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

MicroRNA-Mediated Suppression of the TGF-β Pathway Confers Transmissible and Reversible CDK4/6 Inhibitor Resistance

CDK4/6 inhibition is now part of the standard armamentarium for patients with estrogen receptorpositive (ER⁺) breast cancer, so that defining mechanisms of resistance is a pressing issue. Here, we identify increased CDK6 expression as a key determinant of acquired resistance after palbociclib treatment in ER⁺ breast cancer cells. CDK6 expression is critical for cellular survival during palbociclib exposure. The increased CDK6 expression observed in resistant cells is dependent on TGF-b pathway suppression via miR-432-5p expression. Exosomal miR-432-5p expression mediates the transfer of the resistance phenotype between neighboring cell populations. Levels of miR-432-5p are higher in primary breast cancers demonstrating CDK4/6 resistance compared to those that are sensitive. These data are Furthermore confirmed in pre-treatment and post-progression biopsies from a parotid cancer patient who had responded to ribociclib, demonstrating the clinical relevance of this mechanism. Finally, the CDK4/6 inhibitor resistance phenotype is reversible in vitro and in vivo by a prolonged drug holiday.

Cornell et al. demonstrate a mechanism of acquired CDK4/6 inhibitor resistance that is independent of inherent genetic mutations, is conferred through extracellular signaling, and is reversible in vitro and in vivo. Resistance was mediated by exosomal miRNA, causing increased expression of CDK6 to overcome G1 arrest and promote cell survival.

Dual effects of Ral-activated pathways on p27 localization and TGF-β signaling

Constitutive activation or overactivation of Ras signaling pathways contributes to epithelial tumorigenesis in several ways, one of which is cytoplasmic mislocalization of the cyclin-dependent kinase inhibitor p27(Kip1) (p27). We previously showed that such an effect can be mediated by activation of the Ral-GEF pathway by oncogenic N-Ras. However, the mechanism(s) leading to p27 cytoplasmic accumulation downstream of activated Ral remained unknown. Here, we report a dual regulation of p27 cellular localization by Ral downstream pathways, based on opposing effects via the Ral effectors RalBP1 and phospholipase D1 (PLD1). Because RalA and RalB are equally effective in mislocalizing both murine and human p27, we focus on RalA and murine p27, which lacks the Thr-157 phosphorylation site of human p27. In experiments based on specific RalA and p27 mutants, complemented with short hairpin RNA-mediated knockdown of Ral downstream signaling components, we show that activation of RalBP1 induces cytoplasmic accumulation of p27 and that this event requires p27 Ser-10 phosphorylation by protein kinase B/Akt. Of note, activation of PLD1 counteracts this effect in a Ser-10-independent manner. The physiological relevance of the modulation of p27 localization by Ral is demonstrated by the ability of Ral-mediated activation of the RalBP1 pathway to abrogate transforming growth factor-β-mediated growth arrest in epithelial cells.

Molecular targeting of CSN5 in human hepatocellular carcinoma: a mechanism of therapeutic response

Development of targeted therapy for hepatocellular carcinoma (HCC) remains a major challenge. We have recently identified an elevated expression of the fifth subunit of COP9 signalosome (CSN5) in early HCC as compared to dysplastic stage. In the present study, we explored the possibility of CSN5 being a potential therapeutic target for HCC. Our results show that CSN5 knockdown by small interfering (si) RNA caused a strong induction of apoptosis and inhibition of cell cycle progression in HCC cells in vitro. The downregulation of CSN5 was sufficient to interfere with CSN function as evidenced by the accumulation of neddylated Cullin1 and changes in the protein levels of CSN controlled substrates SKP2, p53, p27 and NF-kB, albeit to a different degree depending on the HCC cell line, which could account for the CSN5 knockdown phenotype. The transcriptomic analysis of CSN5 knockdown signature showed that the anti-proliferative effect was driven by a common subset of molecular alterations including downregulation of CDK6 and ITGB1, which were functionally interconnected with key oncogenic regulators MYC and TGFβ1 involved in the control of proliferation, apoptotic cell death and HCC progression. Consistent with microarray analysis, western blotting revealed that CSN5 depletion increased phosphorylation of Smad 2/3, key mediators of TGFβ1 signaling, decreased the protein levels of ITGB1, CDK6, and cyclin D1 and caused reduced expression of anti-apoptotic Bcl-2 while elevating the levels of pro-apoptotic Bak. A chemically modified variant of CSN5 siRNA was then selected for in vivo application based on the growth inhibitory effect and minimal induction of unwanted immune response. Systemic delivery of the CSN5 3/8 variant by stable-nucleic-acid-lipid-particles (SNALP) significantly suppressed the tumor growth in Huh7-luc⁺ orthotopic xenograft model. Taken together, these results indicate that CSN5 plays a pivotal role in HCC pathogenesis and maybe an attractive molecular target for systemic HCC therapy.

