The Mad protein family links transcriptional repression to cell differentiation

Intestinal remodeling during Xenopus metamorphosis as a model for studying thyroid hormone signaling and adult organogenesis

Intestinal development takes places in two phases, the initial formation of neonatal (mammals)/larval (anurans) intestine and its subsequent maturation into the adult form. This maturation occurs during postembryonic development when plasma thyroid hormone (T3) level peaks. In anurans such as the highly related Xenopus laevis and Xenopus tropicalis, the larval/tadpole intestine is drastically remodeled from a simple tubular structure to a complex, multi-folded adult organ during T3-dependent metamorphosis. This involved complete degeneration of larval epithelium via programmed cell death and de novo formation of adult epithelium, with concurrent maturation of the muscles and connective tissue. Here, we will summarize our current understanding of the underlying molecular mechanisms, with a focus on more recent genetic and genome-wide studies.

The Myc-Like Mlx Network Impacts Aging and Metabolism

The "Mlx" and "Myc" Networks share many common gene targets. Just as Myc's activity depends upon its heterodimerization with Max, the Mlx Network requires that the Max-like factor Mlx associate with the Myc-like factors MondoA or ChREBP. We show here that body-wide Mlx inactivation, like that of Myc, accelerates numerous aging-related phenotypes pertaining to body habitus and metabolism. The deregulation of numerous aging-related Myc target gene sets is also accelerated. Among other functions, these gene sets often regulate ribosomal and mitochondrial structure and function, genomic stability and aging. Whereas "MycKO" mice have an extended lifespan because of a lower cancer incidence, "MlxKO" mice have normal lifespans and a somewhat higher cancer incidence. Like Myc, Mlx, MondoA and ChREBP expression and that of their target genes, deteriorate with age in both mice and humans, underscoring the importance of life-long and balanced cross-talk between the two Networks to maintain normal aging.

PSMA as a Theranostic Target in Hepatocellular Carcinoma: Immunohistochemistry and 68 Ga-PSMA-11 PET Using Cyclotron-Produced 68 Ga

Prostate-specific membrane antigen (PSMA) is a validated target for molecular diagnostics and targeted radionuclide therapy. Our purpose was to evaluate PSMA expression in hepatocellular carcinoma (HCC), cholangiocarcinoma (CCA), and hepatic adenoma (HCA); investigate the genetic pathways in HCC associated with PSMA expression; and evaluate HCC detection rate with 68 Ga-PSMA-11 positron emission tomography (PET). In phase 1, PSMA immunohistochemistry (IHC) on HCC (n = 148), CCA (n = 111), and HCA (n = 78) was scored. In a subset (n = 30), messenger RNA (mRNA) data from the Cancer Genome Atlas HCC RNA sequencing were correlated with PSMA expression. In phase 2, 68 Ga-PSMA-11 PET was prospectively performed in patients with treatment-naïve HCC on a digital PET scanner using cyclotron-produced 68 Ga. Uptake was graded qualitatively and semi-quantitatively using standard metrics. On IHC, PSMA expression was significantly higher in HCC compared with CCA and HCA (P < 0.0001); 91% of HCCs (n = 134) expressed PSMA, which principally localized to tumor-associated neovasculature. Higher tumor grade was associated with PSMA expression (P = 0.012) but there was no association with tumor size (P = 0.14), fibrosis (P = 0.35), cirrhosis (P = 0.74), hepatitis B virus (P = 0.31), or hepatitis C virus (P = 0.15). Overall survival tended to be longer in patients without versus with PSMA expression (median overall survival: 4.2 vs. 1.9 years; P = 0.273). FGF14 (fibroblast growth factor 14) mRNA expression correlated positively (rho = 0.70; P = 1.70 × 10-5 ) and MAD1L1 (Mitotic spindle assembly checkpoint protein MAD1) correlated negatively with PSMA expression (rho = -0.753; P = 1.58 × 10-6 ). Of the 190 patients who met the eligibility criteria, 31 patients with 39 HCC lesions completed PET; 64% (n = 25) lesions had pronounced 68 Ga-PSMA-11 standardized uptake value: SUVmax (median [range] 9.2 [4.9-28.4]), SUVmean 4.7 (2.4-12.7), and tumor-to-liver background ratio 2 (1.1-11). Conclusion: Ex vivo expression of PSMA in neovasculature of HCC translates to marked tumor avidity on 68 Ga-PSMA-11 PET, which suggests that PSMA has the potential as a theranostic target in patients with HCC.

Coordinated Cross-Talk Between the Myc and Mlx Networks in Liver Regeneration and Neoplasia

BACKGROUND & AIMS

The c-Myc (Myc) Basic helix-loop-helix leucine zipper (bHLH-ZIP) transcription factor is deregulated in most cancers. In association with Max, Myc controls target genes that supervise metabolism, ribosome biogenesis, translation, and proliferation. This Myc network crosstalks with the Mlx network, which consists of the Myc-like proteins MondoA and ChREBP, and Max-like Mlx. Together, this extended Myc network regulates both common and distinct gene targets. Here, we studied the consequence of Myc and/or Mlx ablation in the liver, particularly those pertaining to hepatocyte proliferation, metabolism, and spontaneous tumorigenesis.

METHODS

We examined the ability of hepatocytes lacking Mlx (MlxKO) or Myc+Mlx (double KO [DKO]) to repopulate the liver over an extended period of time in a murine model of type I tyrosinemia. We also compared this and other relevant behaviors, phenotypes, and transcriptomes of the livers with those from previously characterized MycKO, ChrebpKO, and MycKO × ChrebpKO mice.

RESULTS

Hepatocyte regenerative potential deteriorated as the Extended Myc Network was progressively dismantled. Genes and pathways dysregulated in MlxKO and DKO hepatocytes included those pertaining to translation, mitochondrial function, and hepatic steatosis resembling nonalcoholic fatty liver disease. The Myc and Mlx Networks were shown to crosstalk, with the latter playing a disproportionate role in target gene regulation. All cohorts also developed steatosis and molecular evidence of early steatohepatitis. Finally, MlxKO and DKO mice showed extensive hepatic adenomatosis.

CONCLUSIONS

In addition to showing cooperation between the Myc and Mlx Networks, this study showed the latter to be more important in maintaining proliferative, metabolic, and translational homeostasis, while concurrently serving as a suppressor of benign tumorigenesis. GEO accession numbers: GSE181371, GSE130178, and GSE114634.

