LukS-PV induces cell cycle arrest and apoptosis through p38/ERK MAPK signaling pathway in NSCLC cells

Non-small-cell lung cancer (NSCLC) accounts for nearly 85% of lung cancer cases. LukS-PV, one of the two components of Panton-Valentine leucocidin (PVL), is produced by Staphylococcus aureus. The present study showed that LukS-PV can induce apoptosis in human acute myeloid leukemia (AML) lines (THP-1 and HL-60). However, the role of LukS-PV in NSCLC is unclear. In this study, we treated NSCLC cell lines A549 and H460 and a normal lung cell line, 16HBE, with LukS-PV and investigated the biological roles of LukS-PV in NSCLC. Cells were treated with varying concentrations of LukS-PV and cell viability was evaluated by CCK8 and EdU assay. Flow cytometry was used to detect cell apoptosis and analyze the cell cycle, and the expression of apoptosis and cell cycle-associated proteins and genes were identified by western blotting analysis and qRT-polymerase chain reaction, respectively. We found that LukS-PV inhibited the proliferation of NSCLC cells but had little cytotoxicity in normal lung cells. LukS-PV induced NSCLC cell apoptosis and increased the BAX/BCL-2 ratio, triggering S-phase arrest in A549 and H460 cells while increasing P21 expression and decreasing CDK2, cyclin D1, and cyclin A2 expression. We also observed increased P-p38 and P-ERK in NSCLC cells treated with LukS-PV. Treatment of NSCLC with LukS-PV combined with p38 and ERK inhibitors reversed the pro-apoptotic and pro-cell cycle arrest effects of LukS-PV. Overall, these findings indicate that LukS-PV has anti-tumor effects in NSCLC and may contribute to the development of anti-cancer agents.     

Allicin induces cell cycle arrest and apoptosis of breast cancer cells in vitro via modulating the p53 pathway

Background

The tumor suppressor protein p53 is a most promising target for the development of anticancer drugs. Allicin (diallylthiosulfinate) is one of the most active components of garlic (Alliium sativum L.) and possesses a variety of health-promoting properties with pharmacological applications. However, whether allicin plays an anti-cancer role against breast cancer cells through the induction of p53-mediated apoptosis remains unknown.

Methods and results

In this study, we investigate the anti-breast cancer effect of allicin in vitro by using MCF-7 and MD-MBA-231 cells. We found that allicin reduces cell viability, induces apoptosis and cell cycle arrest in both cells. Allicin activated p53 and caspase 3 expressions in both cells but produced different effects on the expression of p53-related biomarkers. In MDA-MB-231 cells, allicin up-regulated the mRNA and protein expression of A1BG and THBS1 while down-regulated the expression of TPM4. Conversely, the mRNA and protein expression of A1BG, THBS1 and TPM4 were all reduced in MCF-7 cells. Hence, allicin induces cell cycle arrest and apoptosis in breast cancer cells through p53 activation but it effects on the expression of p53-related biomarkers were dependent upon the specific type of breast cancer involved.

Conclusions

These findings suggest that allicin induces apoptosis and regulates biomarker expression in breast cancer cell lines through modulating the p53 signaling pathway. Furthermore, our results promote the utility of allicin as compound for further studies as an anticancer drug targeting p53.

     

TGEV nucleocapsid protein induces cell cycle arrest and apoptosis through activation of p53 signaling

Our previous studies showed that TGEV infection could induce cell cycle arrest and apoptosis via activation of p53 signaling in cultured host cells. However, it is unclear which viral gene causes these effects. In this study, we investigated the effects of TGEV nucleocapsid (N) protein on PK-15 cells. We found that TGEV N protein suppressed cell proliferation by causing cell cycle arrest at the S and G2/M phases and apoptosis. Characterization of various cellular proteins that are involved in regulating cell cycle progression demonstrated that the expression of N gene resulted in an accumulation of p53 and p21, which suppressed cyclin B1, cdc2 and cdk2 expression. Moreover, the expression of TGEV N gene promoted translocation of Bax to mitochondria, which in turn caused the release of cytochrome c, followed by activation of caspase-3, resulting in cell apoptosis in the transfected PK-15 cells following cell cycle arrest. Further studies showed that p53 inhibitor attenuated TGEV N protein induced cell cycle arrest at S and G2/M phases and apoptosis through reversing the expression changes of cdc2, cdk2 and cyclin B1 and the translocation changes of Bax and cytochrome c induced by TGEV N protein. Taken together, these results demonstrated that TGEV N protein might play an important role in TGEV infection-induced p53 activation and cell cycle arrest at the S and G2/M phases and apoptosis occurrence.     

