Receptor-activated transcription factors and beyond: multiple modes of Smad2/3-dependent transmission of TGF-β signaling

Transforming growth factor β (TGF-β) is a pleiotropic cytokine that is widely distributed throughout the body. Its receptor proteins, TGF-β type I and type II receptors, are also ubiquitously expressed. Therefore, the regulation of various signaling outputs in a context-dependent manner is a critical issue in this field. Smad proteins were originally identified as signal-activated transcription factors similar to signal transducer and activator of transcription proteins. Smads are activated by serine phosphorylation mediated by intrinsic receptor dual specificity kinases of the TGF-β family, indicating that Smads are receptor-restricted effector molecules downstream of ligands of the TGF-β family. Smad proteins have other functions in addition to transcriptional regulation, including post-transcriptional regulation of micro-RNA processing, pre-mRNA splicing, and m6A methylation. Recent technical advances have identified a novel landscape of Smad-dependent signal transduction, including regulation of mitochondrial function without involving regulation of gene expression. Therefore, Smad proteins are receptor-activated transcription factors and also act as intracellular signaling modulators with multiple modes of function. In this review, we discuss the role of Smad proteins as receptor-activated transcription factors and beyond. We also describe the functional differences between Smad2 and Smad3, two receptor-activated Smad proteins downstream of TGF-β, activin, myostatin, growth and differentiation factor (GDF) 11, and Nodal.

In silico analysis of differentially expressed-aberrantly methylated genes in breast cancer for prognostic and therapeutic targets

Breast cancer (BC) is the leading cause of death among women across the globe. Abnormal gene expression plays a crucial role in tumour progression, carcinogenesis and metastasis of BC. The alteration of gene expression may be through aberrant gene methylation. In the present study, differentially expressed genes which may be regulated by DNA methylation and their pathways associated with BC have been identified. Expression microarray datasets GSE10780, GSE10797, GSE21422, GSE42568, GSE61304, GSE61724 and one DNA methylation profile dataset GSE20713 were downloaded from Gene Expression Omnibus database (GEO). Differentially expressed-aberrantly methylated genes were identified using online Venn diagram tool. Based on fold change expression of differentially expressed-aberrantly methylated genes were chosen through heat map. Protein-protein interaction (PPI) network of the hub genes was constructed by Search Tool for the Retrieval of Interacting Genes (STRING). Gene expression and DNA methylation level of the hub genes were validated through UALCAN. Overall survival analysis of the hub genes was analysed through Kaplan-Meier plotter database for BC. A total of 72 upregulated-hypomethylated genes and 92 downregulated-hypermethylated genes were obtained from GSE10780, GSE10797, GSE21422, GSE42568, GSE61304, GSE61724, and GSE20713 datasets by GEO2R and Venn diagram tool. PPI network of the upregulated-hypomethylated hub genes (MRGBP, MANF, ARF3, HIST1H3D, GSK3B, HJURP, GPSM2, MATN3, KDELR2, CEP55, GSPT1, COL11A1, and COL1A1) and downregulated-hypermethylated hub genes were constructed (APOD, DMD, RBPMS, NR3C2, HOXA9, AMKY2, KCTD9, and EDN1). All the differentially expressed hub genes expression was validated in UALCAN database. 4 in 13 upregulated-hypomethylated and 5 in 8 downregulated-hypermethylated hub genes to be significantly hypomethylated or hypermethylated in BC were confirmed using UALCAN database (p < 0.05). MANF, HIST1H3D, HJURP, GSK3B, GPSM2, MATN3, KDELR2, CEP55, COL1A1, APOD, RBPMS, NR3C2, HOXA9, ANKMY2, and EDN1 were significantly (p < 0.05) associated with poor overall survival (OS). The identified aberrantly methylated-differentially expressed genes and their related pathways and function in BC can serve as novel diagnostic and prognostic biomarkers and therapeutic targets.Please confirm if the author names are presented accurately and in the correct sequence (given name, middle name/initial, family name). Author 4 Given name: [Jeewan Ram] Last name [Vishnoi]. Also, kindly confirm the details in the metadata are correct.It is correct.