The development of adult intestinal stem cells: Insights from studies on thyroid hormone-dependent anuran metamorphosis

Vertebrates organ development often takes place in two phases: initial formation and subsequent maturation into the adult form. This is exemplified by the intestine. In mouse, the intestine at birth has villus, where most differentiated epithelial cells are located, but lacks any crypts, where adult intestinal stem cells reside. The crypt is formed during the first 3 weeks after birth when plasma thyroid hormone (T3) levels are high. Similarly, in anurans, the intestine undergoes drastic remodeling into the adult form during metamorphosis in a process completely dependent on T3. Studies on Xenopus metamorphosis have revealed important clues on the formation of the adult intestine during metamorphosis. Here we will review our current understanding on how T3 induces the degeneration of larval epithelium and de novo formation of adult intestinal stem cells. We will also discuss the mechanistic conservations in intestinal development between anurans and mammals.

Infant High-Grade Gliomas Comprise Multiple Subgroups Characterized by Novel Targetable Gene Fusions and Favorable Outcomes

Infant high-grade gliomas appear clinically distinct from their counterparts in older children, indicating that histopathologic grading may not accurately reflect the biology of these tumors. We have collected 241 cases under 4 years of age, and carried out histologic review, methylation profiling, and custom panel, genome, or exome sequencing. After excluding tumors representing other established entities or subgroups, we identified 130 cases to be part of an "intrinsic" spectrum of disease specific to the infant population. These included those with targetable MAPK alterations, and a large proportion of remaining cases harboring gene fusions targeting ALK (n = 31), NTRK1/2/3 (n = 21), ROS1 (n = 9), and MET (n = 4) as their driving alterations, with evidence of efficacy of targeted agents in the clinic. These data strongly support the concept that infant gliomas require a change in diagnostic practice and management. SIGNIFICANCE: Infant high-grade gliomas in the cerebral hemispheres comprise novel subgroups, with a prevalence of ALK, NTRK1/2/3, ROS1, or MET gene fusions. Kinase fusion-positive tumors have better outcome and respond to targeted therapy clinically. Other subgroups have poor outcome, with fusion-negative cases possibly representing an epigenetically driven pluripotent stem cell phenotype.See related commentary by Szulzewsky and Cimino, p. 904.This article is highlighted in the In This Issue feature, p. 890.

The balance of two opposing factors Mad and Myc regulates cell fate during tissue remodeling

Cell proliferation and differentiation are two distinct yet coupled processes in development in diverse organisms. Understanding the molecular mechanisms that regulate this process is a central theme in developmental biology. The intestinal epithelium is a highly complex tissue that relies on the coordination of cell proliferation within the crypts and apoptosis mainly at the tip of the villi, preservation of epithelial function through differentiation, and homeostatic cell migration along the crypt-villus axis. Small populations of adult stem cells are responsible for the self-renewal of the epithelium throughout life. Surprisingly, much less is known about the mechanisms governing the remodeling of the intestine from the embryonic to adult form. Furthermore, it remains unknown how thyroid hormone (T3) affects stem cell development during this postembryonic process, which is around birth in mammals when T3 level increase rapidly in the plasma. Tissue remodeling during amphibian metamorphosis is very similar to the maturation of the mammalian organs around birth in mammals and is regulated by T3. In particular, many unique features of Xenopus intestinal remodeling during metamorphosis has enabled us and others to elucidate how adult stem cells are formed during postembryonic development in vertebrates. In this review, we will focus on recent findings on the role of Mad1/c-Myc in cell death and proliferation during intestinal metamorphosis and discuss how a Mad1-c-Myc balance controls intestinal epithelial cell fate during this T3-dependent process.

A balance of Mad and Myc expression dictates larval cell apoptosis and adult stem cell development during Xenopus intestinal metamorphosis

The Myc/Mad/Max network has long been shown to be an important factor in regulating cell proliferation, death and differentiation in diverse cell types. In general, Myc–Max heterodimers activate target gene expression to promote cell proliferation, although excess of c-Myc can also induce apoptosis. In contrast, Mad competes against Myc to form Mad–Max heterodimers that bind to the same target genes to repress their expression and promote differentiation. The role of the Myc/Mad/Max network during vertebrate development, especially, the so-called postembryonic development, a period around birth in mammals, is unclear. Using thyroid hormone (T3)-dependent Xenopus metamorphosis as a model, we show here that Mad1 is induced by T3 in the intestine during metamorphosis when larval epithelial cell death and adult epithelial stem cell development take place. More importantly, we demonstrate that Mad1 is expressed in the larval cells undergoing apoptosis, whereas c-Myc is expressed in the proliferating adult stem cells during intestinal metamorphosis, suggesting that Mad1 may have a role in cell death during development. By using transcription activator-like effector nuclease-mediated gene-editing technology, we have generated Mad1 knockout Xenopus animals. This has revealed that Mad1 is not essential for embryogenesis or metamorphosis. On the other hand, consistent with its spatiotemporal expression profile, Mad1 knockout leads to reduced larval epithelial apoptosis but surprisingly also results in increased adult stem cell proliferation. These findings not only reveal a novel role of Mad1 in regulating developmental cell death but also suggest that a balance of Mad and Myc controls cell fate determination during adult organ development.

CXCL10 expression induced by Mxi1 inactivation induces mesangial cell apoptosis in mouse Habu nephritis

MAX interactor 1 (Mxi1) proteins are c-myc antagonists that primarily exert their biological functions by inhibiting Myc-dependent gene transcription. In this study, Mxi1(-/-) mice were used to generate a model of mesangial proliferative glomerulonephritis for the first time. In the present study, we demonstrated that Mxi1(-/-) mice exhibited a more typical and severe pathological phenotype, which was displayed primarily as a noticeable dissolution phenotype with a higher proportion of apoptotic cells and higher chemokine CXCL10 expression during the early days of modeling, compared with wild-type mice. Additionally, we determined that IRF3-mediated TLR4 signaling was likely involved in regulating CXCL10 expression, which might participate in the mesangial dissolution process. We also found increases in CXCL10 expression, caspase 3 activation, and the proportion of apoptotic cells when Mxi1 expression was inhibited in mouse mesangial cells. Furthermore, the proportion of apoptotic cells decreased after inhibiting CXCL10 expression. Therefore, we concluded that the mesangial cell apoptosis observed in this mesangial proliferative glomerulonephritis model was related to CXCL10 expression induced by Mxi1 inactivation. This finding provides a new theoretical basis for the mechanism of mesangial proliferative glomerulonephritis progression and reveals potential intervention targets for the early treatment of this disease.

