Hepatocyte growth factor releases mink epithelial cells from transforming growth factor beta1-induced growth arrest by restoring Cdk6 expression and cyclin E-associated Cdk2 activity

Transforming growth factor beta (TGF-beta) potently suppresses Mv1Lu mink epithelial cell growth, whereas hepatocyte growth factor (HGF) counteracts TGF-beta-mediated growth inhibition and induces Mv1Lu cell proliferation (J. Taipale and J. Keski-Oja, J. Biol. Chem. 271:4342-4348, 1996). By addressing the cell cycle regulatory mechanisms involved in HGF-mediated release of Mv1Lu cells from TGF-beta inhibition, we show that increased DNA replication is accompanied by phosphorylation of the retinoblastoma protein and alternative regulation of cyclin-Cdk-inhibitor complexes. While TGF-beta treatment decreased the expression of Cdk6, this effect was counteracted by HGF, followed by partial restoration of cyclin D2-associated kinase activity. Notably, HGF failed to prevent TGF-beta induction of p15 and its association with Cdk6. However, HGF reversed the TGF-beta-mediated decrease in Cdk6-associated p27 and cyclin D2-associated Cdk6, suggesting that HGF modifies the TGF-beta response at the level of G1 cyclin complex formation. Counteraction of TGF-beta regulation of Cdk6 by HGF may in turn affect the association of p27 with Cdk2-cyclin E complexes. Though HGF did not differentially regulate the total levels of p27 in TGF-beta-treated cells, p27 immunodepletion experiments suggested that upon treatment with both growth factors, less p27 is associated with Cdk2-cyclin E complexes, in parallel with restoration of the active form of Cdk2 and the associated kinase activity. The results demonstrate that HGF intercepts TGF-beta cell cycle regulation at multiple points, affecting both G1 and G1-S cyclin kinase activities.     

Activation of phospholipase D activity in transforming growth factor—beta—induced cell growth inhibition

Cells regulate phospholipase D(PLD) activity in response to numerous extracellular signals.Here,we investigated the involvement of PLD activity in transforming growth factor-β(TGF-β1)-mediated growth inhibition of epithelial cells.TGF-β1)-mediated growth inhibition of epithelial cells.TGF-β1 inhibits the growth of MDCK,Mv1Lu,and A-549 cells.In the presence of 0.4% butanol,TGF-β1 induces an increase in the formation of phosphatidylbutanol,a unique product catalyzed by PLD.TGF-β1 also induces an increase in phosphatidic acid (PA) level in A-549 and MDCK cells.TGF-β1 induces an increase in the levels of DAG labeled with [^3H]-myristic acid in A-549 and MDCK cells but not in Mv1Lu cells.No increase of DAG was observed in cells prelabeled with [^3H]-arachidonic acid.The data presented suggest that PLD activation is involved in the TGF-β1-induced cell growth inhibition.     

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.     

The intracellular form of notch blocks transforming growth factor beta-mediated growth arrest in Mv1Lu epithelial cells

Notch signaling influences a variety of cell fate decisions during development, and constitutive activation of the pathway can provoke unbridled cell growth and cancer. The mechanisms by which Notch affects cell growth are not well established. We describe here a novel link between Notch and cell cycle control. We found that Mv1Lu epithelial cells harboring an oncogenic form of Notch (NICD) are resistant to the cell cycle-inhibitory effects of transforming growth factor beta (TGF-beta). NICD did not affect TGF-beta signaling per se but blocked induction of the Cdk inhibitor p15(INK4B). c-Myc, whose down-regulation by TGF-beta is required for p15(INK4B) induction, remained elevated in the NICD-expressing cells. c-Myc expression was also maintained in low serum, indicating that Notch's effects on c-Myc are not specific to TGF-beta. Our results are consistent with a model in which a strong Notch signal indirectly deregulates c-Myc expression and thereby renders Mv1Lu epithelial cells resistant to growth-inhibitory signals.     

A kinase subdomain of transforming growth factor-beta (TGF-beta) type I receptor determines the TGF-beta intracellular signaling specificity.