Receptor binding competition: A paradigm for regulating TGF-β family action

The transforming growth factor (TGF)-β family is a group of structurally related, multifunctional growth factors, or ligands that are crucially involved in the development, regulation, and maintenance of animal tissues. In humans, the family counts over 33 members. These secreted ligands typically form multimeric complexes with two type I and two type II receptors to activate one of two distinct signal transduction branches. A striking feature of the family is its promiscuity, i.e., many ligands bind the same receptors and compete with each other for binding to these receptors. Although several explanations for this feature have been considered, its functional significance has remained puzzling. However, several recent reports have promoted the idea that ligand-receptor binding promiscuity and competition are critical features of the TGF-β family that provide an essential regulating function. Namely, they allow a cell to read and process multi-ligand inputs. This capability may be necessary for producing subtle, distinctive, or adaptive responses and, possibly, for facilitating developmental plasticity. Here, we review the molecular basis for ligand competition, with emphasis on molecular structures and binding affinities. We give an overview of methods that were used to establish experimentally ligand competition. Finally, we discuss how the concept of ligand competition may be fundamentally tied to human physiology, disease, and therapy.

Histone methyltransferase and drug resistance in cancers

A number of novel anticancer drugs have been developed in recent years. However, the mortality of cancer patients remains high because of the emergence of drug resistance. It was reported that drug resistance might involved in changes in gene expression without changing genotypes, which is similar to epigenetic modification. Some studies indicated that targeting histone methyltransferase can reverse drug resistance. Hence, the use of histone methyltransferase inhibitors or histone demethylase inhibitors opens new therapeutic approaches for cancer treatment. While the relationship between histone methyltransferase and tumor resistance has been determined, there is a lack of updated review on the association between them. In this review, we summarized the mechanisms of histone methyltransferases in cancer drug resistance and the therapeutic strategies of targeting histone methyltransferase to reverse drug resistance.

Activins as Dual Specificity TGF-β Family Molecules: SMAD-Activation via Activin- and BMP-Type 1 Receptors

Activins belong to the transforming growth factor (TGF)-β family of multifunctional cytokines and signal via the activin receptors ALK4 or ALK7 to activate the SMAD2/3 pathway. In some cases, activins also signal via the bone morphogenetic protein (BMP) receptor ALK2, causing activation of the SMAD1/5/8 pathway. In this study, we aimed to dissect how activin A and activin B homodimers, and activin AB and AC heterodimers activate the two main SMAD branches. We compared the activin-induced signaling dynamics of ALK4/7-SMAD2/3 and ALK2-SMAD1/5 in a multiple myeloma cell line. Signaling via the ALK2-SMAD1/5 pathway exhibited greater differences between ligands than signaling via ALK4/ALK7-SMAD2/3. Interestingly, activin B and activin AB very potently activated SMAD1/5, resembling the activation commonly seen with BMPs. As SMAD1/5 was also activated by activins in other cell types, we propose that dual specificity is a general mechanism for activin ligands. In addition, we found that the antagonist follistatin inhibited signaling by all the tested activins, whereas the antagonist cerberus specifically inhibited activin B. Taken together, we propose that activins may be considered dual specificity TGF-β family members, critically affecting how activins may be considered and targeted clinically.

Loss of CDKN2B Promotes Fibrosis via Increased Fibroblast Differentiation Rather Than Proliferation

Idiopathic pulmonary fibrosis (IPF) is a devastating lung disease characterized by excessive scarring and fibroblast activation. We previously showed that fibroblasts from patients with IPF are hypermethylated at the CDKN2B gene locus, resulting in decreased CDKN2B expression. Here, we examine how diminished CDKN2B expression in normal and IPF fibroblasts affect fibroblast function, and how loss of CDKN2B contributes to IPF pathogenesis. We first confirmed that protein expression of CDKN2B was diminished in IPF lungs in situ. Loss of CDKN2B was especially notable in regions of increased myofibroblasts and fibroblastic foci. The degree of CDKN2B hypermethylation was particularly elevated in patients with radiographic honeycombing, a marker of more advanced fibrosis, and increased DNA methylation correlated with decreased expression. Although CDKN2B is traditionally considered a cell cycle inhibitor, loss of CDKN2B did not result in an increase in fibroblast proliferation, but instead was associated with an increase in myofibroblast differentiation. An increase in myofibroblast differentiation was not observed when CDKN2A was silenced. Loss of CDKN2B was associated with an increase in the transcription factors serum response factor and myocardin-related transcription factor A, and overexpression of CDKN2B in IPF fibroblasts inhibited myofibroblast differentiation. Finally, decreased CDKN2B expression was noted in fibroblasts from a murine model of fibrosis, and Cdkn2b^-/- mice developed greater histologic fibrosis after bleomycin injury. These findings identify a novel function for CDKN2B that differs from its conventional designation as a cell cycle inhibitor and demonstrate the importance of this protein in pulmonary fibrosis.