The ubiquitin peptidase UCHL1 induces G0/G1 cell cycle arrest and apoptosis through stabilizing p53 and is frequently silenced in breast cancer

Background

Breast cancer (BrCa) is a complex disease driven by aberrant gene alterations and environmental factors. Recent studies reveal that abnormal epigenetic gene regulation also plays an important role in its pathogenesis. Ubiquitin carboxyl- terminal esterase L1 (UCHL1) is a tumor suppressor silenced by promoter methylation in multiple cancers, but its role and alterations in breast tumorigenesis remain unclear.

Methodology/Principal Findings

We found that UCHL1 was frequently downregulated or silenced in breast cancer cell lines and tumor tissues, but readily expressed in normal breast tissues and mammary epithelial cells. Promoter methylation of UCHL1 was detected in 9 of 10 breast cancer cell lines (90%) and 53 of 66 (80%) primary tumors, but rarely in normal breast tissues, which was statistically correlated with advanced clinical stage and progesterone receptor status. Pharmacologic demethylation reactivated UCHL1 expression along with concomitant promoter demethylation. Ectopic expression of UCHL1 significantly suppressed the colony formation and proliferation of breast tumor cells, through inducing G0/G1 cell cycle arrest and apoptosis. Subcellular localization study showed that UCHL1 increased cytoplasmic abundance of p53. We further found that UCHL1 induced p53 accumulation and reduced MDM2 protein level, and subsequently upregulated the expression of p21, as well as cleavage of caspase3 and PARP, but not in catalytic mutant UCHL1 C90S-expressed cells.

Conclusions/Significance

UCHL1 exerts its tumor suppressive functions by inducing G0/G1cell cycle arrest and apoptosis in breast tumorigenesis, requiring its deubiquitinase activity. Its frequent silencing by promoter CpG methylation may serve as a potential tumor marker for breast cancer.     

Folate deprivation induces cell cycle arrest at G0/G1 phase and apoptosis in hippocampal neuron cells through down-regulation of IGF-1 signaling pathway

Folate deficiency contributes to impaired adult hippocampal neurogenesis, yet the mechanisms remain unclear. Here we use HT-22 hippocampal neuron cells as model to investigate the effect of folate deprivation (FD) on cell proliferation and apoptosis, and to elucidate the underlying mechanism. FD caused cell cycle arrest at G0/G1 phase and increased the rate of apoptosis, which was associated with disrupted expression of folate transport and methyl transfer genes. FOLR1 and SLC46A1 were (P < 0.01) down-regulated, while SLC19A1 was up-regulated (P < 0.01) in FD group. FD cells exhibited significantly (P < 0.05) higher protein content of BHMT, MAT2b and DNMT3a, as well as increased SAM/SAH concentrations and global DNA hypermethylation. The expression of the total and all the 3 classes of IGF-1 mRNA variants was significantly (P < 0.01) down-regulated and IGF-1 concentration was decreased (P < 0.05) in the culture media. IGF-1 signaling pathway was also compromised with diminished activation (P < 0.05) of STAT3, AKT and mTOR. CpG hypermethylation was detected in the promoter regions of IGF-1 and FOLR1 genes, while higher SLC19A1 mRNA corresponded to hypomethylation of its promoter. IGF-1 supplementation in FD media significantly abolished FD-induced decrease in cell viability. However, IGF-1 had limited effect in rescuing the cell phenotype when added 24 h after FD. Taken together, down-regulation of IGF-1 expression and signaling is involved in FD-induced cell cycle arrest and apoptosis in HT-22 hippocampal neuron cells, which is associated with an abnormal activation of methyl transfer pathway and hypermethylation of IGF-1 gene promoter.     

