maximal down-regulation of RII. RII and RIII were partially down-regulated when these receptors were coexpressed in the absence of RI (in R-1B and 293 cells). Surprisingly, TGF-beta receptors were partially down-regulated in Mv1Lu, A549, and 293 cells treated with TGF-beta1 at 4 degrees C. Microscopic examination of 293 cells coexpressing RI fused to green fluorescent protein (RI-GFP) and RII indicated that, after treatment with TGF-beta1 at 4 degrees C, RI-GFP formed aggregates at the cell surface at this temperature. RI-GFP was not detected at the surface of these cells after TGF-beta1 treatment at 37 degrees C. Our results suggest a two phase mechanism for TGF-beta1 receptor down-regulation involving receptor modulation (aggregation) at the cell surface and internalization.     

Comparison of Different Cell Substrates on the Measurement of Human Influenza Virus Neutralizing Antibodies

Eight cell lines were systematically compared for their permissivity to primary infection, replication, and spread of seven human influenza viruses. Cell lines were of human origin (Caco-2, A549, HEp-2, and NCI-H292), monkey (Vero, LLC-MK2), mink (Mv1 Lu), and canine (MDCK). The influenza viruses included seasonal types and subtypes and a pandemic virus. The MDCK, Caco-2, and Mv1 Lu cells were subsequently compared for their capacity to report neutralization titers at day one, three and six post-infection. A gradient of sensitivity to primary infection across the eight cell lines was observed. Relative to MDCK cells, Mv1 Lu reported higher titers and the remaining six cell lines reported lower titers. The replication and spread of the seven influenza viruses in the eight cell substrates was determined using hemagglutinin expression, cytopathic effect, and neuraminidase activity. Virus growth was generally concordant with primary infection, with a gradient in virus replication and spread. However, Mv1 Lu cells poorly supported virus growth, despite a higher sensitivity to primary infection. Comparison of MDCK, Caco-2, and Mv1 Lu in neutralization assays using defined animal antiserum confirmed MDCK cells as the preferred cell substrate for influenza virus testing. The results observed for neutralization at one day post-infection showed MDCK cells were similar (<1 log₂ lower) or superior (>1 log₂ higher) for all seven viruses. Relative to Caco-2 and Mv1 Lu cells, MDCK generally reported the highest titers at three and six days post-infection for the type A viruses and lower titers for the type B viruses and the pandemic H9N2 virus. The reduction in B virus titer was attributed to the complete growth of type B viruses in MDCK cells before day three post-infection, resulting in the systematic underestimation of neutralization titers. This phenomenon was also observed with Caco-2 cells.     

The effect of transfection with the oncogenes H-ras and c-myc on the radiosensitivity of a mink epithelial cell line.

We have investigated the effect of transfection with the oncogenes c-myc and H-ras on cellular radiosensitivity. We obtained a mink lung epithelial line, Mv1Lu (ATCC CCL-64), and two sublines which had been transformed by transfection with c-myc and mutated (T24) H-ras 1. The cell survival parameters do not differ significantly between the three lines. These parameters were, for the parent line: D0 = 1.95 Gy, n = 2.0; for the c-myc transfected line: D0 = 2.10 Gy, n = 2.33; and for the H-ras transfected line: D0 = 2.40 Gy, n = 1.77. Although the terminal slope of the survival curve of the cells of the parent line is slightly steeper than that for the cells of either of the transfected lines, the differences are not significant. Nor is there any difference between the cell lines at the clinically relevant dose of 2 Gy. We conclude that neither activated H-ras nor c-myc oncogenes alter the radiosensitivity of the Mv1Lu line.     

Cell surface protein decreases microvilli and ruffles on transformed mouse and chick cells.

Transformation of cultured fibroblasts usually results in a decrease in a high molecular weight cell surface glycoprotein (LETS protein) and often in increased numbers of surface microvilli and ruffles. We have isolated such a major cell surface glycoprotein from chick embryo fibroblasts; this protein, CSP, is decreased after transformation. Treatment of a mouse tumor cell line (SV1), L929 cells, and transformed chick fibroblasts with CSP results in a decrease in the number of microvilli and marginal ruffles, accompanied by restoration of a more normal morphology.     