Palbociclib enhances activin-SMAD-induced cytostasis in estrogen receptor-positive breast cancer

Cyclin‐dependent kinase (CDK) 4 and CDK6 inhibitors are effective therapeutic options for hormone receptor (HR)‐positive, human epidermal growth factor receptor 2 (HER2)‐negative advanced breast cancer. Although CDK4/6 inhibitors mainly target the cyclin D‐CDK4/6‐retinoblastoma tumor suppressor protein (RB) axis, little is known about the clinical impact of inhibiting phosphorylation of other CDK4/6 target proteins. Here, we focused on other CDK4/6 targets, SMAD proteins. We showed that a CDK4/6 inhibitor palbociclib and activin‐SMAD2 signaling cooperatively inhibited cell cycle progression of a luminal‐type breast cancer cell line T47D. Palbociclib enhanced SMAD2 binding to the genome by inhibiting CDK4/6‐mediated linker phosphorylation of the SMAD2 protein. We also showed that cyclin G2 plays essential roles in SMAD2‐dependent cytostatic response. Moreover, comparison of the SMAD2 ChIP‐seq data of T47D cells with those of Hs578T (triple‐negative breast cancer cells) indicated that palbociclib augmented different SMAD2‐mediated functions based on cell type, and enhanced SMAD2 binding to the target regions on the genome without affecting its binding pattern. In summary, palbociclib enhances the cytostatic effects of the activin‐SMAD2 signaling pathway, whereas it possibly strengthens the tumor‐promoting aspect in aggressive breast cancer.

Targeting EZH2 in Multiple Myeloma-Multifaceted Anti-Tumor Activity

The enhancer of zeste homolog 2 (EZH2) is the enzymatic subunit of the polycomb repressive complex 2 (PRC2) that exerts important functions during normal development as well as disease. PRC2 through EZH2 tri-methylates histone H3 lysine tail residue 27 (H3K27me3), a modification associated with repression of gene expression programs related to stem cell self-renewal, cell cycle, cell differentiation, and cellular transformation. EZH2 is deregulated and subjected to gain of function or loss of function mutations, and hence functions as an oncogene or tumor suppressor gene in a context-dependent manner. The development of highly selective inhibitors against the histone methyltransferase activity of EZH2 has also contributed to insight into the role of EZH2 and PRC2 in tumorigenesis, and their potential as therapeutic targets in cancer. EZH2 can function as an oncogene in multiple myeloma (MM) by repressing tumor suppressor genes that control apoptosis, cell cycle control and adhesion properties. Taken together these findings have raised the possibility that EZH2 inhibitors could be a useful therapeutic modality in MM alone or in combination with other targeted agents in MM. Therefore, we review the current knowledge on the regulation of EZH2 and its biological impact in MM, the anti-myeloma activity of EZH2 inhibitors and their potential as a targeted therapy in MM.

BMPR2 inhibits activin and BMP signaling via wild-type ALK2

TGF-β/BMP superfamily ligands require heteromeric complexes of type 1 and 2 receptors for ligand-dependent downstream signaling. Activin A, a TGF-β superfamily member, inhibits growth of multiple myeloma cells, but the mechanism for this is unknown. We therefore aimed to clarify how activins affect myeloma cell survival. Activin A activates the transcription factors SMAD2/3 through the ALK4 type 1 receptor, but may also activate SMAD1/5/8 through mutated variants of the type 1 receptor ALK2 (also known as ACVR1). We demonstrate that activin A and B activate SMAD1/5/8 in myeloma cells through endogenous wild-type ALK2. Knockdown of the type 2 receptor BMPR2 strongly potentiated activin A- and activin B-induced activation of SMAD1/5/8 and subsequent cell death. Furthermore, activity of BMP6, BMP7 or BMP9, which may also signal via ALK2, was potentiated by knockdown of BMPR2. Similar results were seen in HepG2 liver carcinoma cells. We propose that BMPR2 inhibits ALK2-mediated signaling by preventing ALK2 from oligomerizing with the type 2 receptors ACVR2A and ACVR2B, which are necessary for activation of ALK2 by activins and several BMPs. In conclusion, BMPR2 could be explored as a possible target for therapy in patients with multiple myeloma.This article has an associated First Person interview with the first author of the paper.