Non-CDK-bound p27 (p27(NCDK)) is a marker for cell stress and is regulated through the Akt/PKB and AMPK-kinase pathways

p27Kip1 (p27) tumour suppressor protein is regulated by multiple mechanisms including its turnover, localization and complex formation with its key targets, cyclin-dependent kinases (CDK) and cyclins. We have earlier shown that p27 exists in cells in a form that lacks cyclin/CDK interactions (hence non-CDK, p27(NCDK)) but the nature of p27(NCDK) has remained unresolved. Here we demonstrate that the epitope recognized by the p27(NCDK)-specific antibody resides in the p27 CDK-interaction domain and that p27(NCDK) is regulated by the balance of CDK inhibitors and cyclin-CDK complexes. We find that signalling by cellular growth promoting pathways, like phosphoinositol 3-kinase (PI3K) and specifically Akt/PKB kinase, inversely correlates with p27(NCDK) levels whereas total p27 levels are unaffected. p27(NCDK), but not total p27, is increased by cellular perturbations such as hyperosmotic and metabolic stress and activation of AMP-activated protein kinase (AMPK). By using AMPK catalytic subunit proficient and deficient cells we further demonstrate that the AMPK pathway governs p27(NCDK) responses to metabolic stress and PI3K inhibition. These results indicate that p27(NCDK) is a sensitive marker for both cell stress and proliferation over and above p27 and is regulated by Akt/PKB and AMPK pathways.

Oltipraz promotion of liver regeneration after partial hepatectomy: The role of PI3-kinase-dependent C/EBPbeta and cyclin E regulation

Oltipraz, a representative cancer chemopreventive agent, regenerates cirrhotic liver via CCAAT/enhancer binding protein beta (C/EBPbeta). This study examined the effect of oltipraz on liver regeneration after partial hepatectomy (PH) and explored the role of phosphatidylinositol 3-kinase (PI3K) pathway responsible in liver regeneration. Oltipraz treatment (30 mg/kg/day, for 3 days) promoted liver regeneration in PH rats, but did not increase hepatocyte growth factor production. Subcellular fractionation and electrophoretic mobility shift assays showed that oltipraz treatment increased C/EBPbeta-DNA binding activity in the liver of sham control rats and further enhanced PH-mediated nuclear translocation of C/EBPbeta. The expression of cyclin E and the activity of cyclin E-dependent kinase were both enhanced by oltipraz treatment of PH rats. The signaling pathway that controls C/EBPbeta and cyclin E were studied in H4IIE cells, a rat-derived hepatocyte cell line. Oltipraz potentiated the nuclear accumulation of C/ EBPbeta and C/EBPbeta-DNA binding activity in cells incubated in a medium containing serum. PI3K and its downstream kinase, p70S6 kinase, were both required for C/EBPbeta-dependent induction of cyclin E by oltipraz, as shown by chemical inhibition and plasmid transfection experiments. The results of this study demonstrate that oltipraz treatment enhances liver regeneration after PH, which involves activation of C/EBPbeta and C/EBPbeta-dependent cyclin E expression via the PI3K-p70S6 kinase pathway.

Pathway- and expression level-dependent effects of oncogenic N-Ras: p27(Kip1) mislocalization by the Ral-GEF pathway and Erk-mediated interference with Smad signaling

Overactivation of Ras pathways contributes to oncogenesis and metastasis of epithelial cells in several ways, including interference with cell cycle regulation via the CDK inhibitor p27(Kip1) (p27) and disruption of transforming growth factor beta (TGF-beta) anti-proliferative activity. Here, we show that at high expression levels, constitutively active N-Ras induces cytoplasmic mislocalization of murine and human p27 via the Ral-GEF pathway and disrupts TGF-beta-mediated Smad nuclear translocation by activation of the Mek/Erk pathway. While human p27 could also be mislocalized via the phosphatidylinositol 3-kinase/Akt pathway, only Ral-GEF activation was effective for murine p27, which lacks the Thr157 Akt phosphorylation site of human p27. This establishes a novel role for the Ral-GEF pathway in regulating p27 localization. Interference with either Smad translocation or p27 nuclear localization was sufficient to disrupt TGF-beta growth inhibition. Moreover, expression of activated N-Ras or specific effector loop mutants at lower levels using retroviral vectors induced p27 mislocalization but did not inhibit Smad2/3 translocation, indicating that the effects on p27 localization occur at lower levels of activated Ras. These findings have important implications for the contribution of activated Ras to oncogenesis and for the conversion of TGF-beta from an inhibitory to a metastatic factor in some epithelial tumors.

Amelioration of pulmonary emphysema by in vivo gene transfection with hepatocyte growth factor in rats

BACKGROUND

Hepatocyte growth factor (HGF) is an important mitogen and morphogen that contributes to the repair process after lung injury. The goal of the present study was to characterize its role in pulmonary emphysema, which may lead to the development of new treatment strategies with HGF.

