Formation of alpha-Pal/Max heterodimers synergistically activates the eIF2-alpha promoter

2-Aminopurine inhibits lipid accumulation induced by apolipoprotein E-deficient lipoprotein in macrophages: potential role of eukaryotic initiation factor-2alpha phosphorylation in foam cell formation

We previously reported that apolipoprotein (Apo) E-deficient, ApoB48-containing (E(-)/B48) lipoproteins inhibited expression of lysosomal hydrolase and transformed mouse peritoneal macrophages (MPMs) into foam cells. The present study examined the effect of 2-aminopurine (2-AP), an inhibitor of eukaryotic initiation factor (eIF)-2alpha phosphorylation, on E(-)/B48 lipoprotein-induced changes in gene expression and foam cell formation. Our data demonstrated that E(-)/B48 lipoproteins enhanced phosphorylation of eIF-2alpha in macrophages. Incubation of MPMs with E(-)/B48 lipoproteins inhibited the translation efficiency of mRNAs encoding lysosomal acid lipase, cathepsin B, and cation-dependent mannose 6 phosphate receptor, with a parallel reduction in the level of these proteins. Addition of 2-AP to the culture media alleviated the suppressive effect of E(-)/B48 lipoproteins on lysosomal hydrolase mRNA translation, increased macrophage degradation of E(-)/B48 lipoproteins, and inhibited foam cell formation. Transfection of MPMs with a nonphosphorylatable eIF-2alpha mutant also attenuated the suppressive effect of E(-)/B48 lipoproteins on expression of lysosomal acid lipase, associated with a reduced accumulation of cellular cholesterol esters. This is the first demonstration that ApoE-deficient lipoproteins inhibit lysosomal hydrolase synthesis and transform macrophages into foam cells through induction of eIF-2alpha phosphorylation.

Myc/Max/Mad in invertebrates: the evolution of the Max network

The Myc proto-oncogenes, their binding partner Max and their antagonists from the Mad family of transcriptional repressors have been extensively analysed in vertebrates. However, members of this network are found in all animals examined so far. Several recent studies have addressed the physiological function of these proteins in invertebrate model organisms, in particular Drosophila melanogaster. This review describes the structure of invertebrate Myc/Max/Mad genes and it discusses their regulation and physiological functions, with special emphasis on their essential role in the control of cellular growth and proliferation.

The roles of Sp1, Sp3, USF1/USF2 and NRF-1 in the regulation and three-dimensional structure of the Fragile X mental retardation gene promoter

Expansion of a CGG.CCG-repeat tract in the 5'-untranslated region of the FMR1 (Fragile X mental retardation 1) gene causes its aberrant transcription. This produces symptoms ranging from premature ovarian failure and Fragile X associated tremor and ataxia syndrome to FMR syndrome, depending on the size of the expansion. The promoter from normal alleles shows four protein-binding regions in vivo. We had previously shown that in mouse brain extracts two of these sites are bound by USF1/USF2 (upstream stimulatory factors 1 and 2) heterodimers and NRF-1 (nuclear respiratory factor-1). We also showed that these sites are involved in the positive regulation of FMR1 transcription in neuronally derived cells. In the present study, we show that Sp1 (specificity protein 1) and Sp3 are also strong positive regulators of FMR1 promoter activity. We also show that, like Sp1 and E-box-binding proteins such as USF1 and USF2, NRF-1 causes DNA bending, in this case producing a bend of 57 degrees towards the major groove. The combined effect of the four protein-induced bends on promoter geometry is the formation of a highly compact arch-like structure in which the 5' end of the promoter is brought in close proximity to the 3' end. We had previously shown that while point mutations in the GC-boxes decrease promoter activity, deletion of either one of them leads to an increase in promoter activity. We can reconcile these observations with the positive effect of Sp1 and Sp3 if protein-induced bending acts, at least in part, to bring together distally spaced factors important for transcription initiation.

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.

The role of c-myc in cellular growth control

Cell division is coupled to cell growth. Since some c-myc target genes are regulators of cell growth while others function in cell division pathways, c-myc is apparently poised at the interface of these processes. Cell culture systems have shown specific myc-associated growth phenotypes. Increased cell growth precedes DNA synthesis after myc activation in cells expressing myc-estrogen receptor fuson constructs and cells lacking c-myc exhibit a marked loss of protein synthesis. A number of candidate c-myc target genes regulate processes required for cell growth including rRNA transcription and processing, ribosomal protein transcription and translation, and translation initiation. These interactions all have the potential to account for the growth phenotypes in c-myc mutant cells. The ability of translation initiation factors, including eIF4E, to transform cells makes them particularly interesting targets of c-myc. Further evaluation of these target genes will provide important insights into growth control and c-myc's functions in cellular proliferation.