Dysregulation of EZH2/miR-138 axis contributes to drug resistance in multiple myeloma by downregulating RBPMS

EZH2 is highly expressed in multiple myeloma (MM). However, the molecular mechanisms underlying EZH2 overexpression and its role in drug resistance of MM remain undefined. Here we show that EZH2 is upregulated in drug-resistant MM cells and its aberrant overexpression is associated with poor prognosis of MM patients. Overexpression of EZH2 in parental MM cells renders them resistant to anti-myeloma drugs and suppression of EZH2 displays the opposite effects. Using miRNA target scan algorithms, we identify miR-138 as a regulator of EZH2, which is conversely repressed by EZH2-induced H3K27 trimethylation in MM-resistant cell lines and primary tumor cells. Analysis of ChIP-seq dataset and H3K27me3 ChIP reveals that RBPMS is a direct and functionally relevant target of EZH2. RBPMS silencing confers resistance to MM cells and restoration of RBPMS by miR-138 overexpression re-sensitizes the resistant cells to drug. Importantly, in vivo delivery of miR-138 mimics or pharmacological inhibitor of EZH2 in combination with a proteasome inhibitor, bortezomib, induces significant regression of tumors in xenograft model. This study establishes EZH2/miR-138 axis as a potential therapeutic target for MM.

TGF-β contamination of purified recombinant GDF15

Purified recombinant proteins for use in biomedical research are invaluable to investigate protein function. However, purity varies in protein batches made in mammalian expression systems, such as CHO-cells or HEK293-cells. This study points to caution while investigating effects of proteins related to the transforming growth factor (TGF)-β superfamily. TGF-β itself is a very potent cytokine and has effects on cells in the femtomolar range. Thus, even very small amounts of contaminating TGF-β in purified protein batches may influence the experimental results given that receptors for TGF-β are present. When we attempted to characterize possible receptors for the TGF-β superfamily ligand GDF15, striking similarities between GDF15-induced activities and known TGF-β activities were found. However, differences between batches of GDF15 were a concern and finally led us to the conclusion that the measured effects were caused by TGF-β and not by GDF15. Our results emphasize that purified recombinant proteins must be used with caution and warrant proper controls. Notably, some conclusions made about GDF15 in already published papers may not be supported by the results shown. Awareness about this issue in the scientific community may prevent spreading of false positive results.

TGF-β induces growth suppression in multiple myeloma MM.1S cells via E2F1

Transforming growth factor-β (TGF-β) has an important role in multiple target genes and signaling pathways. The E2F family of transcription factors is a group of DNA-binding proteins that are involved in cell-cycle progression, and therefore have a key role in proliferation. The present study demonstrates that inhibition of cell growth by TGF-β occurs in the multiple myeloma cell line MM.1S. However, the growth-suppressive effects of TGF-β may be reversed by small interfering (si)RNA to reduce the expression of E2F1. TGF-β1 and E2F1 siRNA were manipulated in MM.1S cells to investigate the association between these genes. FACScan Flow Cytometer, western blot analysis and other methods were adopted to confirm such interrelation. The present data showed that TGF-β mediated growth suppression in MM.1S cells, while inducing E2F1 protein expression levels rapidly and transiently. The present data support the hypothesis that E2F1 is a central mediator of TGF-β-induced growth suppression in MM.1S cells and control of E2F1 may be a downstream event of TGF-β action, at least in one multiple myeloma cell line.

Phosphorylation of Smads by Intracellular Kinases

Smad proteins transduce the TGF-ß family signal at the cell surface into gene regulation in the nucleus. In addition to being phosphorylated by the TGF-ß family receptors, Smads are phosphorylated by a variety of intracellular kinases. The most studied are by cyclin-dependent kinases, the MAP kinase family members, and GSK-3. Phosphorylation by these kinases regulates Smad activities, leading to various biological effects. This chapter describes the methods for analyzing Smad phosphorylation by these kinases.