Taming of the beast: shaping Myc-dependent amplification

Myc deregulation is a hallmark oncogenic event where overexpression of the transcription factor gives rise to numerous tumorigenic phenotypes. The complex consequences of Myc deregulation have prevented clear mechanistic interpretations of its function. A synthesis of recent experimental observations offers a consensus on the direct transcriptional function of Myc: when overexpressed, Myc broadly engages the established euchromatic cis-regulatory landscape of the cell, where the factor generally amplifies transcription. The level of Myc binding at target genes and the transcriptional output are differentially modulated by additional regulators, including Miz1. Targeting Myc oncogenic activity will require an understanding of whether amplification promotes tumorigenesis and the consequences of amplification in tumors adapted to oncogenic Myc.

The transcription factor FOXM1 (Forkhead box M1): proliferation-specific expression, transcription factor function, target genes, mouse models, and normal biological roles

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor, which stimulates cell proliferation and exhibits a proliferation-specific expression pattern. Accordingly, both the expression and the transcriptional activity of FOXM1 are increased by proliferation signals, but decreased by antiproliferation signals, including the positive and negative regulation by protooncoproteins or tumor suppressors, respectively. FOXM1 stimulates cell cycle progression by promoting the entry into S-phase and M-phase. Moreover, FOXM1 is required for proper execution of mitosis. Accordingly, FOXM1 regulates the expression of genes, whose products control G1/S-transition, S-phase progression, G2/M-transition, and M-phase progression. Additionally, FOXM1 target genes encode proteins with functions in the execution of DNA replication and mitosis. FOXM1 is a transcriptional activator with a forkhead domain as DNA binding domain and with a very strong acidic transactivation domain. However, wild-type FOXM1 is (almost) inactive because the transactivation domain is repressed by three inhibitory domains. Inactive FOXM1 can be converted into a very potent transactivator by activating signals, which release the transactivation domain from its inhibition by the inhibitory domains. FOXM1 is essential for embryonic development and the foxm1 knockout is embryonically lethal. In adults, FOXM1 is important for tissue repair after injury. FOXM1 prevents premature senescence and interferes with contact inhibition. FOXM1 plays a role for maintenance of stem cell pluripotency and for self-renewal capacity of stem cells. The functions of FOXM1 in prevention of polyploidy and aneuploidy and in homologous recombination repair of DNA-double-strand breaks suggest an importance of FOXM1 for the maintenance of genomic stability and chromosomal integrity.

Crystal structure of the minimalist Max-E47 protein chimera

Max-E47 is a protein chimera generated from the fusion of the DNA-binding basic region of Max and the dimerization region of E47, both members of the basic region/helix-loop-helix (bHLH) superfamily of transcription factors. Like native Max, Max-E47 binds with high affinity and specificity to the E-box site, 5'-CACGTG, both in vivo and in vitro. We have determined the crystal structure of Max-E47 at 1.7 Å resolution, and found that it associates to form a well-structured dimer even in the absence of its cognate DNA. Analytical ultracentrifugation confirms that Max-E47 is dimeric even at low micromolar concentrations, indicating that the Max-E47 dimer is stable in the absence of DNA. Circular dichroism analysis demonstrates that both non-specific DNA and the E-box site induce similar levels of helical secondary structure in Max-E47. These results suggest that Max-E47 may bind to the E-box following the two-step mechanism proposed for other bHLH proteins. In this mechanism, a rapid step where protein binds to DNA without sequence specificity is followed by a slow step where specific protein:DNA interactions are fine-tuned, leading to sequence-specific recognition. Collectively, these results show that the designed Max-E47 protein chimera behaves both structurally and functionally like its native counterparts.

Solution structure of the mSin3A PAH2-Pf1 SID1 complex: a Mad1/Mxd1-like interaction disrupted by MRG15 in the Rpd3S/Sin3S complex

Histone deacetylation constitutes an important mechanism for silencing genes. The histone-deacetylase-associated mammalian Rpd3S/Sin3S corepressor complex plays key roles in repressing aberrant gene transcription from cryptic transcription initiation sites and in mitigating RNA polymerase II progression in intragenic regions of actively transcribed genes. The Sin3 corepressor functions as a molecular adaptor linking histone deacetylases on the one hand, with the chromatin targeting subunits Pf1 and MRG15 on the other. Pf1 also functions as an adaptor by interacting with MRG15 and engaging in multivalent interactions with Sin3 targeting among other domains the two N-terminal paired amphipathic helix (PAH) domains that serve as sites of interaction with sequence-specific DNA-binding transcription factors. Here, we structurally and functionally evaluate the interaction between the PAH2 domain of mSin3A and the Sin3 interaction domain 1 (SID1) motif of Pf1 and find the structural aspects to be reminiscent of the interaction between the Mad1/Mxd1 transcription factor and Sin3. Pf1 residues within a highly conserved sequence motif immediately C-terminal to SID1 appear not to be important for the interaction with Sin3 PAH2. Unexpectedly, the MRG15 subunit competes, rather than collaborates, with Sin3 for the Pf1 segment encompassing the two conserved motifs, implying competition between two subunits for another subunit of the same chromatin-modifying complex.

Global regulation of nucleotide biosynthetic genes by c-Myc

Background

The c-Myc transcription factor is a master regulator and integrates cell proliferation, cell growth and metabolism through activating thousands of target genes. Our identification of direct c-Myc target genes by chromatin immunoprecipitation (ChIP) coupled with pair-end ditag sequencing analysis (ChIP-PET) revealed that nucleotide metabolic genes are enriched among c-Myc targets, but the role of Myc in regulating nucleotide metabolic genes has not been comprehensively delineated.

Methodology/Principal Findings

Here, we report that the majority of genes in human purine and pyrimidine biosynthesis pathway were induced and directly bound by c-Myc in the P493-6 human Burkitt's lymphoma model cell line. The majority of these genes were also responsive to the ligand-activated Myc-estrogen receptor fusion protein, Myc-ER, in a Myc null rat fibroblast cell line, HO.15 MYC-ER. Furthermore, these targets are also responsive to Myc activation in transgenic mouse livers in vivo. To determine the functional significance of c-Myc regulation of nucleotide metabolism, we sought to determine the effect of loss of function of direct Myc targets inosine monophosphate dehydrogenases (IMPDH1 and IMPDH2) on c-Myc-induced cell growth and proliferation. In this regard, we used a specific IMPDH inhibitor mycophenolic acid (MPA) and found that MPA dramatically inhibits c-Myc-induced P493-6 cell proliferation through S-phase arrest and apoptosis.

Conclusions/Significance

Taken together, these results demonstrate the direct induction of nucleotide metabolic genes by c-Myc in multiple systems. Our finding of an S-phase arrest in cells with diminished IMPDH activity suggests that nucleotide pool balance is essential for c-Myc's orchestration of DNA replication, such that uncoupling of these two processes create DNA replication stress and apoptosis.