Apoptosis of mink lung epithelial cells by co-treatment of low-dose staurosporine and transforming growth factor-beta1 depends on the enhanced TGF-beta signaling and requires the decreased phosphorylation of PKB/Akt

We demonstrate how co-treatment of low-dose staurosporine (STS) and TGF-beta1, which alone have little effect on cell death, markedly induces apoptosis in Mv1Lu mink lung epithelial cells, but not in its clonal variant R1B cells lacking functional TGF-beta signaling. This process was associated with mitochondria-dependent apoptosis and the enhanced TGF-beta/Smad signaling in Mv1Lu cells. When R1B cells were infected with adenovirus carrying wild-type ALK5, a functional TGF-beta type I receptor gene, the apoptotic cell death was significantly restored in these cells following co-treatment of low-dose STS and TGF-beta1. Treatment of Mv1Lu cells with both low-dose STS and TGF-beta1 decreased the activity of phospho-Akt, which is involved in cell survival signal. In addition, pre-treatments of PI3 kinase inhibitors, LY294002 and wortmannin, further increased the apoptosis of MvlLu cells induced by co-treatment of low-dose STS and TGF-beta1. And overexpression of constitutively active Akt (myr-Akt) using adenoviral expression system inhibited the apoptotic cell death of Mv1Lu cells by about 50% upon co-treatment of low-dose STS and TGF-beta1. These results suggest that co-treatment of low-dose STS and TGF-beta1 induces apoptosis of mink lung epithelial cells by enhancing TGF-beta signaling and in part suppressing cytoprotective signaling.     

Colloidal iron hydroxide-binding to the surfaces of chick embryo fibroblasts transformed by wild-type and a temperature-sensitive mutant of Rous sarcoma virus.

The densities of colloidal iron hydroxide (CIH) particles binding to the surfaces of chick embryo fibroblasts were determined before and after transformation with wild type Rous sarcoma virus and a temperature sensitive (ts) mutant of this virus. On the basis of in vitro behavior, cells transformed by the ts virus manifest a malignant phenotype at 36 degrees C (permissive temperature) and appear normal at 41 degrees C (non-permissive temperature). At the permissive temperatures there is a significant increase in CIH particle-binding to spaces of cell surface between microvilli on the wild type and ts transformed cells. At the non-permissive temperature this significant increase in binding is only observed on the wild type transformant, while the density found on the ts transformant is not significantly different from the untransformed state. Therefore, in vitro characteristics of normalcy and malignancy are reflected in changes in the CIH binding properties of the cell surface spaces between microvilli. The CIH densities observed on the microvilli are significantly different from the density on the spaces between them for each of the classes of cells studied at either temperature. The microvilli are found to bind a lower density of particles in five of the six cases. No correlations between microvilli particle density and transformation to in vitro malignant characteristics were observed.     

Role of microvilli in surface changes of synchronized P815Y mastocytoma cells

The surface morphology of synchronized P815Y mastocytoma cells has been examined by scanning electron microscopy. Early G1 cells are comparatively smooth or light villated, whereas at later stages the surface becomes progressively more villated. In G1 cell most microvilli have a uniform diameter, whereas in S and G2 cells, many microvilli show branching and often originate from much larger surface protuberances. Small "blebs" are seen on the surface of many cells but these structures do not appear to be a characteristic feature of cells at any one stage of the cycle. The presence of microvilli increases the total surface of the cell to such an extent that the ratio of volume to surface area remains constant throughout the cell cycle. The mechanism of cytokinesis is thus a physical one, involving the unfolding of previously accumulated microvilli.     

Down-regulation of Transforming Growth Factor-β Receptors: Cooperativity between the Types I, II, and III Receptors and Modulation at the Cell Surface

The types I, II, and III receptors (RI, RII, RIII) for transforming growth factor-β (TGF-β) become down-regulated in response to ligand, presumably via their internalization from the cell surface. This report examines the down-regulation of full-length RI, RII, and RIII in cells endogenously or transiently expressing these receptors. Down-regulation occurred rapidly (within 2 h after TGF-β1 treatment at 37°C) and showed a dose response, between 10 and 200 pM TGF-β1, in cells expressing RI, RII, and RIII (Mv1lu and A549 cells). A comparison between Mv1Lu and mutant cell derivatives R-1B (lacking RI) or DR-26 (lacking RII) indicated that all three receptors were necessary for efficient down-regulation. Down-regulation experiments, utilizing TGF-β-treated 293 cells transiently expressing different combinations of these receptors indicated that neither RII or RIII were down-regulated when expressed alone and that RI was required for maximal down-regulation of RII. RII and RIII were partially down-regulated when these receptors were coexpressed in the absence of RI (in R-1B and 293 cells). Surprisingly, TGF-β receptors were partially down-regulated in Mv1Lu, A549, and 293 cells treated with TGF-β1 at 4°C. Microscopic examination of 293 cells coexpressing RI fused to green fluorescent protein (RI–GFP) and RII indicated that, after treatment with TGF-β1 at 4°C, RI–GFP formed aggregates at the cell surface at this temperature. RI–GFP was not detected at the surface of these cells after TGF-β1 treatment at 37°C. Our results suggest a two phase mechanism for TGF-β1 receptor down-regulation involving receptor modulation (aggregation) at the cell surface and internalization.     