METHODS AND RESULTS

HGF mRNA and protein levels in lung tissue and plasma from elastase-induced emphysema rats transiently increased, then declined significantly to below the basal level in a time-dependent manner (P<0.01). Furthermore, changes in HGF were correlated with histologically progressive emphysematous changes and deterioration in pulmonary physiology. Use of the HVJ (hemagglutinating virus of Japan) envelope method resulted in successful transfection of cDNA encoding human HGF, as demonstrated by an efficient expression of HGF in alveolar endothelial and epithelial cells. Transfection of HGF resulted in a more extensive pulmonary vasculature and inhibition of alveolar wall cell apoptosis, and those effects led to improved exercise tolerance and gas exchange (P<0.05), which persisted for more than 1 month.

CONCLUSIONS

Decreased HGF expression due to a failure in sustained endogenous production after injury was associated with emphysema-related histopathologic and physiological changes in the present rat model. In addition, induction of HGF expression by a gene-transfection method resulted in improved pulmonary function via inhibition of alveolar cell apoptosis, enhancement of alveolar regeneration, and promotion of angiogenesis.

TGF β1 induces prolonged mitochondrial ROS generation through decreased complex IV activity with senescent arrest in Mv1Lu cells

Transforming growth factor beta1 (TGF beta1) is a well-characterized cytokine that suppresses epithelial cell growth. We report here that TGF beta1 arrested lung epithelial Mv1Lu cells at G1 phase of the cell cycle with acquisition of senescent phenotypes in the presence of 10% serum, whereas it gradually induced apoptosis with lower concentrations of serum. The senescent arrest was accompanied by prolonged generation of reactive oxygen species (ROS) and persistent disruption of mitochondrial membrane potential (DeltaPsim). We demonstrated that the sustained ROS overproduction was derived from mitochondrial respiratory defect via decreased complex IV activity and was involved in the arrest. Moreover, we verified that hepatocyte growth factor released Mv1Lu cells from the arrest by protecting mitochondrial respiration, thereby preventing both the DeltaPsim disruption and the ROS generation. Our present results suggest the TGF beta1-induced senescent arrest as another plausible mechanism to suppress cellular growth in vivo and provide a new biochemical association between the mitochondrial functional defects and the cytokine-induced senescent arrest, emphasizing the importance of maintenance of mitochondrial function in cellular protection from the arrest.

Mirk/dyrk1B kinase destabilizes cyclin D1 by phosphorylation at threonine 288

The phosphorylation of cyclin D1 at threonine 286 by glycogen synthase kinase 3beta (GSK3beta) has been shown to be required for the ubiquitination and nuclear export of cyclin D1 and its subsequent degradation in the proteasome. The mutation of the nearby residue, threonine 288, to nonphosphorylatable alanine has also been shown to reduce the ubiquitination of cyclin D1, suggesting that phosphorylation at threonine 288 may also lead to degradation of cyclin D1. We now demonstrate that the G(0)/G(1)-active arginine-directed protein kinase Mirk/dyrk1B binds to cyclin D1 and phosphorylates cyclin D1 at threonine 288 in vivo and that the cyclin D1-T288A construct is more stable than wild-type cyclin D1. Transient overexpression of Mirk in nontransformed Mv1Lu lung epithelial cells blocked cells in G(0)/G(1). Depletion of endogenous Mirk by RNA interference increased cyclin D1 protein levels but not mRNA levels, indicating that Mirk destabilizes cyclin D1 protein. Destabilization was confirmed by induction of a stable Mirk transfectant of Mv1Lu cells, which blocked cell migration (Zou, Y., Lim, S., Lee, K., Deng, X., and Friedman, E. (2003) J. Biol. Chem. 278, 49573-49581), and caused a decrease in the half-life of endogenous cyclin D1, concomitant with an increase in Mirk expression. In vitro cyclin D1 was phosphorylated in an additive fashion by Mirk and GSK3beta. Mirk-phosphorylated cyclin D1 mutated at the GSK3beta phosphorylation site and was capable of phosphorylating cyclin D1 in the presence of the GSK3beta inhibitor LiCl. Mirk may function together with GSK3beta to assist cell arrest in G(0)/G(1) by destabilizing cyclin D1.

Role of p12(CDK2-AP1) in transforming growth factor-beta1-mediated growth suppression