Magnolol induces apoptosis in osteosarcoma cells via G0/G1 phase arrest and p53-mediated mitochondrial pathway

Osteosarcoma is a highly invasive primary malignancy of bone. Magnolol is biologically active, which shows antitumor effects in a variety of cancer cell lines. However, it has not been elucidated magnolol's effects on human osteosarcoma cells (HOC). This study aimed to determine antitumor activity of magnolol and illustrate the molecular mechanism in HOC. Magnolol showed significant inhibition effect of growth on MG-63 and 143B cells and induced apoptosis and cell cycle arrest at G0/G1. In osteosarcoma cells, magnolol upregulated expressions of proapoptosis proteins and suppressed expressions of antiapoptosis proteins. Additionally, under the pretreatment of pifithrin-a (PFT-a, a p53 inhibitor), the magnolol-induced apoptosis was significantly reversed. The results above indicated that magnolol induces apoptosis in osteosarcoma cells may via G0/G1 phase arrest and p53-mediated mitochondrial pathway.     

Viral oncoprotein LMP1 disrupts p53-induced cell cycle arrest and apoptosis through modulating K63-linked ubiquitination of p53

Disruption of the gatekeeper p53 tumor suppressor is involved in various virus-associated tumorigeneses, with aberrant ubiquitination as the major cause of p53 abnormalities in virus-associated tumors. Of note, wild-type p53 is accumulated in Epstein-Barr virus (EBV)-associated tumors, especially in nasopharyngeal carcinoma (NPC). We have previously identified that p53 is accumulated and phosphorylated by EBV oncoprotein latent membrane protein 1 (LMP1) in NPC. Here, we further found that LMP1 promoted p53 accumulation via two distinct ubiquitin modifications. LMP1 promoted p53 stability and accumulation by suppressing K48-linked ubiquitination of p53 mediated by E3 ligase MDM2, which is associated with its phosphorylation at Ser20, while increasing the levels of total cellular ubiquitinated p53. LMP1 also induced K63-linked ubiquitination of p53 by interacting with tumor necrosis factor receptor-associated factor 2 (TRAF2), thus contributing to p53 accumulation. Furthermore, LMP1 rescued tumor cell apoptosis and cell cycle arrest mediated by K63-linked ubiquitination of p53. Collectively, these results demonstrate aberrant ubiquitin modifications of p53 and its biological functions by viral protein LMP1, which has broad implications to the pathogenesis of multiple EBV-associated tumors.     

Caudatin induces cell cycle arrest and caspase-dependent apoptosis in HepG2 cell

In the present study, we investigate the anti-cancer activity and mechanism of caudatin, the C-21 steroidal glycosides, on human hepatoma cell line HepG2. The MTT assay and flow cytometry were used to evaluate HepG2 cell proliferation and cell cycle. Annexin-V/PI and DAPI staining were used to investigate cell apoptosis. Western blotting analysis was used to evaluate the expression levels of proteins. It is found that caudatin inhibits HepG2 cell growth and induces of G₀/G₁ phase arrest in a dose dependent manner, which is associated with a decreased in the expression of cyclinD₁ and increased the levels of p21 and p53. HepG2 cells dealing with caudatin showed typical characteristics of apoptosis. Western blotting analysis indicated that the levels of Bcl-2 were down-regulated after caudatin treatment, whereas the expression of Bax was up-regulated. Furthermore, caudatin-induced apoptosis was accompanied by activation of caspase-3, -9, and poly(ADP-Ribose) Polymerase (PARP). Treatment with caudatin also induced phosphorylation of extracellular-signal regulating kinase (ERK) and c-Jun N-terminal kinase (JNK). These results demonstrate that caudatin inhibits cell proliferation via DNA synthesis reduction and induces caspase-dependent apoptosis in HepG2 cell. Activation of ERK and JNK may be involved in caudatin-induced hepatoma cell apoptosis.     

Fucoxanthin induces cell cycle arrest at G0/G1 phase in human colon carcinoma cells through up-regulation of p21WAF1/Cip1

Fucoxanthin, a natural carotenoid, has been reported to have antitumorigenic activity in mouse colon, skin and duodenum models. The present study was designed to evaluate the molecular mechanisms of fucoxanthin against colon cancer using the human colon adenocarcinoma cell lines. Fucoxanthin reduced the viability of WiDr cells in a dose-dependent manner accompanied by the induction of cell cycle arrest during the G0/G1 phase at 25 microM and apoptosis at 50 microM. Fucoxanthin at 25 microM inhibited the phosphorylation of the retinoblastoma protein (pRb) at Ser780 and Ser807/811 24 h after treatment without changes in the protein levels of the D-types of cyclin and cyclin-dependent kinase (cdk) 4, whose complexes are responsible for the phosphorylation of pRb at these sites. A cdk inhibitory protein, p21WAF1/Cip1 increased 24 h after the treatment with 25 microM of fucoxanthin, but not p27Kip1. In addition, the mRNA of p21WAF1/Cip1 also increased in a dose-dependent manner. According to the experiments using the isogenic human colon adenocarcinoma cell lines, fucoxanthin failed to induce G0/G1 arrest in the p21-deficient HCT116 cells, but not in HCT116 wild-type cells. All of these findings showed that fucoxanthin inhibited proliferation of colon cancer cells. The inhibitory mechanism is due to the cell cycle arrest during the G0/G1 phase mediated through the up-regulation of p21WAF1/Cip1, which may be related to the antitumorigenic activity.     

