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.

The Myc/Max/Mad Network and the Transcriptional Control of Cell Behavior

Deregulated expression of c-MYC occurs in a broad range of human cancers and is often associated with poor prognosis, indicating a key role for this oncogene in tumour progression. However, as established human tumours often bear multiple genetic lesions, it is difficult to determine whether c-MYC is instrumental in the initiation/progression of the tumour, or indeed whether inactivating c-MYC would lead to tumour regression. Regulatable transgenic mouse models of oncogenesis have shed light on these issues and provide hope for effective cancer therapies.

c-MYC: more than just a matter of life and death.

Myc genes are key regulators of cell proliferation, and their deregulation contributes to the genesis of most human tumours. Recently, a wealth of data has shed new light on the biochemical functions of Myc proteins and on the mechanisms through which they function in cellular transformation.

Transcriptional regulation and transformation by Myc proteins

The Ras-dependent extracellular signal-regulated kinase (ERK)1/2 mitogen-activated protein (MAP) kinase pathway plays a central role in cell proliferation control. In normal cells, sustained activation of ERK1/ERK2 is necessary for G1- to S-phase progression and is associated with induction of positive regulators of the cell cycle and inactivation of antiproliferative genes. In cells expressing activated Ras or Raf mutants, hyperactivation of the ERK1/2 pathway elicits cell cycle arrest by inducing the accumulation of cyclin-dependent kinase inhibitors. In this review, we discuss the mechanisms by which activated ERK1/ERK2 regulate growth and cell cycle progression of mammalian somatic cells. We also highlight the findings obtained from gene disruption studies.

The ERK1/2 mitogen-activated protein kinase pathway as a master regulator of the G1- to S-phase transition

Mechanisms linking mitogenic and growth inhibitory cytokine signaling and the cell cycle have not been fully elucidated in either cancer or in normal cells. Here we show that activation of protein kinase B (PKB)/Akt, contributes to resistance to antiproliferative signals and breast cancer progression in part by impairing the nuclear import and action of p27. Akt transfection caused cytoplasmic p27 accumulation and resistance to cytokine-mediated G1 arrest. The nuclear localization signal of p27 contains an Akt consensus site at threonine 157, and p27 phosphorylation by Akt impaired its nuclear import in vitro. Akt phosphorylated wild-type p27 but not p27T157A. In cells transfected with constitutively active AktT308DS473D (PKBDD), p27WT mislocalized to the cytoplasm, but p27T157A was nuclear. In cells with activated Akt, p27WT failed to cause G1 arrest, while the antiproliferative effect of p27T157A was not impaired. Cytoplasmic p27 was seen in 41% (52 of 128) of primary human breast cancers in conjunction with Akt activation and was correlated with a poor patient prognosis. Thus, we show a novel mechanism whereby Akt impairs p27 function that is associated with an aggressive phenotype in human breast cancer. NOTE: In the version of the article initially published online, the abstract contained one extraneous sentence. This error has been corrected in the HTML and PDF versions. The abstract will appear correctly in the forthcoming print issue.

PKB/Akt phosphorylates p27, impairs nuclear import of p27 and opposes p27-mediated G1 arrest.

Ectopic expression of Myc induces Cdk2 kinase activity in quiescent cells and antagonizes association of p27 kip1 with Cdk2. The target gene(s) by which Myc mediates this effect is largely unknown. We now show that p27 is rapidly and transiently sequestered by cyclin D2–Cdk4 complexes upon activation of Myc and that cyclin D2 is a direct target gene of Myc. The cyclin D2 promoter is repressed by Mad–Max complexes and de‐repressed by Myc via a single highly conserved E‐box element. Addition of trichostatin A to quiescent cells mimics activation of Myc and induces cyclin D2 expression, suggesting that cyclin D2 is repressed in a histone deacetylase‐dependent manner in quiescent cells. Inhibition of cyclin D2 function in established cell lines, either by ectopic expression of p16 or by antibody injection, inhibits Myc‐dependent dissociation of p27 from Cdk2 and Myc‐induced cell cycle entry. Primary mouse fibroblasts that are cyclin D2‐deficient undergo accelerated senescence in culture and are not immortalized by Myc; induction of apoptosis by Myc is unimpaired in such cells. Our data identify a downstream effector pathway that links Myc directly to cell cycle progression.

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Antje Maier

Papers

Direct induction of cyclin D2 by Myc contributes to cell cycle progression and sequestration of p27.