p12(CDK2-AP1) (p12) is a growth suppressor isolated from normal keratinocytes. Ectopic expression of p12 in squamous carcinoma cells reversed the malignant phenotype of these cells, in part due an ability of p12 to bind to both DNA polymerase alpha/primase and to cyclin-dependent kinase 2 (CDK2), thereby inhibiting their activities. We report in this article that in normal epithelial cells, transforming growth factor beta1 (TGF-beta1) induces p12 expression transcriptionally, which, in turn, mediates the growth inhibitory activity of TGF-beta1. We created inducible p12 antisense HaCaT cell lines [ip12 (-) HaCaT] and showed that selective reduction of cellular p12 resulted in an increase in: (a) CDK2-associated kinase activity; (b) protein retinoblastoma (pRB) phosphorylation; and (c) [(3)H]thymidine incorporation, and partially reversed TGF-beta1-mediated inhibition of CDK2 kinase activity, pRB phosphorylation, and cell proliferation. Furthermore, we generated p12-deficient mouse oral keratinocytes (MOK(p12-/-)) and compared their growth characteristics and response to TGF-beta1 with that of wild-type mouse oral keratinocytes (MOK(WT)). Under normal culture conditions, the number of MOK(p12-/-) in S phase is 2-fold greater than that of MOK(WT). Concomitantly, fewer cells are in G(2) phase in MOK(p12-/-) than that in MOK(WT). Moreover, response to TGF-beta1-mediated growth suppression is compromised in MOK(p12-/-) cells. Mechanistic studies showed that MOK(p12-/-) have increased CDK2 activity and reduced sensitivity to inhibition by TGF-beta1. Collectively our data suggest that p12 plays a role in TGF-beta1-mediated growth suppression by modulating CDK2 activities and pRB phosphorylation.

Negative regulation of SCFSkp2 ubiquitin ligase by TGF-β signaling

TGF-beta is a multifunctional growth factor whose best-known function is to inhibit cell growth and suppress tumor formation. TGF-beta causes cells to accumulate in mid-to-late G1 phase by blocking the transition from G1 to S. It has been shown that TGF-beta inhibits Cdk2-cyclin E kinase activity by promoting the binding of cell cycle inhibitor p27Kip1 to the kinase complexes. Here, we show that TGF-beta treatment leads to stabilization of p27Kip1 during G1 to S transition. We found that TGF-beta negatively regulates components of the SCF complex, which degrades the p27Kip1 during the G1 to S transition, through two distinct mechanisms. Using a pulse-chase analysis, we demonstrated that the stability of Skp2 decreases in the presence of TGF-beta. Destabilization of Skp2 by ubiquitin-mediated proteolysis was also demonstrated that in an in vitro degradation system, using cell extracts prepared from TGF-beta-treated cultured cells. In addition, TGF-beta treatment decreases the levels of Cks1 mRNA. The deficiency of Cks1 in TGF-beta-treated cells likely contributes to the stabilization of p27Kip1 and destabilization of Skp2, because in the absence of Cks1, SCFSkp2 cannot ubiquitinate p27Kip1; instead, self-ubiquitination of Skp2 occurs. Thus, stabilization of the cell cycle inhibitor p27Kip1 and cell growth inhibition in response to TGF-beta occur in part through limiting the threshold of the SCFSkp2 ubiquitin ligase by transcriptional and post-transcriptional mechanisms.

The cyclin D3-CDK4-p27kip1 holoenzyme in thyroid epithelial cells: activation by TSH, inhibition by TGFbeta, and phosphorylations of its subunits demonstrated by two-dimensional gel electrophoresis

The cAMP-dependent mitogenic stimulation elicited by thyroid-stimulating hormone (TSH) in primary cultures of canine thyroid epithelial cells is unique as it upregulates the cyclin-dependent kinase (CDK) inhibitor p27kip1 but not D-type cyclins. TSH and cAMP promote the assembly of required cyclin D3-CDK4 complexes and their nuclear import. Here, the nuclear translocation of these complexes strictly correlated in individual cells with the enhanced presence of nuclear p27. p27, like cyclin D3, supported the TSH-stimulated pRb-kinase activity of the CDK4 complex and, as demonstrated using the high-resolution power of the two-dimensional (2D) gel electrophoresis, the phosphorylation of CDK4, presumably by the nuclear CDK-activating kinase. In the presence of TSH, transforming growth factor beta (TGFbeta) did not affect the assembly of cyclin D3-CDK4, but it strongly inhibited the pRb-kinase activity associated with both cyclin D3 and p27, not only by preventing the nuclear import of cyclin D3-CDK4 and its binding to p27, but also by inhibiting CDK4 phosphorylation within residual p27-bound cyclin D3-CDK4 complexes. No alterations of the relative abundance of multiple (un)phosphorylated forms of cyclin D3 and p27 demonstrated by 2D-gel electrophoresis were associated with these processes. This study suggests a crucial positive role of p27 in the TSH-stimulated nuclear import, phosphorylation, and catalytic activity of cyclin D3-bound CDK4. Moreover, it demonstrates a technique to directly assess the in vivo phosphorylation of endogenous CDK4, which might appear as a last regulated step targeted by the antagonistic cell cycle effects of TSH and TGFbeta.