Transforming growth factor (TGF)-beta 1 internalization: modulation by ligand interaction with TGF-beta receptors types I and II and a mechanism that is distinct from clathrin-mediated endocytosis

Transforming growth factor-beta (TGF-beta) internalization was studied by monitoring the uptake of (125)I-TGF-beta1 in Mv1Lu cells, which endogenously express TGF-beta receptors types I (RI), II (RII), and III (RIII), and 293 cells transfected with RI and RII. At 37 degrees C internalization occurred rapidly, within 10 min of ligand addition. Internalization was optimal in 293 cells expressing both RI and RII. Internalization was prevented by phenylarsine oxide, a nonspecific inhibitor of receptor internalization, but was not affected by reagents that interfere with clathrin-mediated endocytosis such as monodansylcadaverine, K44A dynamin, and inhibitors of endosomal acidification. Electron microscopic examination of Mv1Lu cells treated with (125)I- TGF-beta1 at 37 degrees C indicated that internalization occurred via a noncoated vesicular mechanism. Internalization was prevented by prebinding cells with TGF-beta1 at 4 degrees C for 2 h prior to switching the cells to 37 degrees C. This was attributed to a loss of receptor binding, as indicated by a rapid decrease in the amount of TGF-beta1 bound to the cell surface at 37 degrees C and by a reduction in the labeling intensities of RI and RII in (125)I-TGF-beta1-cross-linking experiments. Mv1Lu or 293 (RI+RII) cells, prebound with TGF-beta1 at 4 degrees C and subsequently stripped of ligand by an acid wash, nevertheless initiated a signaling response upon transfer to 37 degrees C, suggesting that prebinding promotes formation of stable RI.RII complexes that can signal independently of ligand.     

Susceptibility of mink (Mustera vision)-derived cells to replication by human immunodeficiency virus type 1

In vivo studies for understanding viral transmission and replication, host immune responses, and pathogenesis of human immunodeficiency virus type 1 (HIV-1) infection would greatly benefit from the establishment of a small-animal model. In this study, we explored the potential of American mink (Mustera vison) as a susceptible host. We found that primary cells and cell lines derived from this species efficiently supported trans-activation of the HIV-1 long terminal repeat by Tat. Accordingly, the cysteine residue at position 261, which has been shown to be important for interaction of the human cyclin T1 with the HIV-1 regulatory protein Tat, is conserved in the mink homologue. No species-specific defect in Rev function could be detected in mink cells. In addition, primary splenocytes, fibroblasts, and the Mv.1.Lu cell line from American mink supported early as well as late HIV-1 gene expression following infection with vesicular stomatitis G protein-pseudotyped HIV-1 viruses, at levels comparable to those seen with permissive human cells. Furthermore, the mink Mv.1.Lu cell line stably expressing human CD4 and CCR5 receptors supported a spreading HIV-1 infection with few, if any, deficiencies compared to findings in human cell lines. This indicates the potential of HIV-1 to replicate in these cells once the blockade at the stage of virus entry has been removed. These results clearly show that cells from American mink generally pose no functional intracellular block to HIV-1 replication, and collectively they raise the possibility that this animal species could be engineered to support HIV-1 infection, providing a useful small-animal model for evaluating de novo infection by HIV-1.     

Surface morphology and agglutinability with concanavalin A in normal and transformed murine fibroblasts

The surface morphology of attached and suspended normal and transformed fibroblasts has been studied with the scanning electron microscope. Normal murine fibroblasts (3T3) grow in vitro with widely extended leading lamellae. During most parts of the cell cycle the surfaces of these cells are practically free of microvilli. When the cells round up for mitosis, their cell surfaces become adorned with many microvilli. In contrast, simian virus 40-transformed fibroblasts (SV3T3) grow more compact, and their cell surfaces remain smooth throughout the life cycle. When confluent 3T3 and SV3T3 cells are suspended with ethylenediaminetetraacetic acid (EDTA) for agglutination assays, similar differences in surface morphology are found: 3T3 cells always bear many microvilli, whereas most SV3T3 cells are essentially free of microvilli. The addition of concanavalin A (Con A) does not influence the surface morphology of the suspended cells. The morphological differences described here may be important for the agglutination process of the normal and transformed 3T3 cells, because they affect the real cell surface area and thus the density of Con A-binding sites.     