Solution NMR studies of apo-mSin3A and -mSin3B reveal that the PAH1 and PAH2 domains are structurally independent

The evolutionarily conserved mammalian Sin3 (mSin3) transcriptional corepressor interacts with a diverse array of transcription factors mainly through two PAH (paired amphipathic helix) domains located near the N terminus. Previous studies suggested the possibility of interdomain interactions involving the PAH domains. Here, we show that the domains are structurally independent and the properties of the individual domains, such as the conformational heterogeneity and the ability of mSin3A PAH2 to homodimerize, are preserved in constructs that span both PAH domains. Our results thus suggest that the N-terminal segments of the Sin3 proteins are broadly available for interactions with other proteins and that the PAH domains are organized into structurally independent modules. Our data also rule out any heterotypic association between the paralogous mSin3A and mSin3B proteins via interactions involving the mSin3A PAH2 domain.

The interplay between MYC and HIF in the Warburg effect

c-MYC and the hypoxia-inducible factors (HIFs) are critical factors for tumorigenesis in a large number of human cancers. While the normal function of MYC involves the induction of cell proliferation and enhancement of cellular metabolism, the function of HIF, particularly HIF-1, involves adaptation to the hypoxic microenvironment, including activation of anaerobic glycolysis. When MYC-dependent tumors grow, the hypoxic tumor microenvironment elevates the levels of HIF, such that oncogenic MYC and HIF collaborate to enhance the cancer cell's metabolic needs through increased uptake of glucose and its conversion to lactate. HIF is also able to attenuate mitochondrial respiration through the induction of pyruvate dehydrogenase kinase 1 (PDK1), which in part accounts for the Warburg effect that describes the propensity for cancers to avidly take up glucose and convert it to lactate with the concurrent decrease in mitochondrial respiration. Target genes that are common to both HIF and MYC, such as PDK1, LDHA, HK2, and TFRC, are therefore attractive therapeutic targets, because their coordinate induction by HIF and MYC widens the therapeutic window between cancer and normal tissues.

Interaction of hepatitis B viral oncoprotein with cellular target HBXIP dysregulates centrosome dynamics and mitotic spindle formation

Hepatitis B virus infection is associated with hepatocellular carcinoma, claiming 1 million lives annually worldwide. To understand the carcinogenic mechanism of hepatitis B virus-encoded oncoprotein HBx, we explored the function of HBx interaction with its cellular target HBXIP. Previously, we demonstrated that viral HBx and cellular HBXIP control mitotic spindle formation, regulating centrosome splitting. By using various fragments of HBx, we determined that residues (137)CRHK(140) within HBx are necessary for binding HBXIP. Mutation of the (137)CRHK(140) motif in HBx abolished its ability to bind HBXIP and to dysregulate centrosome dynamics in HeLa and immortal diploid RPE-1 cells. Unlike wild-type HBx, which targets to centrosomes as determined by subcellular fractionation and immunofluorescence microscopy, HBx mutants failed to localize to centrosomes. Overexpression of viral HBx wild-type protein and knockdown of endogenous HBXIP altered centrosome assembly and induced modifications of pericentrin and centrin-2, two essential proteins required for centrosome formation and function, whereas HBXIP nonbinding mutants of HBx did not. Overexpression of HBXIP or fragments of HBXIP that bind HBx neutralized the effects of viral HBx on centrosome dynamics and spindle formation. These results suggest that HBXIP is a critical target of viral HBx for promoting genetic instability through formation of defective spindles and subsequent aberrant chromosome segregation.

A novel role of the Mad family member Mad3 in cerebellar granule neuron precursor proliferation

During development, Sonic hedgehog (Shh) regulates the proliferation of cerebellar granule neuron precursors (GNPs) in part via expression of Nmyc. We present evidence supporting a novel role for the Mad family member Mad3 in the Shh pathway to regulate Nmyc expression and GNP proliferation. Mad3 mRNA is transiently expressed in GNPs during proliferation. Cultured GNPs express Mad3 in response to Shh stimulation in a cyclopamine-dependent manner. Mad3 is necessary for Shh-dependent GNP proliferation as measured by bromodeoxyuridine incorporation and Nmyc expression. Furthermore, Mad3 overexpression, but not that of other Mad proteins, is sufficient to induce GNP proliferation in the absence of Shh. Structure-function analysis revealed that Max dimerization and recruitment of the mSin3 corepressor are required for Mad3-mediated GNP proliferation. Surprisingly, basic-domain-dependent DNA binding of Mad3 is not required, suggesting that Mad3 interacts with other DNA binding proteins to repress transcription. Interestingly, cerebellar tumors and pretumor cells derived from patched heterozygous mice express high levels of Mad3 compared with adjacent normal cerebellar tissue. Our studies support a novel role for Mad3 in cerebellar GNP proliferation and possibly tumorigenesis, and they challenge the current paradigm that Mad3 should antagonize Nmyc by competition for direct DNA binding via Max dimerization.

Structural aspects of interactions within the Myc/Max/Mad network

Recently determined structures of a number of Myc family proteins have provided significant insights into the molecular nature of complex assembly and DNA binding. These structures illuminate the details of specific interactions that govern the assembly of nucleoprotein complexes and, in doing so, raise more questions regarding Myc biology. In this review, we focus on the lessons provided by these structures toward understanding (1) interactions that govern transcriptional repression by Mad via the Sin3 pathway, (2) homodimerization of Max, (3) heterodimerization of Myc-Max and Mad-Max, and (4) DNA recognition by each of the Max-Max, Myc-Max, and Mad-Max dimers.

Thermodynamics of protein-protein interactions of cMyc, Max, and Mad: effect of polyions on protein dimerization

The Myc-Max-Mad network of proteins activates or represses gene transcription depending on whether the dimerization partner of Max is c-Myc or Mad. To elucidate the physical properties of these protein-protein interactions, fluorescence anisotropy of TRITC-labeled Max was used. The binding affinities and thermodynamics of dimerization of the Max-Max homodimer and c-Myc-Max and Mad-Max heterodimers were determined. Our results indicate that c-Myc and Max form the most stable heterodimer. Previous work [Kohler, J. J., Metallo, S. J., Schneider, T. L., and Schepartz, A. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 11735-9] has shown that instead of dimerizing first and then binding to DNA, these proteins use a monomer pathway in which a monomer binds to DNA followed by dimerization on the surface of the DNA. The DNA E-box affects the dimerization, but nonspecific effects may also play a role. The influence of polyions, poly-L-lysine and poly-L-glutamic acid, were investigated to determine the effects of charged polymers other than DNA on homodimerization and heterodimerization. While the positively charged poly-L-lysine, PLL, did not show any significant effect, negatively charged poly-L-glutamic acid, PLG, stabilized both heterodimers and homodimers by 2-3 kJ/mol. These data suggest that in the cell nucleus the presence of negatively charged DNA or RNA could nonspecifically aid in association of these proteins. Calculations of DeltaH degrees and DeltaS degrees from the temperature dependence of K(d) indicated that although the thermodynamic parameters for the dimer are different, the reactions for all three dimers are driven by negative (favorable) enthalpic and negative (unfavorable) entropic contributions. In the presence of PLG, entropy became more negative with the effect being largest for c-Myc-Max heterodimers. This suggests that van der Waals and H-bonding interactions are predominant in dimerization of these proteins.