Serine/threonine kinase Mirk/Dyrk1B is an inhibitor of epithelial cell migration and is negatively regulated by the Met adaptor Ran-binding protein M

Minibrain-related kinase (Mirk)/Dyrk1B is an arginine-directed serine/threonine kinase that is active in skeletal muscle development but is also expressed in various carcinomas. In the current study, the Met adaptor protein Ran-binding protein M (RanBPM) was identified as a Mirk-binding protein by yeast two-hybrid analysis. The Mirk-RanBPM association was confirmed by glutathione S-transferase pull-down assays, co-immunoprecipitation studies, and in vivo cross-linking. Met plays an important role in tumor cell invasion and cell migration. RanBPM has been reported to bind to the tyrosine kinase domain of the hepatocyte growth factor (HGF) receptor Met, enhance Met downstream signaling, and enhance HGF-induced A704 kidney carcinoma cell invasion (Wang, D., Li, Z., Messing, E. M., and Wu, G. (2002) J. Biol. Chem. 277, 36216-36222). We made a stable Mirk-inducible subline from nontransformed Mv1Lu lung epithelial cells and now demonstrate that induction of Mirk inhibited the migration of these cells in wounding experiments and inhibited their invasion through polycarbonate Transwell filters. Furthermore the ability of Mirk to inhibit Mv1Lu cell migration was attenuated when cells were exposed to HGF or to elevated levels of transiently expressed RanBPM. RanBPM inhibited the kinase activity of Mirk/Dyrk1B and Dyrk1A. In addition, RanBPM and HGF inhibited the function of Mirk as a transcriptional coactivator. Our findings suggest that Mirk plays a role in modulating cell migration through opposing the action of the Met signaling cascade adaptor protein RanBPM.

A naturally occurring point substitution in Cdc25A, and not Fv2/Stk, is associated with altered cell-cycle status of early erythroid progenitor cells

The Friend virus susceptibility gene 2 (Fv2) controls the polyclonal expansion of infected cells that occurs early during Friend erythroleukemia virus infection. Fv2 has recently been shown to encode a truncated form of the Stk receptor tyrosine kinase (Sf-Stk). This observation, coupled with earlier work, suggested that Sf-Stk drives the expansion of infected cells by forming a complex with the Friend virus envelope glycoprotein, gp55, and the erythropoietin receptor. Fv2 has also been implicated in the control of cell cycling in early erythroid progenitors (erythroid blast-forming units [BFU-Es]). Mouse strains that are homozygous for the resistant allele of Fv2 (Fv2(rr)) have few actively cycling BFU-Es. In this report, we demonstrate that the control of BFU-E cycling is encoded by a gene linked to, but distinct from, Fv2, and suggest that this gene is the dual-specific protein phosphatase Cdc25A, which regulates the G1- to S-phase transition of the cell cycle. We show that a naturally occurring allele of Cdc25A, which increases Cdc25A phosphatase activity and promotes cell-cycle progression, segregates in mouse strains that exhibit high levels of BFU-E cell cycling. In wild-type mice, this allele of Cdc25A does not overtly affect erythropoiesis; however, when this allele is combined with a mutation of the Kit receptor (Kit(WV)), the anemia of the mice is enhanced. Furthermore, overexpression of Cdc25A in bone marrow cells causes a defect in the BFU-E colony formation. These results suggest that proper regulation of the cell cycle through Cdc25A is required for normal erythropoiesis.

The human papilloma virus E7 oncoprotein inhibits transforming growth factor-beta signaling by blocking binding of the Smad complex to its target sequence

The human papillomavirus (HPV) oncoprotein E7 is implicated in the etiology of cervical cancer associated with infection by HPV. HPV-positive cells develop resistance to TGF-beta growth inhibitory activity through the inhibition of hypophosphorylation of pRb by papillomavirus type 16 E7 oncoprotein. In this study, we examined whether E7, in addition to its well known effects on pRb, might directly target the Smad proteins that mediate TGF-beta signaling. Here, we show that E7 significantly blocks both Smad transcriptional activity and the ability of TGF-beta to inhibit DNA synthesis. We found that E7 interacts constitutively with Smad2, Smad3, and Smad4. Confocal microscopic studies confirm that E7 and Smads co-localize in vivo. Using a canonical Smad DNA binding sequence, we found that E7 blocks Smad3 binding to its target sequence on DNA. These results suggest that suppression of Smad-mediated signaling by E7 may contribute to HPV-associated carcinogenesis.

Proteasomal activity modulates TGF-ss signaling in a gene-specific manner

Transforming growth factor-beta (TGF-beta) signaling relies on Smad-signaling pathway controlled in part by the proteasome. Here we demonstrate that inhibition of the proteasome function in mink epithelial cells accumulates both positive and negative modulators of TGF-beta signaling, phospho-Smad2 and SnoN. Inhibition of the proteasome led to abrogation of TGF-beta target gene regulation in a gene-specific manner. While regulation of p15Ink4b and myc by TGF-beta are lost, PAI-1 induction, previously shown to occur in a Smad3-dependent manner, was not affected by treatment of the cells with the proteasomal inhibitor MG132. The results suggest that proteasomal activity is required for TGF-beta signaling in a gene-specific manner.