Eimeria tenella ROP kinase EtROP1 induces G0/G1 cell cycle arrest and inhibits host cell apoptosis

Coccidia are obligate intracellular protozoan parasites responsible for human and veterinary diseases. Eimeria tenella, the aetiologic agent of caecal coccidiosis, is a major pathogen of chickens. In Toxoplasma gondii, some kinases from the rhoptry compartment (ROP) are key virulence factors. ROP kinases hijack and modulate many cellular functions and pathways, allowing T. gondii survival and development. E. tenella's kinome comprises 28 putative members of the ROP kinase family; most of them are predicted, as pseudokinases and their functions have never been characterised. One of the predicted kinase, EtROP1, was identified in the rhoptry proteome of E. tenella sporozoites. Here, we demonstrated that EtROP1 is active, and the N‐terminal extension is necessary for its catalytic kinase activity. Ectopic expression of EtROP1 followed by co‐immunoprecipitation identified cellular p53 as EtROP1 partner. Further characterisation confirmed the interaction and the phosphorylation of p53 by EtROP1. E. tenella infection or overexpression of EtROP1 resulted both in inhibition of host cell apoptosis and G0/G1 cell cycle arrest. This work functionally described the first ROP kinase from E. tenella and its noncanonical structure. Our study provides the first mechanistic insight into host cell apoptosis inhibition by E. tenella. EtROP1 appears as a new candidate for coccidiosis control.     

Polycystin-1 induced apoptosis and cell cycle arrest in G0/G1 phase in cancer cells

Studies have shown that polycystin-1, encoded by PKD1, the major ADPKD, may have a central role in regulating both apoptosis and proliferation, which could prevent the malignant transformation of affected cells. However, as a putative tumor suppressor, direct studies on the possibility that polycystin-1 may play a role in cancer cells' biological properties have not yet been reported. We have demonstrated that the apoptosis of cancer cells was induced by overexpression of polycystin-1. After transfection with polycystin-1, three cancer cell lines, HepG2, A549, and SW480, showed significantly increased apoptosis compared with the respective control groups. This was accompanied by cell cycle arrest at G(0)/G(1) phase, whereas cell proliferation was not significantly affected. Overexpression of polycystin-1 induces apoptosis in cancer cells, at least partially, through Wnt and a caspase-dependent pathway.     

Murine coronavirus replication induces cell cycle arrest in G0/G1 phase

Mouse hepatitis virus (MHV) replication in actively growing DBT and 17Cl-1 cells resulted in the inhibition of host cellular DNA synthesis and the accumulation of infected cells in the G₀/G₁ phase of the cell cycle. UV-irradiated MHV failed to inhibit host cellular DNA synthesis. MHV infection in quiescent 17Cl-1 cells that had been synchronized in the G₀ phase by serum deprivation prevented infected cells from entering the S phase after serum stimulation. MHV replication inhibited hyperphosphorylation of the retinoblastoma protein (pRb), the event that is necessary for cell cycle progression through late G₁ and into the S phase. While the amounts of the cellular cyclin-dependent kinase (Cdk) inhibitors p21^Cip1, p27^Kip1, and p16^INK4a did not change in infected cells, MHV infection in asynchronous cultures induced a clear reduction in the amounts of Cdk4 and G₁ cyclins (cyclins D1, D2, D3, and E) in both DBT and 17Cl-1 cells and a reduction in Cdk6 levels in 17Cl-1 cells. Infection also resulted in a decrease in Cdk2 activity in both cell lines. MHV infection in quiescent 17Cl-1 cells prevented normal increases in Cdk4, Cdk6, cyclin D1, and cyclin D3 levels after serum stimulation. The amounts of cyclin D2 and cyclin E were not increased significantly after serum stimulation in mock-infected cells, whereas they were decreased in MHV-infected cells, suggesting the possibility that MHV infection may induce cyclin D2 and cyclin E degradation. Our data suggested that a reduction in the amounts of G₁ cyclin-Cdk complexes in MHV-infected cells led to a reduction in Cdk activities and insufficient hyperphosphorylation of pRb, resulting in inhibition of the cell cycle in the G₀/G₁ phase.     