The modulation of apoptosis by cyclic AMP involves Akt and epidermal growth factor receptor

Adenosine 3',5'-cyclic monophosphate (cAMP) and transforming growth factor-beta are important regulators of many biological processes. In this study we investigated the effect and its potential mechanism of cAMP on transforming growth factor-beta1- and serum deprivation-induced apoptosis in Mv1Lu cells. Transforming growth factor-beta1 treatment or serum deprivation induces apoptotic response in Mv1Lu cells. Forskolin, a cAMP-elevating agent, or 8-Bromo-cAMP (8-B-cAMP), a cell permeable cAMP analogue, inhibited the cell proliferation and markedly enhanced apoptosis induced by transforming growth factor-beta1, but completely suppressed serum deprivation-induced apoptosis. Furthermore, forskolin decreased the Akt phosphorylation, and the inhibition of phosphatidylinositol-3 kinase by LY294002 sensitized Mv1Lu cells to transforming growth factor-beta1-induced apoptosis. In addition, forskolin treatment induced tyrosine phosphorylation of epidermal growth factor receptor. Inhibition of epidermal growth factor receptor by specific inhibitor PD153035 blocked the cAMP-mediated suppression of serum deprivation-induced apoptosis. The results indicate that cAMP exerts its opposite effects in transforming growth factor-beta1- and serum deprivation-induced apoptosis via a mechanism involving the modulation of signaling components of phosphatidylinositol-3-kinase/Akt and epidermal growth factor receptor in Mv1Lu cells.     

Effects of growth conditions on cyclic nucleotide phosphodiesterases of cultured fibroblasts.

Cyclic nucleotide phosphodiesterase activities of baby hamster kidney cells (BHK) grown in surface cultures were altered by modifying growth conditions. The untransformed BHK cells grown in medium containing 10% fetal calf serum showed non-linear LineweaverBurk plots for cyclic AMP phosphodiesterase activity with apparent Michaelis constants for cyclic AMP of approximately 5 and 30 muM. When these cells were placed in medium containing 1% fetal calf serum, linear kinetic plots for cyclic AMP phosphodiesterase with an apparent Km for cyclic AMP of approximately 20 muM were obtained. Modification of the apparent Km of BHK cell phosphodiesterase was detectable within 20 minutes after dillution of cells grown in 10% serum into fresh medium containing 1% serum. With the BHK cell line transformed with Rous sarcoma virus, differences in enzyme kinetics were not seen when these cells were diluted in 1% or 10% serum. In addition to the serum induced differences in the apparent Km of cyclic AMP phosphodiesterases of BHK cells, total cyclic AMP and cyclic GMP phosphodiesterase activities were also modified by growth conditions. BHK cells grown to high cell densities had three to five-fold higher total cyclic AMP activity than did the cells in less dense cultures. When the dense cell cultures were diluted into fresh medium containing 10% serum, total enzyme activities fell to levels comparable to those found in the rapidly growing cells at low cell densities. The reduction in total enzyme activity after dilution of BHK cells occurred rapidly and was influenced by cell density. A similar reduction of total enzyme activity was also seen in diluted RSV cells; however, the time course of the response differed from that seen in the untransformed cells.     

PKCδ phosphorylation is an upstream event of GSK3 inactivation-mediated ROS generation in TGF-β1-induced senescence

Abstract

Transforming growth factor β1 (TGF-β1) induces Mv1Lu cell senescence through inactivating glycogen synthase kinase 3 (GSK3), thereby inactivating complex IV and increasing intracellular ROS. In the present study, we identified protein kinase C delta (PKCδ) as an upstream regulator of GSK3 inactivation in this mechanism of TGF-β1-induced senescence. When Mv1Lu cells were exposed to TGF-β1, PKCδ phosphorylation simultaneously increased with GSK3 phosphorylation, and then AKT and ERK were phosphorylated. AKT phosphorylation and Smad signaling were independent of GSK3 phosphorylation, but ERK phosphorylation was downstream of GSK3 inactivation. TGF-β1-triggered GSK3 phosphorylation was blocked by inhibition of PKCδ, using its pharmacological inhibitor, Rottlerin, or overexpression of a dominant negative PKCδ mutant, but GSK3 inhibition with SB415286 did not alter PKCδ phosphorylation. Activation of PKCδ by PMA delayed cell growth and increased intracellular ROS level, but did not induce senescent phenotypes. In addition, overexpression of wild type or a constitutively active PKCδ mutant was enough to delay cell growth and decrease the mitochondrial oxygen consumption rate and complex IV activity, but weakly induce senescence. However, PMA treatment on Mv1Lu cells, which overexpress wild type and constitutively active PKCδ mutants, effectively induced senescence. These results indicate that PKCδ plays a key role in TGF-β1-induced senescence of Mv1Lu cells through the phosphorylation of GSK3, thereby triggering mitochondrial complex IV dysfunction and intracellular ROS generation.