Phosphorylation of pRb is required for HGF-induced muscle cell proliferation and is p27kip1-dependent

Hepatocyte growth factor (HGF) plays a crucial role in the differentiation of skeletal muscle cells, where a process in which the retinoblastoma protein (pRb) has been implicated. We addressed the role of pRb in HGF-mediated effects on the proliferation and differentiation of adult skeletal muscle myoblasts. HGF shifted pRb to its hyperphosphorylation forms and increased the transactivation of E2F1, a transcription factor required for S phase entry. A constitutively active pRb mutant blocked HGF-dependent pRb phosphorylation and transactivation of E2F1 and increased cell proliferation. Accordingly, this mutant reversed the inhibitory effects of HGF on the expression of the cyclin-dependent kinase (CDK) inhibitor p27 and myogenic differentiation markers. HGF-mediated pRb phosphorylation was reversed by ectopic expression of p27, but neither the myogenic regulatory factor, MEF2, nor the myogenic inhibitory protein Twist had that effect. These results suggest that in response to HGF signaling, there is a decrease in p27 expression that results in an accumulation of hyperphosphorylated Rb protein, and subsequent progression of myoblasts into the G1 phase of the cell cycle.

Scatter factor/hepatocyte growth factor stimulation of glioblastoma cell cycle progression through G(1) is c-Myc dependent and independent of p27 suppression, Cdk2 activation, or E2F1-dependent transcription

Scatter factor/hepatocyte growth factor (SF/HGF) expression has been linked to malignant progression in glial neoplasms. Using two glioma cell lines, U373MG and SNB-19, we have demonstrated that SF/HGF stimulation allows cells to escape G(1)/G(0) arrest induced by contact inhibition or serum withdrawal. SF/HGF induced effects on two mechanisms of cell cycle regulation: suppression of the cyclin-dependent kinase inhibitor p27 and induction of the transcription factor c-Myc. Regulation of p27 by SF/HGF was posttranslational and is associated with p27 nuclear export. Transient transfections of U373MG and SNB-19 with wild-type p27 and a degradation-resistant p27T187A mutant were insufficient to induce cell cycle arrest, and SF/HGF downregulation of p27 was not necessary for cell cycle reentry. Analysis of Cdk2 kinase activity and p27 binding to cyclin E complexes in the presence of exogenous wild-type p27 or p27T187A demonstrated that Cdk2 activity was not necessary for SF/HGF-mediated G(1)/S transition. Similarly, overexpression of dominant-negative forms of Cdk2 did not block SF/HGF-triggered cell cycle progression. In contrast, SF/HGF transcriptionally upregulated c-Myc, and overexpression of c-Myc was able to prevent G(1)/G(0) arrest in the absence of SF/HGF. Transient overexpression of MadMyc, a dominant-negative chimera for c-Myc, caused G(1)/G(0) arrest in logarithmically growing cells and blocked SF/HGF-mediated G(1)/S transition. c-Myc did not exert its effects through p27 downregulation in these cell lines. SF/HGF induced E2F1-dependent transcription, the inhibition of which did not block SF/HGF-induced cell cycle progression. We conclude that SF/HGF prevents G(1)/G(0) arrest in glioma cell lines by a c-myc-dependent mechanism that is independent of p27, Cdk2, or E2F1.

Interferon-gamma inhibits hepatocyte growth factor-stimulated cell proliferation of human bronchial epithelial cells: upregulation of p27(kip1) cyclin-dependent kinase inhibitor

Proliferation of bronchial epithelial cells is an important biologic process in a variety of physiologic and pathologic conditions. In this study, we demonstrate that hepatocyte growth factor (HGF) stimulates proliferation of human bronchial epithelial cells obtained from healthy volunteers. The mitogenic effect of HGF is dependent on costimulation with serum and is completely abrogated by interferon-gamma (IFN-gamma). In the absence of serum, HGF is capable of inducing activation of extracellular signal-regulated kinases (ERK)1 and ERK2, but fails to stimulate proliferation by itself. These effects of HGF and IFN-gamma were reproduced faithfully in BEAS-2B cells, which are an immortalized cell line derived from human bronchial epithelial cells. Further, we investigated the molecular mechanisms underlying the effects of HGF and IFN-gamma in BEAS-2B cells and found that the MEK1 inhibitor PD98059, but not the p38 M-associated protein kinase inhibitor SB203580, abrogates HGF-induced ERK activation and proliferation in response to HGF and serum. In addition, LY294002, which is the specific inhibitor of phosphatidyl inositol 3-kinase, partially inhibited HGF- and serum-stimulated proliferation. We also found that HGF by itself is capable of inducing a G1 cyclin, cyclin D1, but fails to downregulate p27(kip1) cyclin-dependent kinase (CDK) inhibitor, which is a requisite for G1 to S phase cell cycle progression. IFN-gamma does not interfere with the effects of HGF on either ERK activation or cyclin D1 induction; however, it prevents the downregulation of p27(kip1) CDK inhibitor that takes place in response to a combination of HGF and serum. These results indicate that the MEK-ERK signaling pathway is necessary but not sufficient for human bronchial epithelial cell proliferation, and implicate the significance of HGF and IFN-gamma in the repair processes of injured human bronchial epithelial cells.