Brk/Protein tyrosine kinase 6 phosphorylates p27KIP1, regulating the activity of cyclin D-cyclin-dependent kinase 4

Cyclin D and cyclin-dependent kinase 4 (cdk4) are overexpressed in a variety of tumors, but their levels are not accurate indicators of oncogenic activity because an accessory factor such as p27(Kip1) is required to assemble this unstable dimer. Additionally, tyrosine (Y) phosphorylation of p27 (pY88) is required to activate cdk4, acting as an "on/off switch." We identified two SH3 recruitment domains within p27 that modulate pY88, thereby modulating cdk4 activity. Via an SH3-PXXP interaction screen, we identified Brk (breast tumor-related kinase) as a high-affinity p27 kinase. Modulation of Brk in breast cancer cells modulates pY88 and increases resistance to the cdk4 inhibitor PD 0332991. An alternatively spliced form of Brk (Alt Brk) which contains its SH3 domain blocks pY88 and acts as an endogenous cdk4 inhibitor, identifying a potentially targetable regulatory region within p27. Brk is overexpressed in 60% of breast carcinomas, suggesting that this facilitates cell cycle progression by modulating cdk4 through p27 Y phosphorylation. p27 has been considered a tumor suppressor, but our data strengthen the idea that it should also be considered an oncoprotein, responsible for cyclin D-cdk4 activity.

Bortezomib inhibits expression of TGF-β1, IL-10, and CXCR4, resulting in decreased survival and migration of cutaneous T cell lymphoma cells

Increased expression of the immunosuppressive cytokines, TGF-β1 and IL-10, is a hallmark of the advanced stages of cutaneous T cell lymphoma (CTCL), where it has been associated with suppressed immunity, increased susceptibility to infections, and diminished antitumor responses. Yet, little is known about the transcriptional regulation of TGF-β1 and IL-10 in CTCL, and about their function in regulating the CTCL cell responses. In this article, we show that TGF-β1 and IL-10 expression in CTCL cells is regulated by NF-κB and suppressed by bortezomib (BZ), which has shown promising results in the treatment of CTCL. However, although the TGF-β1 expression is IκBα dependent and is regulated by the canonical pathway, the IL-10 expression is IκBα independent, and its inhibition by BZ is associated with increased promoter recruitment of p52 that characterizes the noncanonical pathway. TGF-β1 suppression decreases CTCL cell viability and increases apoptosis, and adding exogenous TGF-β1 increases viability of BZ-treated CTCL cells, indicating TGF-β1 prosurvival function in CTCL cells. In addition, TGF-β1 suppression increases expression of the proinflammatory cytokines IL-8 and IL-17 in CTCL cells, suggesting that TGF-β1 also regulates the IL-8 and IL-17 expression. Importantly, our results demonstrate that BZ inhibits expression of the chemokine receptor CXCR4 in CTCL cells, resulting in their decreased migration, and that the CTCL cell migration is mediated by TGF-β1. These findings provide the first insights into the BZ-regulated TGF-β1 and IL-10 expression in CTCL cells, and indicate that TGF-β1 has a key role in regulating CTCL survival, inflammatory gene expression, and migration.

The role of bone morphogenetic proteins in myeloma cell survival

Multiple myeloma is characterized by slowly growing clones of malignant plasma cells in the bone marrow. The malignant state is frequently accompanied by osteolytic bone disease due to a disturbed balance between osteoblasts and osteoclasts. Bone morphogenetic proteins (BMPs) are present in the bone marrow and are important for several aspects of myeloma pathogenesis including growth and survival of tumor cells, bone homeostasis, and anemia. Among cancer cells, myeloma cells are particularly sensitive to growth inhibition and apoptosis induced by BMPs and therefore represent good models to study BMP receptor usage and signaling. Our review highlights and discusses the current knowledge on BMP signaling in myeloma.

Bone morphogenetic protein-9 suppresses growth of myeloma cells by signaling through ALK2 but is inhibited by endoglin

Multiple myeloma is a malignancy of plasma cells predominantly located in the bone marrow. A number of bone morphogenetic proteins (BMPs) induce apoptosis in myeloma cells in vitro, and with this study we add BMP-9 to the list. BMP-9 has been found in human serum at concentrations that inhibit cancer cell growth in vitro. We here show that the level of BMP-9 in serum was elevated in myeloma patients (median 176 pg/ml, range 8–809) compared with healthy controls (median 110 pg/ml, range 8–359). BMP-9 was also present in the bone marrow and was able to induce apoptosis in 4 out of 11 primary myeloma cell samples by signaling through ALK2. BMP-9-induced apoptosis in myeloma cells was associated with c-MYC downregulation. The effects of BMP-9 were counteracted by membrane-bound (CD105) or soluble endoglin present in the bone marrow microenvironment, suggesting a mechanism for how myeloma cells can evade the tumor suppressing activity of BMP-9 in multiple myeloma.