Phthalocyanine-mediated photodynamic therapy induces cell death and a G0/G1 cell cycle arrest in cervical cancer cells

We have developed a series of novel photosensitizers which have potential for anticancer photodynamic therapy (PDT). Photosensitizers include zinc phthalocyanine tetra-sulphonic acid and a family of derivatives with amino acid substituents of varying alkyl chain length and degree of branching. Subcellular localization of these photosensitizers at the phototoxic IC(50) concentration in human cervical carcinoma cells (SiHa Cells) was similar to that of the lysosomal dye Lucifer Yellow. Subsequent nuclear relocalization was observed following irradiation with 665nm laser light. The PDT response was characterized using the Sulforhodamine B cytotoxicity assay. Flow cytometry was used for both DNA cell cycle and dual Annexin V-FITC/propidium iodide analysis. Phototoxicity of the derivatives was of the same order of magnitude as for tetrasulphonated phthalocyanine but with an overall trend of increased phototoxicity with increasing amino acid chain length. Our results demonstrate cell death, inhibition of cell growth, and G(0)/G(1) cell cycle arrest during the phthalocyanine PDT-mediated response.     

5′-Nitro-indirubinoxime induces G1 cell cycle arrest and apoptosis in salivary gland adenocarcinoma cells through the inhibition of Notch-1 signaling

Background

5′-Nitro-indirubinoxime (5′-NIO) is a new derivative of indirubin that exhibits anti-cancer activity in a variety of human cancer cells. However, its mechanism has not been fully clarified.

Methods

Human salivary gland adenocarcinoma (SGT) cells were used in this study. Western blot and RT-PCR analyses were performed to determine cellular Notch levels. The cell cycle stage and level of apoptosis were analyzed using flow cytometry analysis.

Results

5′-NIO significantly inhibited the mRNA levels of Notch-1 and Notch-3 and their ligands (Delta1, 2, 3, and Jagged-2) in SGT cells. Immunocytochemistry analysis showed that 5′-NIO specifically decreased the level of Notch-1 in the nucleus. In addition, 5′-NIO induced G1 cell cycle arrest by reducing levels of CDK4 and CDK6 in SGT cells. Using flow cytometry and immunoblotting analysis, we found that 5′-NIO induces apoptosis following the secretion of cytochrome c and the activation of caspase-3 and caspase-7. Intracellular Notch-1 overexpression led to a decrease in G1 phase arrest and an inhibition of 5′-NIO-induced apoptosis.

Conclusion

These observations suggest that 5′-NIO induces cell cycle arrest and apoptosis by down-regulating Notch-1 signaling.

General significance

This study identifies a new mechanism of 5′-NIO-mediated anti-tumor properties. Thus, 5′-NIO could be used as a candidate for salivary gland adenocarcinoma therapeutics.     

Ganoderic acid Me induces G1 arrest in wild-type p53 human tumor cells while G1/S transition arrest in p53-null cells

The mechanism of cell cycle arrest of tumor cells induced by ganoderic acid Me (GA-Me) is not understood. In this work, GA-Me was found to possess remarkable cytotoxicity on highly metastatic lung carcinoma 95-D cell line in both dose- and time-dependent manners. The effect of GA-Me on cell cycle arrest was found in 95-D, p53-null lung cancer cells H1299, HCT-116 p53^+/+ and HCT-116 p53^?/? human colon cancer cells. To obtain an insight into the role of p53 in cell cycle arrest by GA-Me, 95-D, H1299, HCT-116 p53^+/+ and HCT-116 p53^?/? cells were used for further investigation. GA-Me arrested cell cycle at G₁ phase in 95-D and HCT-116 p53^+/+ cells while S phase or G₁/S transition arrest in H1299 and HCT-116 p53^?/? cells. The results suggested that p53 may be a target of GA-Me, and it may be looked at as a new promising candidate for the treatment of carcinoma cells.     