Transforming growth factor-beta (TGF-beta) signals through a heteromeric complex of related type I and type II serine/threonine kinase receptors. In Mv1Lu cells the type I receptor TbetaRI mediates TGF-beta-induced gene expression and growth inhibition, while the closely related type I receptors Tsk7L and TSR1 are inactive in these responses. Using chimeras between TbetaRI and Tsk7L or TSR1, we have defined the structural requirements for TGF-beta signaling by TbetaRI. The extracellular/transmembrane or cytoplasmic domains of TbetaRI and Tsk7L were functionally not equivalent. The juxtamembrane domain, including the GS motif, and most regions in the kinase domain can functionally substitute for each other, but the alphaC-beta4-beta5 region from kinase subdomains III to V conferred a distinct signaling ability. Replacement of this sequence in TbetaRI by the corresponding domain of Tsk7L inactivated TGF-beta signaling, whereas its introduction into Tsk7L conferred TGF-beta signaling. The differential signaling associated with this region was narrowed down to a sequence of eight amino acids, the L45 loop, which is exposed in the three-dimensional kinase structure and diverges highly between TbetaRI and Tsk7L or TSR1. Replacement of the L45 sequence in Tsk7L with that of TbetaRI conferred TGF-beta responsiveness to the Tsk7L cytoplasmic domain in Mv1Lu cells. Thus, the L45 sequence between kinase subdomains IV and V specifies TGF-beta responsiveness of the type I receptor.     

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.     

Concomitant loss of transforming growth factor (TGF)-beta receptor types I and II in TGF-beta-resistant cell mutants implicates both receptor types in signal transduction

A panel of 71 chemically mutagenized Mv1Lu mink lung epithelial cell clones were selected based on their resistance to the growth inhibitory action of transforming growth factor beta 1 (TGF-beta 1) and TGF-beta 2. Characterization of TGF-beta receptors in these mutants indicates that the TGF-beta-binding membrane proteoglycan, betaglycan, is apparently normal in all of them. However, 14 of the mutant clones are defective in TGF-beta receptor type I, and 22 clones are simultaneously defective in receptor types I and II. The clones with type I receptor defects fall into two distinct phenotypes, called R and LR. The R phenotype is characterized by the lack of detectable type I receptors, and has been previously described (Boyd, F. T., and Massagué, J. (1989) J. Biol. Chem. 264, 2272-2278). LR mutants are characterized by expression of low levels of type I receptor and are, like the R mutants, completely resistant to growth inhibition by TGF-beta 1 or -beta 2. Mutant clones that are simultaneously defective in receptor types I and II fall into three distinct phenotypes. These included DRa mutants which are characterized by lack of detectable receptor types I and II, DRb mutants which are characterized by low expression of both receptor types and an anomalously fast electrophoretic mobility of the type II receptor protein. All mutants that have a low level of type II receptor are also defective in type I receptor. In addition to the loss of growth inhibitory response, the receptor-defective mutants described here have lost all other responses to TGF-beta 1 and -beta 2 known to occur in parental Mv1Lu cells. The defects present in these mutant clones are not encountered in clones isolated from nonmutagenized parental Mv1Lu cells or in mutagenized cells that had not been exposed to selection with TGF-beta. The results implicate TGF-beta receptor types I and II in the mediation of a common set of cellular responses to TGF-beta. Furthermore, the high relative frequency of isolation of DR mutants raises the possibility that receptor types I and II interact as part of a common signaling TGF-beta receptor complex.     

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.     

Responsiveness to transforming growth factor-beta (TGF-beta) restored by genetic complementation between cells defective in TGF-beta receptors I and II.

Selection of mutant Mv1Lu mink lung epithelial cells resistant to growth inhibition by transforming growth factor-beta (TGF-beta) has led to the isolation of cell clones with distinct alterations in type I and II TGF-beta receptors. Certain mutant clones present a decreased number or complete loss of detectable type I receptor. Other clones show a loss and/or altered electrophoretic mobility of the type II receptor, with concomitant loss of the type I receptor. Using somatic cell hybridization analysis we demonstrate the recessive nature of these mutants with respect to the wild-type phenotype and define various mutant complementation groups. Among these, hybrids between cells that express only type II receptor (R mutants) and cells that express neither receptor type (DRa mutants) rescue wild-type expression of type I receptors. Moreover, these hybrids regain full responsiveness to TGF-beta 1, as measured by inhibition of DNA synthesis as well as stimulation of fibronectin and plasminogen activator inhibitor-1 production. These results provide evidence for an interaction between TGF-beta receptor components I and II and show that, in Mv1Lu cells, expression of both receptor types is required for mediation of biological responses to TGF-beta 1.     

Distinct transforming growth factor-beta (TGF-beta) receptor subsets as determinants of cellular responsiveness to three TGF-beta isoforms.