Oncogenic PAK4 regulates Smad2/3 axis involving gastric tumorigenesis

The alteration of p21-activated kinase 4 (PAK4) and transforming growth factor-beta (TGF-β) signaling effector Smad2/3 was detected in several types of tumors, which acts as oncogenic factor and tumor suppressor, but the relationship between these events has not been explored. Here, we demonstrate that PAK4 interacts with and modulates phosphorylation of Smad2/3 via both kinase-dependent and kinase-independent mechanisms, which attenuate Smad2/3 axis transactivation and TGF-β-mediated growth inhibition in gastric cancer cells. First, PAK4 interaction with Smad2/3, which is independent of PAK4 kinase activity, blocks TGF-β1-induced phosphorylation of Smad2 Ser465/467 or Smad3 Ser423/425 and the consequent activation. In addition, PAK4 phosphorylates Smad2 on Ser465, leading to the degradation of Smad2 through ubiquitin-proteasome-dependent pathway under hepatocyte growth factor (HGF) stimulation. Interestingly, PAK4 expression correlates negatively with phospho-Ser465/467 Smad2 but positively with phospho-Ser465 Smad2 in gastric cancer tissues. Furthermore, the expressions of HGF, phospho-Ser474 PAK4 and phospho-Ser465 Smad2 are markedly increased in gastric cancer tissues, and the expression of Smad2 is decreased in gastric cancer tissues. Our results document an oncogenic role of PAK4 in repression of Smad2/3 transactivation that involved in tumorigenesis, and suggest PAK4 as a potential therapeutic target for gastric cancer.

CDK inhibitors suppress Th17 and promote iTreg differentiation, and ameliorate experimental autoimmune encephalomyelitis in mice

Th17 cells, which have been implicated in autoimmune diseases, require IL-6 and TGF-β for early differentiation. Several Smad-independent pathways including the JNK and the RhoA-ROCK pathways have been implicated in the induction of RORγt, the master regulator of Th17, however, molecular mechanisms underlying Smad-independent pathway remain largely unknown. To identify novel pathways involved in Th17 differentiation, we screened 285 chemical inhibitors for known signaling pathways. Among them, we found that Kenpaullone, a GSK3-β and CDK inhibitor, efficiently suppressed TGF-β-mediated RORγt induction and enhanced Foxp3 induction in primary T cells. Another CDK inhibitor, Roscovitine, but not other GSK3-β inhibitors, suppressed Th17 differentiation and enhanced iTreg development. Kenpaullone and Roscovitine suppressed experimental autoimmune encephalomyelitis (EAE), a typical Th17-mediated autoimmune disease model. These two compounds enhanced STAT5 phosphorylation and restored IL-2 production in the presence of TGF-β. These data suggest that CDK inhibitors modulate TGF-β-signaling pathways, which restore TGF-β-mediated suppression of IL-2 production, thereby modifying the Th17/iTreg balance.

Constitutive Smad linker phosphorylation in melanoma: a mechanism of resistance to transforming growth factor-β-mediated growth inhibition

Melanoma cells are resistant to transforming growth factor-β (TGFβ)-induced cell-cycle arrest. In this study, we investigated a mechanism of resistance involving a regulatory domain, called linker region, in Smad2 and Smad3, main downstream effectors of TGFβ. Melanoma cells in culture and tumor samples exhibited constitutive Smad2 and Smad3 linker phosphorylation. Treatment of melanoma cells with the MEK1/2 inhibitor, U0126, or the two pan-CDK and GSK3 inhibitors, Flavopiridol and R547, resulted in decreased linker phosphorylation of Smad2 and Smad3. Overexpression of the linker phosphorylation-resistant Smad3 EPSM mutant in melanoma cells resulted in an increase in expression of p15(INK4B) and p21(WAF1) , as compared with cells transfected with wild-type (WT) Smad3. In addition, the cell numbers of EPSM Smad3-expressing melanoma cells were significantly reduced compared with WT Smad3-expressing cells. These results suggest that the linker phosphorylation of Smad3 contributes to the resistance of melanoma cells to TGFβ-mediated growth inhibition.