Ectopic expression of Cdk6 circumvents transforming growth factor-beta mediated growth inhibition

Transforming growth factor-beta (TGF-beta) induced growth arrest of cells involves regulation of the activities of both D- and E-type cyclin kinase complexes thought to be mediated primarily by the regulation of p15(Ink4b) and p27(Kip1) cyclin kinase inhibitors. We show here that TGF-beta downregulates Cdk6 and that transient and stable expression of Cdk6 in Mv1Lu mink epithelial cells overrides TGF-beta mediated arrest. The main effect of the ectopic Cdk6 expression was to sequester TGF-beta induced p15(Ink4b) and to maintain more p27(Kip1) in cyclin D-complexes preventing the complete shift of p27(Kip1) to Cdk2 invoked by TGF-beta. This led to the presence of an active cyclinD-Cdk6-p27(Kip1) complex and partially active cyclin E-Cdk2 complex and resulted in the failure of TGF-beta to fully arrest Mv1Lu cell growth. Though dominant negative Cdk6, expressed similarly in the cells, sequestered both p15(Ink4b) and p27(Kip1), it lacks kinase activity and was unable to override the TGF-beta arrest. The results demonstrate that downregulation of Cdk6 kinase is required for the enforcement of the G(1)-phase arrest by TGF-beta and results in changes in association of the p15(Ink4b) and p27(Kip1) inhibitors with D- and E-type cyclin kinase complexes.

Caspase-mediated Cdk2 activation is a critical step to execute transforming growth factor-beta1-induced apoptosis in human gastric cancer cells

Although TGF-beta1, a growth inhibitor, is known to also induce apoptosis, the molecular mechanism of this apoptosis is largely undefined. Here, we identify the mechanism of TGF-beta1-induced apoptosis in SNU-16 human gastric cancer cells. Cell cycle and TUNEL analysis showed that, upon TGF-beta1 treatment, cells were initially arrested at the G1 phase and then driven into apoptosis. Of note, caspase-3 was activated in accordance with TGF-beta1-induced G1 arrest. Activated caspase-3 is targeted to cleave p21(cip1), p27(kip1), and Rb, which play important roles in TGF-beta-induced G1 arrest, into inactive fragments. Subsequently, Cdk2 was aberrantly activated due to the cleavage of p21 and p27. We found that the inhibition of Cdk2 activity efficiently blocks TGF-beta1-induced apoptosis, whereas it did not prevent caspase-3 activation or the subsequent cleavage of target proteins. In contrast, the suppression of caspase-3 activity inhibited the cleavage of target proteins, the activation of Cdk2, and the induction of apoptosis. Taken together, our results suggest that activation of caspase-3 by TGF-beta1 may initiate the conversion from G1 cell cycle arrest to apoptosis via the cleavage of p21, p27 and Rb, which in turn causes Cdk2 activation and, most significantly, Cdk2 activation as a downstream effector of caspase is a critical step for the execution of TGF-beta1-induced apoptosis.

Ski/Sno and TGF-beta signaling

Transforming growth factor-beta is a potent inhibitor of epithelial cell proliferation. Proteins involved in TGF-beta signaling are bona fide tumor suppressors and many tumor cells acquire the ability to escape TGF-beta growth inhibition through the loss of key signaling transducers in the pathway or through the activation of oncogenes. Recent studies indicate that there is a specific connection between the TGF-beta signaling pathway and the Ski/SnoN family of oncoproteins. We summarize evidence that Ski and SnoN directly associate with Smad proteins and block the ability of the Smads to activate expression of many if not all TGF-beta-responsive genes. This appears to cause abrogation of TGF-beta growth inhibition in epithelial cells.

Disruption of TGF-beta growth inhibition by oncogenic ras is linked to p27Kip1 mislocalization

Expression of oncogenic Ras in epithelial tumor cells is linked to the loss of transforming growth factor-beta (TGF-beta) anti-proliferative activity, and was proposed to involve inhibition of Smad2/3 nuclear translocation. Here we studied several epithelial cell lines expressing oncogenic N-RasK61 and show that TGF-beta-induced nuclear translocation of and transcriptional activation by Smad2/3 were unaffected. In contrast, oncogenic Ras mediated nuclearto-cytoplasmic mislocalization of p27KiP1 (p27) and of the cyclin-dependent kinase (CDK) CDK6, but not CDK2. Concomitantly, oncogenic Ras abrogated the ability of TGF-beta to release p27 from CDK6, to enhance its binding to CDK2 and to inhibit CDK2 activity. Inactivation of Ras by a specific antagonist restored the growth inhibitory response to TGF-beta with concurrent normalization of p27 and CDK6 localization. Therefore, the disruption of TGF-beta-mediated growth inhibition by oncogenic Ras appears to be due to lack of inhibition of CDK2, caused by the sequestration of p27 and CDK2 in different subcellular compartments and by the loss of TGF-beta-induced partner switching of p27 from CDK6 to CDK2.