The transcriptional repressor dMnt is a regulator of growth in Drosophila melanogaster

The Myc-Max-Mad/Mnt network of transcription factors has been implicated in oncogenesis and the regulation of proliferation in vertebrate cells. The identification of Myc and Max homologs in Drosophila melanogaster has demonstrated a critical role for dMyc in cell growth control. In this report, we identify and characterize the third member of this network, dMnt, the sole fly homolog of the mammalian Mnt and Mad family of transcriptional repressors. dMnt possesses two regions characteristic of Mad and Mnt proteins: a basic helix-loop-helix-zipper domain, through which it dimerizes with dMax to form a sequence-specific DNA binding complex, and a Sin-interacting domain, which mediates interaction with the dSin3 corepressor. Using the upstream activation sequence/GAL4 system, we show that expression of dMnt results in an inhibition of cellular growth and proliferation. Furthermore, we have generated a dMnt null allele, which results in flies with larger cells, increased weight, and decreased life span compared to wild-type flies. Our results demonstrate that dMnt is a transcriptional repressor that regulates D. melanogaster body size.

Id2 Is Dispensable for Myc-Induced Lymphomagenesis

The Emu-Myc transgenic mouse appears to be an accurate model of human Burkitt's lymphoma that bears MYC/Immunoglobulin gene translocations. Id2, a negative regulator of basic helix-loop-helix transcription factors, has also been proposed as a Myc target gene that drives the proliferative response of Myc by binding to and overriding the checkpoint functions of the retinoblastoma tumor suppressor protein. Targeted deletion of Id2 in mice results in defects in B-cell development and prevents the development of peripheral lymphoid nodes. In precancerous B cells and lymphomas that arise in Emu-Myc transgenic mice and in Burkitt's lymphomas, Id2 is overexpressed, suggesting that it plays a regulatory role in lymphoma development. Surprisingly, despite these connections, Emu-Myc mice lacking Id2 succumb to lethal B-cell lymphoma at rates comparable with wild-type Emu-Myc transgenics. Furthermore, precancerous splenic B cells lacking Id2 do not exhibit any significant defects in Myc-induced target gene transactivation and proliferation. However, due to their lack of secondary lymph nodes, Emu-Myc mice lacking Id2 rather succumb to disseminated lymphoma with an associated leukemia, with pronounced infiltrates of the bone marrow and other major organs. Collectively these findings argue that targeting Id2 functions may be ineffective in preventing Myc-associated malignancies.

c-Myc as a therapeutic target in cancer

c-Myc is frequently deregulated in human cancers. The c-Myc oncoprotein is a transcription factor, with many of its target genes encoding proteins that initiate and maintain the transformed state. c-Myc is also part of a dynamic network whose members interact selectively with one another and with various transcriptional coregulators and histone-modifying enzymes. This knowledge highlights several points that might be amenable to attack. This review summarizes progress in controlling the extent of c-Myc transcription, translation, interaction with other myc network members, DNA binding and transcriptional activation. Inhibition of c-Myc can be achieved with many of these approaches; however, clinical efficacy will likely require intervention at several levels, perhaps in combination with traditional chemotherapeutic drugs or agents that target other oncoproteins.

Functional analysis of the Mad1-mSin3A repressor-corepressor interaction reveals determinants of specificity, affinity, and transcriptional response

The recruitment of corepressors by DNA-bound repressors is likely to be a critical rate-limiting step in the transcriptional regulation of many genes. An excellent paradigm for such an interaction is the association of the basic helix-loop-helix zipper protein Mad1 with the corepressor mSin3A. When bound together, the Sin3 interaction domain (SID) of Mad1 forms extensive hydrophobic contacts with the four-helix bundle formed by the paired amphipathic helix 2 (PAH2) domain of mSin3A. Using the costructure to predict the principle residues required for binding, we have carried out an extensive mutational analysis to examine the Mad1 SID-mSin3A PAH2 interaction in vitro and in vivo. Bulky hydrophobic residues in the alpha1 (I308 and V311) and alpha2 (L329 and L332) helices of the PAH2 domain are necessary to accommodate the precise arrangement of bulky (L12) and short (A15 and A16) hydrophobic residues in the amphipathic Mad1 SID. We have also used phage display to derive an optimal SID, which shows an essentially identical arrangement of key residues. By manipulating these key residues, we have generated altered-specificity Mad1 SID mutants that bind only to a PAH2 domain with a reciprocal mutation, permitting us to demonstrate for the first time that these domains interact directly in vivo. We have also found that the integrity of the PAH1 domain affects the Mad1 SID-PAH2 interaction. It is conceivable that cross talk between different PAH domains and their binding partners helps to determine the subunit composition and order of assembly of mSin3A complexes.

Identification of target genes of the p16INK4A-pRB-E2F pathway

Deregulation of the retinoblastoma protein (pRB) pathway is a hallmark of human cancer. The core members of this pathway include the tumor suppressor protein, pRB, which through binding to a number of cellular proteins, most notably members of the E2F transcription factor family, regulates progression through the cell division cycle. With the aim of identifying transcriptional changes provoked by deregulation of the pRB pathway, we have used cell lines that conditionally express a constitutively active phosphorylation site mutant of pRB (pRBDeltaCDK) or p16INK4A (p16). The expression of pRBDeltaCDK and p16 resulted in significant repression and activation of a large number of genes as measured by high density oligonucleotide array analysis. Transcriptional changes were found in genes that are essential for DNA replication and cell proliferation. In agreement with previous results, we found a high degree of overlap between genes regulated by p16 and pRB. Data we have obtained previously for E2F family members showed that 74 of the genes repressed by pRB and p16 were induced by the E2Fs and 23 genes that were induced by pRB and p16 were repressed by the E2Fs. Thus, we have identified 97 genes as physiological targets of the pRB pathway, and the further characterization of these genes should provide insights into how this pathway controls proliferation. We show that Gibbs sampling detects enrichment of several sequence motifs, including E2F consensus binding sites, in the upstream regions of these genes and use this enrichment in an in silico filtering process to refine microarray derived gene lists.