The role of cell cycle in retinal development: cyclin-dependent kinase inhibitors co-ordinate cell-cycle inhibition, cell-fate determination and differentiation in the developing retina

The mature retina is formed through multi-step developmental processes, including eye field specification, optic vesicle evagination, and cell-fate determination. Co-ordination of these developmental events with cell-proliferative activity is essential to achieve formation of proper retinal structure and function. In particular, the molecular and cellular dynamics of the final cell cycle significantly influence the identity that a cell acquires, since cell fate is largely determined at the final cell cycle for the production of postmitotic cells. This review summarizes our current understanding of the cellular mechanisms that underlie the co-ordination of cell-cycle and cell-fate determination, and also describes a molecular role of cyclin-dependent kinase inhibitors (CDKIs) as co-ordinators of cell-cycle arrest, cell-fate determination and differentiation.

Cyclin-dependent kinase 4-mediated phosphorylation inhibits Smad3 activity in cyclin D-overexpressing breast cancer cells

Smad3, a component of the transforming growth factor β signaling cascade, contributes to G(1) arrest in breast cancer cells. Cyclin D1/cyclin-dependent kinase 4 (CDK4) promotes G(1)-S-phase transition, and CDK phosphorylation of Smad3 has been associated with inhibition of Smad3 activity. We hypothesized that overexpression of cyclin D1 exerts tumorigenic effects in breast cancer cells through CDK4-mediated phosphorylation and inhibition of Smad3 and release of G(1) arrest. Real-time quantitative reverse transcription-PCR and immunoblotting were used to evaluate expression of study proteins in cyclin D1-overexpressing breast cancer cells. Smad3 transcriptional activity and cell cycle control were examined in cells transfected with wild-type (WT) Smad3 or Smad3 with single or multiple CDK phosphorylation site mutations (M) in the presence or absence of the CDK4 inhibitor or cotransfection with cdk4 small interfering RNA (siRNA). Transfection of the Smad3 5M construct resulted in decreased c-myc and higher p15(INK4B) expression. Compared with WT Smad3, overexpression of the Smad3 T8, T178, 4M, or 5M mutant constructs resulted in higher Smad3 transcriptional activity. Compared with cells transfected with WT Smad3, Smad3 transcriptional activity was higher in cells overexpressing Smad3 mutant constructs and treated with the CDK4 inhibitor or transfected with cdk4 siRNA. Cells transfected with Smad3 T8 or T178 and treated with the CDK4 inhibitor showed an increase in the G(1) cell population. Inhibition of CDK-mediated Smad3 phosphorylation released cyclin D1-regulated blockade of Smad3 transcriptional activity and recovered cell cycle arrest in breast cancer cells. Targeted inhibition of CDK4 activity may have a role in the treatment of cyclin D-overexpressing breast cancers.

Gene clustering analysis in human osseous remodeling

BACKGROUND

Tentative bioinformatic predictions were performed to comprehend the complexity of the gene interaction networks of the T lymphocyte cell cycle and of human periodontitis. This study aims to identify and rank genes involved in osseous augmentation or bone remodeling to obtain groups with more numerous predicted associations called the leader gene clusters.

METHODS

An iterative search (consisting of a consecutive expansion-filtering loop) was performed for which only genes involved in a specific process were identified. For each gene, predicted associations with all other involved genes were obtained from a Web-available database (Search Tool for the Retrieval of Interacting Genes/Proteins) and the weighted number of links (WNL), given by the sum of only high-confidence predicted associations (results with a score > or =0.9), allowing gene ranking. Genes belonging to higher clustering classes were identified.

RESULTS

A total of 161 genes potentially involved in bone-volume augmentation and 128 genes connected with the bone-remodeling phenomenon were identified. For the bone-volume augmentation process, only one gene belonged to the leader gene group, whereas six other genes were classified as cluster B genes; for the bone-remodeling phenomenon, three leader genes were identified, whereas six other genes formed the cluster B group. No one gene belonged to leader gene clusters of both processes, whereas one gene of each higher cluster group belonged to the immediately lower cluster of the opposite process. Only three genes of the higher clusters were experimentally involved in both analyses.

CONCLUSIONS

A de novo identification was performed based on the data mining of leader genes involved in bone-volume augmentation or bone remodeling to acquire primeval information about their molecular basis and to plan future ad hoc targeted experiments. For several genes of the upper clusters, an active role in the bone processes was already known, but the present analysis suggested that they play a major role in the analyzed phenomena. The role of the transcription factors as leader genes and the numerous orphan genes (genes with WNL = 0) recovered probably attest to a lack of information regarding these processes, which could be further clarified through specific DNA microarray experiments.