Myostatin regulation during skeletal muscle regeneration

Myostatin, a member of the TGF-beta superfamily, is a key negative regulator of skeletal muscle growth. The role of myostatin during skeletal muscle regeneration has not previously been reported. In the present studies, normal Sprague-Dawley and growth hormone (GH)-deficient (dw/dw) rats were administered the myotoxin, notexin, in the right M. biceps femoris on day 0. The dw/dw rats then received either saline or human-N-methionyl GH (200microg/100g body weight/day) during the ensuing regeneration. Normal and dw/dw M. biceps femoris were dissected on days 1, 2, 3, 5, 9 and 13, formalin-fixed, then immunostained for myostatin protein. Immunostaining for myostatin revealed high levels of protein within necrotic fibres and connective tissue of normal and dw/dw damaged muscles. Regenerating myotubes contained no myostatin at the time of fusion (peak fusion on day 5), and only low levels of myostatin were observed during subsequent myotube enlargement. Fibres which survived assault by notexin (survivor fibres) contained moderate to high myostatin immunostaining initially. The levels in both normal and dw/dw rat survivor fibres decreased on days 2-3, then increased on days 9-13. In dw/dw rats, there was no observed effect of GH administration on the levels of myostatin protein in damaged muscle. The low level of myostatin observed in regenerating myotubes in these studies suggests a negative regulatory role for myostatin in muscle regeneration.

Accumulation of a form of p27(Kip1) not associated with Cdk-cyclin complexes in transforming growth factor-beta-arrested Mv1Lu cells

The p27(Kip1) cyclin-dependent kinase inhibitor translocates in response to transforming growth factor-beta to a Cdk2-cyclin E complex inhibiting its catalytic activity, but the p27(Kip1) protein levels are unaffected [1]. We show here that transforming growth factor-beta induces the accumulation of a form of p27(Kip1) representing a subpopulation of total p27(Kip1) in growth-arrested Mv1Lu epithelial cells. The inducible p27(Kip1) is detectable only by a specific p27(Kip1) monoclonal antibody recognizing a native form of p27(Kip1). The increase in this subset of p27(Kip1) correlates with G(1) arrest and withdrawal of the cells from the cycle induced by transforming growth factor-beta, serum starvation, or contact inhibition. In contrast to the majority of p27(Kip1) in the cells, the transforming growth factor-beta-inducible p27(Kip1) is devoid of cyclin-dependent kinase/cyclin interactions. The results indicate that growth arresting treatments induce the accumulation of non-cyclin-dependent kinase-bound p27(Kip1), which may function as a reservoir for inhibition of Cdk2-cyclin E activities.

Transforming growth factor beta(1) selectively inhibits the cyclic AMP-dependent proliferation of primary thyroid epithelial cells by preventing the association of cyclin D3-cdk4 with nuclear p27(kip1)

Dog thyroid epithelial cells in primary culture constitute a physiologically relevant model of positive control of DNA synthesis initiation and G0-S prereplicative phase progression by cAMP as a second messenger for thyrotropin (thyroid-stimulating hormone [TSH]). As previously shown in this system, the cAMP-dependent mitogenic pathway differs from growth factor cascades as it stimulates the accumulation of p27(kip1) but not cyclins D. Nevertheless, TSH induces the nuclear translocations and assembly of cyclin D3 and cdk4, which are essential in cAMP-dependent mitogenesis. Here we demonstrate that transforming growth factor beta(1) (TGFbeta(1)) selectively inhibits the cAMP-dependent cell cycle in mid-G1 and various cell cycle regulatory events, but it weakly affects the stimulation of DNA synthesis by epidermal growth factor (EGF), hepatocyte growth factor, serum, and phorbol esters. EGF+serum and TSH did not interfere importantly with TGFbeta receptor signaling, because they did not affect the TGFbeta-induced nuclear translocation of Smad 2 and 3. TGFbeta inhibited the phosphorylation of Rb, p107, and p130 induced by TSH, but it weakly affected the phosphorylation state of Rb-related proteins in EGF+serum-treated cells. TGFbeta did not inhibit c-myc expression. In TSH-stimulated cells, TGFbeta did not affect the expression of cyclin D3, cdk4, and p27(kip1), nor the induced formation of cyclin D3-cdk4 complexes, but it prevented the TSH-induced relocalization of p27(kip1) from cdk2 to cyclin D3-cdk4. It prevented the nuclear translocations of cdk4 and cyclin D3 without altering the assembly of cyclin D3-cdk4 complexes probably formed in the cytoplasm, where they were prevented from sequestering nuclear p27(kip1) away from cdk2. This study dissociates the assembly of cyclin D3-cdk4 complexes from their nuclear localization and association with p27(kip1). It provides a new mechanism of regulation of proliferation by TGFbeta, which points out the subcellular location of cyclin D-cdk4 complexes as a crucial factor integrating mitogenic and antimitogenic regulations in an epithelial cell in primary culture.