Characterization of the three mammalian transforming growth factor-beta (TGF-beta) isoforms, TGF-beta 1, -beta 2, and -beta 3, indicates that TGF-beta 3 is somewhat more potent (ED50 = 0.5 pM versus 2 pM) than TGF-beta 1 and TGF-beta 2 as a growth inhibitor of the Mv1Lu mink lung epithelial cell line. In the fetal bovine heart endothelial (FBHE) cell line, however, TGF-beta 1 and -beta 3 are at least 50-fold more potent than TGF-beta 2 which is a very weak growth inhibitor (ED50 greater than or equal to 0.5 nM). Thus, as growth inhibitors, TGF-beta 1 and -beta 3 resemble each other more than TGF-beta 2. The presence of serum alpha 2-macroglobulin in the FBHE cell assays decreases the biological potency of TGF-beta s, in particular TGF-beta 2. This effect of alpha 2-macroglobulin, however, is not sufficient to explain the low responsiveness of FBHE cells to TGF-beta 2. Evaluation of the role of TGF-beta receptors as determinants of cell-specific responsiveness to TGF-beta isoforms indicates that TGF-beta 1, -beta 2, and -beta 3 have similar affinity for the membrane proteoglycan, betaglycan. They differ, however, in their ability to bind to receptor types I and II which are implicated in TGF-beta signal transduction. TGF-beta 1 is similar, albeit not identical, to TGF-beta 3 and much more potent than TGF-beta 2 as a competitor for binding to the overall population of receptors I and II in all cell lines tested. A subset of receptors I and II has been identified in Mv1Lu cells which has high affinity for TGF-beta 2 (KD approximately 10 pM) and binds this factor at concentrations that are biologically active in Mv1Lu cells. This receptor subset could not be detected in FBHE cells, suggesting that cell-specific differences in the level of high affinity of TGF-beta 2 receptors may lead to cell-specific differences in responsiveness to this isoform. Thus, despite their structural and biological similarities, TGF-beta 1, -beta 2, and -beta 3 diverge in their ability to bind to receptors in a manner that correlates with their potency as growth inhibitors.     

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.     

Novel Cdk inhibitors restore TGF-beta sensitivity in cdk4 overexpressing epithelial cells

Transforming growth factor-beta (TGF-beta) is a potent mitogen that effects a wide variety of cells by blocking cell growth. TGF-beta acts by interacting with components of cell cycle machinery to cause G1 arrest and in mink lung epithelial cells (Mv1Lu) it does so by inhibiting Cdk4 synthesis. Overexpression of Cdk4 in these cells (B7) renders them resistant to the effects of TGF-beta. Here we report that two novel Cdk inhibitors (pyridopyrimidines) that not only inhibit Cdk4 and Cdk2 in an in vitro kinase assay but also, in the absence of TGF-beta, block growth of Mv1Lu cells in G1 more efficiently than their B7 (overexpressing Cdk4) counterparts. Interestingly, these inhibitors restored sensitivity of B7 cells towards TGF-beta. This may have implications for the treatment of tumors that have lost TGF-beta responsiveness due to deregulated cellular growth in vivo. These Cdk inhibitors could therefore be used in conjunction with TGF-beta to understand the mechanism of growth arrest in normal versus tumour cells.     

Overexpression of human transforming growth factor-beta1 using a recombinant CHO cell expression system

Transforming growth factor-beta1 (TGF-beta1) is secreted by most cells as a high molecular weight latent complex, which consists of latent TGF-beta1 disulfide bonded to latent TGF-beta1-binding protein (LTBP). Current recombinant expression systems yield less than 1-2 mg of the mature TGF-beta1 per liter of cell culture medium. In an effort to produce large quantities of the recombinant cytokine for structural studies, we have constructed a mammalian expression system based on a modified pcDNA3.1(+) vector with a glutamine synthetase gene inserted for gene amplification. The leader peptide of TGF-beta1 was replaced with that of rat serum albumin, and an eight-histidine tag was inserted immediately after the leader sequence to facilitate protein purification. In addition, Cys 33 of TGF-beta1, which forms a disulfide bond with LTBP, was replaced by a serine residue. The resulting expression construct produced a stable clone expressing 30 mg of mature TGF-beta1 per liter of spent medium. Purified TGF-beta1 bound with high affinity to its type II receptor with a solution dissociation constant of approximately 70 nM, and was fully active in both a Mv1Lu cell growth inhibition assay and in a PAI-1 luciferase reporter assay. Owing to similarities in the synthesis, secretion, and structure of TGF-beta family members, this recombinant expression system may also be applied to the overexpression of other TGF-beta isomers and even to members of the TGF-beta superfamily to facilitate their preparation.