Liriodenine induces G1/S cell cycle arrest in human colon cancer cells via nitric oxide- and p53-mediated pathway

Liriodenine is an aporphine alkaloid compound extracted from the leaves of Michelia compressa var. lanyuensis. It had been reported to have an anti-colon cancer effect, but the mechanism remains unclear. In the present study, the antiproliferative mechanisms of liriodenine were investigated in the human colon cancer SW480 cells. Flow cytometry analysis indicated that liriodenine notably induced the G1/S phase arrest. The G1/S phase cycle-related proteins analysis illustrated that the expressions of cyclin-dependent kinase (CDK) 2, CDK4 and CDK6, and of cyclin D1 and A, as well as the phosphorylation of retinoblastoma tumor suppressor protein (ppRB) were found to be markedly reduced by liriodenine, whereas the protein levels of the CDK inhibitors (CKIs), p21 and p27 were increased. Moreover, the intracellular nitric oxide (NO) production, protein levels of inducible NO synthase (iNOS) and, p53 were increased. The p53 overexpression was a downstream event of NO production in liriodenine-induced G1/S-arrested SW480 cells, and the up-regulation of p21 and p27 was found to be mediated by a p53-dependent pathway. The inhibition of p53 by pifithrin-α (PFT-α), down-regulation of p21 and p27 by siRNA, or NO reduction by S-ethylisothiourea (ETU) entirely abolished the liriodenine-induced G1/S phase arrest. We concluded that liriodenine potently inhibited the cell cycle of SW480 cancer cells via NO- and p53-dependent G1/S phase arrest pathway. These results suggest that liriodenine might be a powerful agent against colon cancer.     

ERK activation mediates cell cycle arrest and apoptosis after DNA damage independently of p53

In response to DNA damage, ataxia-telangiectasia mutant and ataxia-telangiectasia and Rad-3 activate p53, resulting in either cell cycle arrest or apoptosis. We report here that DNA damage stimuli, including etoposide (ETOP), adriamycin (ADR), ionizing irradiation (IR), and ultraviolet irradiation (UV) activate ERK1/2 (ERK) mitogen-activated protein kinase in primary (MEF and IMR90), immortalized (NIH3T3) and transformed (MCF-7) cells. ERK activation in response to ETOP was abolished in ATM-/- fibroblasts (GM05823) and was independent of p53. The MEK1 inhibitor PD98059 prevented ERK activation but not p53 stabilization. Maximal ERK activation in response to DNA damage was not attenuated in MEF(p53-/-). However, ERK activation contributes to either cell cycle arrest or apoptosis in response to low or high intensity DNA insults, respectively. Inhibition of ERK activation by PD98059 or U0126 attenuated p21(CIP1) induction, resulting in partial release of the G(2)/M cell cycle arrest induced by ETOP. Furthermore, PD98059 or U0126 also strongly attenuated apoptosis induced by high dose ETOP, ADR, or UV. Conversely, enforced activation of ERK by overexpression of MEK-1/Q56P sensitized cells to DNA damage-induced apoptosis. Taken together, these results indicate that DNA damage activates parallel ERK and p53 pathways in an ATM-dependent manner. These pathways might function cooperatively in cell cycle arrest and apoptosis.     

Smad7 induces G0/G1 cell cycle arrest in mesenchymal cells by inhibiting the expression of G1 cyclins

The major Smad pathways serve in regulating the expression of genes downstream of TGFbeta signals. In this study, we examined the effects of sustained Smad7 expression in cultured cells. Interestingly, Smad7 caused various mesenchymal cells, including NIH3T3 fibroblast and ST2 bone-marrow stromal cells, to undergo a marked morphological alteration into a flattened cell shape, but kept them alive for as long as 60 days. Furthermore, Smad7 arrested the proliferation of the cells even before they reached confluence. These cells became quiescent in G0/G1 phase and accumulated a hypophosphorylated form of retinoblastoma. The cytostatic effect of Smad7 was closely associated with a preceding decrease in the levels of G1 cyclins, such as cyclin D1 and cyclin E. Accordingly, ectopic cyclin E was able to overcome the Smad7-induced arrest of proliferation. These results indicate that Smad7 functions upstream of G1 cyclins and suggest a novel role for Smad7 as an antiproliferative factor. In contrast to the growth of mesenchymal cells, that of epithelial cells was little susceptible to Smad7. The present findings raise the possibility that a link between Smad7 and the G1 to S phase transition may also contribute to the cell cycle control by certain Smad7-inducing stimuli in a cell-type-dependent fashion.