Identification of the molecular requirements for an RAR alpha-mediated cell cycle arrest during granulocytic differentiation

Retinoids are potent inducers of cell cycle arrest and differentiation of numerous cell types, notably granulocytes. However the mechanisms by which retinoids mediate cell cycle arrest during differentiation remain unclear. We have used myeloid differentiation to characterize the molecular pathways that couple cell cycle withdrawal to terminal differentiation. Using primary cells from mice deficient for either the cyclin-dependent kinase inhibitor (CDKi) p27(Kip1), the Myc antagonist Mad1, or both Mad1 and p27(Kip1), we observed that signals mediated through retinoic acid receptor alpha (RAR alpha), but not RAR beta or gamma, required both Mad1 and p27(Kip1) to induce cell cycle arrest and to accelerate terminal differentiation of granulocytes. Although RAR alpha did not directly regulate Mad1 or p27(Kip1), the RAR alpha target gene C/EBP epsilon directly regulated transcription of Mad1. Induction of C/EBP epsilon activity in granulocytic cells led to rapid induction of Mad1 protein and transcript, with direct binding of C/EBP epsilon to the Mad1 promoter demonstrated through chromatin immunoprecipitation assay. These data demonstrate that cell cycle arrest in response to RAR alpha specifically requires Mad1 and p27(Kip1) and that Mad1 is transcriptionally activated by CCAAT/enhancer-binding protein epsilon (C/EBP epsilon). Moreover, these data demonstrate selectivity among the RARs for cell cycle arrest pathways and provide a direct mechanism to link differentiation induction and regulation of the Myc antagonist Mad1.

TGF beta1 induces multiple independent signals to regulate human trophoblastic differentiation: mechanistic insights

Transforming growth factor-beta 1 (TGF beta1) plays a crucial role in controlling trophoblast growth and invasion. Loss of this key regulatory function provides the pathophysiological basis for several tumors, which are characterized by uncontrolled telomerase activity. We have shown earlier that telomerase activity is negatively regulated during terminal differentiation of human trophoblasts, and that TGF beta1 may be an important factor governing the transcription of human telomerase reverse transcriptase (hTERT) (the catalytic subunit of the telomerase complex) during this process. In the present study, we extend these observations to identify possible functional effectors of TGF beta1-induced loss in telomerase activity during human trophoblastic differentiation. We show that this regulation may involve the suppression of c-Myc and an increased production of Mad1. We also observed a simultaneous increase in the expression of cyclin-dependent-kinase inhibitors, p21, p27, p15 and p16, associated with a loss in expression of Cyclin-A2 and Cyclin-E. Thus, TGF beta1 may induce multiple independent signals to check the proliferative potential of human trophoblastic cells and allow their functional differentiation.

X-ray structures of Myc-Max and Mad-Max recognizing DNA. Molecular bases of regulation by proto-oncogenic transcription factors

X-ray structures of the basic/helix-loop-helix/leucine zipper (bHLHZ) domains of Myc-Max and Mad-Max heterodimers bound to their common DNA target (Enhancer or E box hexanucleotide, 5'-CACGTG-3') have been determined at 1.9 A and 2.0 A resolution, respectively. E box recognition by these two structurally similar transcription factor pairs determines whether a cell will divide and proliferate (Myc-Max) or differentiate and become quiescent (Mad-Max). Deregulation of Myc has been implicated in the development of many human cancers, including Burkitt's lymphoma, neuroblastomas, and small cell lung cancers. Both quasisymmetric heterodimers resemble the symmetric Max homodimer, albeit with marked structural differences in the coiled-coil leucine zipper regions that explain preferential homo- and heteromeric dimerization of these three evolutionarily related DNA-binding proteins. The Myc-Max heterodimer, but not its Mad-Max counterpart, dimerizes to form a bivalent heterotetramer, which explains how Myc can upregulate expression of genes with promoters bearing widely separated E boxes.

MAD1 and p27(KIP1) cooperate to promote terminal differentiation of granulocytes and to inhibit Myc expression and cyclin E-CDK2 activity

To understand how cellular differentiation is coupled to withdrawal from the cell cycle, we have focused on two negative regulators of the cell cycle, the MYC antagonist MAD1 and the cyclin-dependent kinase inhibitor p27(KIP1). Generation of Mad1/p27(KIP1) double-null mice revealed a number of synthetic effects between the null alleles of Mad1 and p27(KIP1), including embryonic lethality, increased proliferation, and impaired differentiation of granulocyte precursors. Furthermore, with granulocyte cell lines derived from the Mad1/p27(KIP1) double-null mice, we observed constitutive Myc expression and cyclin E-CDK2 kinase activity as well as impaired differentiation following treatment with an inducer of differentiation. By contrast, similar treatment of granulocytes from Mad1 or p27(KIP1) single-null mice resulted in differentiation accompanied by downregulation of both Myc expression and cyclin E-CDK2 kinase activity. In the double-null granulocytic cells, addition of a CDK2 inhibitor in the presence of differentiation inducer was sufficient to restore differentiation and reduce Myc levels. We conclude that Mad1 and p27(KIP1) operate, at least in part, by distinct mechanisms to downregulate CDK2 activity and Myc expression in order to promote cell cycle exit during differentiation.

Functional domains of histone deacetylase-3

Post-translational modifications of histones, in general, and acetylation/deacetylation, in particular, can dramatically alter gene expression in eukaryotic cells. In humans, four highly homologous class I HDAC enzymes (HDAC1, HDAC2, HDAC3, and HDAC8) have been identified to date. Although HDAC3 shares some structural and functional similarities with other class I HDACs, it exists in multisubunit complexes separate and different from other known HDAC complexes, implying that individual HDACs might function in a distinct manner. In this current study, to understand further the cellular function of HDAC3 and to uncover possible unique roles this protein may have in gene regulation, we performed a detailed analysis of HDAC3 using deletion mutations. Surprisingly, we found that the non-conserved C-terminal region of HDAC3 is required for both deacetylase and transcriptional repression activity. In addition, we discovered that the central portion of the HDAC3 protein possesses a nuclear export signal, whereas the C-terminal part of HDAC3 contributes to the protein's localization in the nucleus. Finally, we found that HDAC3 forms oligomers in vitro and in vivo and that the N-terminal portion of HDAC3 is necessary for this property. These data indicate that HDAC3 comprises separate, non-overlapping domains that contribute to the unique properties and function of this protein.

Histone deacetylase inhibitors in cancer treatment

Histone deacetylase (HDAC) inhibitors are emerging as an exciting new class of potential anticancer agents for the treatment of solid and hematological malignancies. In recent years, an increasing number of structurally diverse HDAC inhibitors have been identified that inhibit proliferation and induce differentiation and/or apoptosis of tumor cells in culture and in animal models. HDAC inhibition causes acetylated nuclear histones to accumulate in both tumor and normal tissues, providing a surrogate marker for the biological activity of HDAC inhibitors in vivo. The effects of HDAC inhibitors on gene expression are highly selective, leading to transcriptional activation of certain genes such as the cyclin-dependent kinase inhibitor p21WAF1/CIP1 but repression of others. HDAC inhibition not only results in acetylation of histones but also transcription factors such as p53, GATA-1 and estrogen receptor-alpha. The functional significance of acetylation of non-histone proteins and the precise mechanisms whereby HDAC inhibitors induce tumor cell growth arrest, differentiation and/or apoptosis are currently the focus of intensive research. Several HDAC inhibitors have shown impressive antitumor activity in vivo with remarkably little toxicity in preclinical studies and are currently in phase I clinical trial. The focus of this review is the development and clinical application of HDAC inhibitors for the treatment of cancer.

The FK506-binding protein 25 functionally associates with histone deacetylases and with transcription factor YY1

FK506-binding proteins (FKBPs) are cellular receptors for immunosuppressants that belong to a subgroup of proteins, known as immunophilins, with peptidylprolyl cis-trans isomerase (PPIase) activity. Sequence comparison suggested that the HD2-type histone deacetylases and the FKBP-type PPIases may have evolved from a common ancestor enzyme. Here we show that FKBP25 physically associates with the histone deacetylases HDAC1 and HDAC2 and with the HDAC-binding transcriptional regulator YY1. An FKBP25 immunoprecipitated complex contains deacetylase activity, and this activity is associated with the N-terminus of FKBP25, distinct from the FK506/rapamycin-binding domain. Furthermore, FKBP25 can alter the DNA-binding activity of YY1. Together, our data firmly establish a relationship between histone deacetylases and the FKBP enzymes and provide a novel and critical function for the FKBPs.

The Myc/Max/Mad network and the transcriptional control of cell behavior

The Myc/Max/Mad network comprises a group of transcription factors whose distinct interactions result in gene-specific transcriptional activation or repression. A great deal of research indicates that the functions of the network play roles in cell proliferation, differentiation, and death. In this review we focus on the Myc and Mad protein families and attempt to relate their biological functions to their transcriptional activities and gene targets. Both Myc and Mad, as well as the more recently described Mnt and Mga proteins, form heterodimers with Max, permitting binding to specific DNA sequences. These DNA-bound heterodimers recruit coactivator or corepressor complexes that generate alterations in chromatin structure, which in turn modulate transcription. Initial identification of target genes suggests that the network regulates genes involved in the cell cycle, growth, life span, and morphology. Because Myc and Mad proteins are expressed in response to diverse signaling pathways, the network can be viewed as a functional module which acts to convert environmental signals into specific gene-regulatory programs.

Identification of the Sin3-binding site in Ume6 defines a two-step process for conversion of Ume6 from a transcriptional repressor to an activator in yeast

The DNA-binding protein Ume6 is required for both repression and activation of meiosis-specific genes, through interaction with the Sin3 corepressor and Rpd3 histone deacetylase and the meiotic activator Ime1. Here we show that fusion of a heterologous activation domain to Ume6 is unable to convert it into a constitutive activator of early meiotic gene transcription, indicating that an additional function is needed to overcome repression at these promoters. Mutations in UME6 allowing the fusion to activate lie in a predicted amphipathic alpha helix and specifically disrupt interaction with Sin3 but not with Teal, an activator of Ty transcription also found to interact with Ume6 in a two-hybrid screen. The mutations cause a loss of repression by Ume6 and precisely identify the Ume6 Sin3-binding domain, which we show interacts with the paired amphipathic helix 2 region of Sin3. Analysis of these mutants indicates that conversion of Ume6 to an activator involves two genetically distinct steps that act to relieve Sin3-mediated repression and provide an activation domain to Ume6. The mutants further demonstrate that premature expression and lack of subsequent rerepression of Ume6-Sin3-regulated genes are not deleterious to meiotic progression and suggest that the essential role of Sin3 in meiosis is independent of Ume6. The model for Ume6 function arising from these studies indicates that Ume6 is similar in many respects to metazoan regulators that utilize Sin3, such as the Myc-Mad-Max system and nuclear hormone receptors, and provides new insights into the control of transcriptional repression and activation by the Ume6-URS1 regulatory complex in yeast.

Targeted deletion of the S-phase-specific Myc antagonist Mad3 sensitizes neuronal and lymphoid cells to radiation-induced apoptosis

The Mad family comprises four basic-helix-loop-helix/leucine zipper proteins, Mad1, Mxi1, Mad3, and Mad4, which heterodimerize with Max and function as transcriptional repressors. The balance between Myc-Max and Mad-Max complexes has been postulated to influence cell proliferation and differentiation. The expression patterns of Mad family genes are complex, but in general, the induction of most family members is linked to cell cycle exit and differentiation. The expression pattern of mad3 is unusual in that mad3 mRNA and protein were found to be restricted to proliferating cells prior to differentiation. We show here that during murine development mad3 is specifically expressed in the S phase of the cell cycle in neuronal progenitor cells that are committed to differentiation. To investigate mad3 function, we disrupted the mad3 gene by homologous recombination in mice. No defect in cell cycle exit and differentiation could be detected in mad3 homozygous mutant mice. However, upon gamma irradiation, increased cell death of thymocytes and neural progenitor cells was observed, implicating mad3 in the regulation of the cellular response to DNA damage.

Cytokine-induced restoration of differentiation and cell cycle arrest in v-Myc transformed U-937 monoblasts correlates with reduced Myc activity

Deregulated expression of the myc-family of oncogenes in hematopoietic and other cell types plays an important role in tumorigenesis, and results in increased proliferative potential and block of cellular differentiation. We have previously shown that IFN-gamma restores phorbol ester-induced differentiation and cell cycle arrest in v-myc transformed human U-937 monoblasts. To investigate whether other cytokine signals could also abrogate such a block, IL-1, IL-3, IL-4, IL-6, IL-7, IL-10, IL-11, LIF, oncostatin M, M-CSF, G-CSF and GM-CSF, and TGFbeta1, TNF-alpha, IFN-alpha were examined. We show that GM-CSF and IL-6, in combination with the phorbol ester 12-O-tetradecanoyl-phorbol acetate (TPA), restored differentiation and cell cycle arrest. In contrast, treatment by TGFbeta1 +/- TPA resulted in an efficient G1/G0 arrest, but did not appear to induce terminal differentiation. Restoration of differentiation and cell cycle arrest was accomplished despite maintained expression of the v-Myc protein. Our results show that the cytokine-induced signals reduced Myc-dependent transcription of an artificial target promoter/reporter gene construct, correlating in most, but not all, cases with decreased association of v- and c-Myc with its essential partner, Max. Thus, cytokine-induced signals may counteract the activity of deregulated Myc, and contribute to the normalization of differentiation, arrest in the G1/G0 phase of the cell cycle, or both.

MondoA, a novel basic helix-loop-helix-leucine zipper transcriptional activator that constitutes a positive branch of a max-like network

Max is a common dimerization partner for a family of transcription factors (Myc, Mad [or Mxi]), and Mnt [or Rox] proteins) that regulate cell growth, proliferation, and apoptosis. We recently characterized a novel Max-like protein, Mlx, which interacts with Mad1 and Mad4. Here we describe the cloning and functional characterization of a new family of basic helix-loop-helix-leucine zipper heterodimeric partners for Mlx termed the Mondo family. MondoA forms homodimers weakly and does not interact with Max or members of the Myc or Mad families. MondoA and Mlx associate in vivo, and surprisingly, they are localized primarily to the cytoplasm of cultured mammalian cells. Treatment of cells with the nuclear export inhibitor leptomycin B results in the nuclear accumulation of MondoA and Mlx, demonstrating that they shuttle between the cytoplasmic and nuclear compartments rather than having exclusively cytoplasmic localization. MondoA preferentially forms heterodimers with Mlx, and this heterocomplex can bind to, and activate transcription from, CACGTG E-boxes when targeted to the nucleus via a heterologous nuclear localization signal. The amino termini of the Mondo proteins are highly conserved among family members and contain separable and autonomous cytoplasmic localization and transcription activation domains. Therefore, Mlx can mediate transcriptional repression in conjunction with the Mad family and can mediate transcriptional activation via the Mondo family. We propose that Mlx, like Max, functions as the center of a transcription factor network.

The transcriptional signature of dioxin in human hepatoma HepG2 cells

We have used a high density microarray hybridization approach to characterize the transcriptional response of human hepatoma HepG2 cells to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We find that exposure to 10 nM TCDD for 8 hr alters by at least a factor of 2.1 the expression of 310 known genes and of an equivalent number of expressed sequence tags. Treatment with TCDD in the presence of 20 microg/mL of cycloheximide blocked the effect on 202 of these genes, allowing us to distinguish between primary effects of TCDD exposure, which take place whether cycloheximide is present or not, and secondary effects, which are blocked by inhibition of protein synthesis. Of the 310 known genes affected by TCDD, 30 are up-regulated and 78 are down-regulated regardless of cycloheximide treatment, and 84 are up-regulated and 118 are down-regulated only when protein synthesis is not inhibited. Functional clustering of genes regulated by TCDD reveals many potential physiological interactions that might shed light on the multiple biological effects of this compound. Our results, however, suggest that arriving at a sound understanding of the molecular mechanisms governing the biological outcome of TCDD exposure promises to be orders of magnitude more complicated than might have been previously imagined.

Histone deacetylases, transcriptional control, and cancer

A key event in the regulation of eukaryotic gene expression is the posttranslational modification of nucleosomal histones, which converts regions of chromosomes into transcriptionally active or inactive chromatin. The most well studied posttranslational modification of histones is the acetylation of epsilon-amino groups on conserved lysine residues in the histones' amino-terminal tail domains. Significant advances have been made in the past few years toward the identification of histone acetyltransferases and histone deacetylases. Currently, there are over a dozen cloned histone acetyltransferases and at least eight cloned human histone deacetylases. Interestingly, many histone deacetylases can function as transcriptional corepressors and, often, they are present in multi-subunit complexes. More intriguing, at least some histone deacetylases are associated with chromatin-remodeling machines. In addition, several studies have pointed to the possible involvement of histone deacetylases in human cancer. The availability of the cloned histone deacetylase genes has provided swift progress in the understanding of the mechanisms of deacetylases, their role in transcription, and their possible role in health and disease.

Analysis of Myc/Max/Mad network members in adipogenesis: inhibition of the proliferative burst and differentiation by ectopically expressed Mad1

Transcription factors of the Myc/Max/Mad network affect multiple aspects of cellular behavior, including proliferation, differentiation, and apoptosis. Recent studies have shown that Mad proteins can inhibit cellular growth and transformation and thus antagonize the function of Myc proteins. To define further the contribution of these proteins to cellular growth control, we have studied the expression of the respective genes and proteins in 3T3-L1 cells, both upon serum stimulation of quiescent cells and during adipocytic differentiation in response to insulin, dexamethasone, and isobutylmethylxanthine. We found distinct expression patterns for the mad genes. Mad4 was induced when cells exit the cell cycle and, together with mad1, during the late phase of differentiation. In contrast, mad3 expression was associated with progression through S phase and the proliferative burst of differentiating preadipocytes, overlapping in part c-myc expression. DNA binding analyses revealed that the most prominent network complex both in cycling and in differentiating cells was Mnt/Max, whereas c-Myc/Max complexes were detectable only during peak c-Myc expression periods. Ectopic expression of Mad1 in preadipocytes resulted in the inhibition of S phase and the proliferation associated with the proliferative burst; as a consequence, adipocytic differentiation was significantly inhibited. Our findings suggest that the precise temporal regulation of Myc/Max/Mad network proteins is critical for determining cellular behavior.

Two MAD tails: what the recent knockouts of Mad1 and Mxi1 tell us about the MYC/MAX/MAD network

Members of the MAD/MXI protein family heterodimerize with MAX and repress transcription by recruiting a chromatin-modifying co-repressor complex to specific DNA target genes. Repression mediated by MAD is thought to antagonize the transcriptional activation and proliferation-promoting functions of MYC-MAX heterodimers. Because they are induced during differentiation, it has been suggested that MAD proteins act to limit cell proliferation during terminal differentiation. There is also controversial evidence that these proteins may function as tumor suppressors. Recently, targeted gene deletions of two members of this gene family, Mad1 and Mxi1, have been carried out in mice. Although these animals display what appear to be quite different phenotypes, further analysis supports the view that both these proteins function in cell-cycle exit during terminal differentiation, and that at least MXI1 can act as a tumor suppressor.