Gene structure and chromosomal localization of mouse S-adenosylmethionine decarboxylase

Magnesium Supplementation Attenuates Ultraviolet-B-Induced Damage Mediated through Elevation of Polyamine Production in Human HaCaT Keratinocytes

Magnesium ions (Mg²⁺) have favorable effects such as the improvement of barrier function and the reduction of inflammation reaction in inflammatory skin diseases. However, its mechanisms have not been fully understood. Microarray analysis has shown that the gene expressions of polyamine synthases are upregulated by MgCl₂ supplementation in human HaCaT keratinocytes. Here, we investigated the mechanism and function of polyamine production. The mRNA and protein levels of polyamine synthases were dose-dependently increased by MgCl₂ supplementation, which were inhibited by U0126, a MEK inhibitor; CHIR-99021, a glycogen synthase kinase-3 (GSK3) inhibitor; and Naphthol AS-E, a cyclic AMP-response-element-binding protein (CREB) inhibitor. Similarly, reporter activities of polyamine synthases were suppressed by these inhibitors, suggesting that MEK, GSK3, and CREB are involved in the transcriptional regulation of polyamine synthases. Cell viability was reduced by ultraviolet B (UVB) exposure, which was rescued by MgCl₂ supplementation. The UVB-induced elevation of reactive oxygen species was attenuated by MgCl₂ supplementation, which was inhibited by cysteamine, a polyamine synthase inhibitor. Our data indicate that the expression levels of polyamine synthases are upregulated by MgCl₂ supplementation mediated through the activation of the MEK/GSK3/CREB pathway. MgCl₂ supplementation may be useful in reducing the UVB-induced oxidative stress in the skin.

A perspective on mammalian upstream open reading frame function

Post-transcriptional control makes a major contribution to the overall regulation of gene expression pathway. Within the cytoplasm this is mediated by a combination of regulatory RNA motifs within the 5′ and 3′ untranslated regions of mRNAs and their interacting protein/RNA partners. One of the most common regulatory RNA elements in mammalian transcripts (present in approximately 40% of all mRNAs) are upstream open reading frames (uORFs). However, despite the prevalence of these RNA elements how they function is not well understood. In general, they act to repress translation of the physiological ORF under control conditions, and under certain pathophysiological stresses this repression can be alleviated. It is known that re-initiation following the translation of an uORF is utilised in some situations however there are numerous alternative mechanisms that control the synthesis of a protein whose mRNA contains uORFs. Moreover, the trans-acting factors that are also involved in this process are not well defined. In this review we summarise our current understanding of this area and highlight some common features of these RNA motifs that have been discovered to date.

Chapter 2: Clusterin (CLU): From one gene and two transcripts to many proteins

Clusterin (CLU) has kept many researchers engaged for a long time since its first discovery and characterization in the attempt to unravel its biological role in mammals. Although there is a general consensus on the fact that CLU is supposed to play important roles in nearly all fundamental biological phenomena and in many human diseases including cancer, after about 10 years of work CLU has been defined as an "enigmatic" protein. This sense of frustration among the researchers is originated by the fact that, despite considerable scientific production concerning CLU, there is still a lack of basic information about the complex regulation of its expression. The CLU gene is a single 9-exon gene expressed at very different levels in almost all major tissues in mammals. The gene produces at least three protein forms with different subcellular localization and diverse biological functions. The molecular mechanism of production of these protein forms remains unclear. The best known is the glycosylated mature form of CLU (sCLU), secreted with very big quantitative differences at different body sites. Hormones and growth factors are the most important regulators of CLU gene expression. Before 2006, it was believed that a unique transcript of about 1.9 kb was originated by transcription of the CLU gene. Now we know that alternative transcriptional initiation, possibly driven by two distinct promoters, may produce at least two distinct CLU mRNA isoforms differing in their unique first exon, named Isoform 1 and Isoform 2. A third transcript, named Isoform 11036, has been recently found as one of the most probable mRNA variants. Approaches like cloning, expression, and functional characterization of the different CLU protein products have generated a critical mass of information teaching us an important lesson about CLU gene expression regulation. Nevertheless, further studies are necessary to better understand the tissue-specific regulation of CLU expression and to identify the specific signals triggering the expression of different/alternative transcript isoforms and protein forms in different cell types at appropriate time.

Genomic characterization of the S-adenosylmethionine decarboxylase genes from soybean

A full-length gene GmSAMDC1, encoding the S-adenosylmethionine decarboxylase (SAMDC), a key enzyme involved in polyamine biosynthesis, was identified from soybean expressed sequence tags and was characterized. GmSAMDC1 encoded a peptide of 355 amino acids. When compared with other plant SAMDCs, the GmSAMDC1 protein had several highly conserved regions including a putative pro-enzyme cleavage site and a PEST sequence. The 5' leader sequence of the the GmSAMDC1 mRNA contained two additional open reading frames (ORFs), which may regulate the translational process. The genomic sequence of the GmSAMDC1 gene contained three introns in the 5' leader sequence, but no intron in the 3'-UTR or the main pro-enzyme ORF. A simple sequence repeat (SSR) was found in intron 2, and the GmSAMDC1 gene was mapped to linkage group D1 using this SSR. The genomic organization of the GmSAMDC1 gene in the subgenus Glycine and the subgenus Soja was found to be different by Southern-blot and PCR analysis. A pseudogene, GmSAMDC2, was also identified. This gene contained no intron and lost its two uORFs. Northern-blot analysis showed that the GmSAMDC1 gene expression was induced by salt, drought and cold, but not induced by wounding; suggesting that the gene was implicated in response to multiple-stress conditions.

Essential role of S-adenosylmethionine decarboxylase in mouse embryonic development

BACKGROUND

S-Adenosylmethionine decarboxylase (AdoMetDC) is one of the key enzymes involved in the biosynthesis of spermidine and spermine, which are essential for normal cell growth. To examine the role of polyamines in embryogenesis, we carried out targeted disruption of the mouse Amd1 gene, encoding AdoMetDC, to generate mice that can not synthesize spermidine and spermine.

RESULTS

Amd1 heterozygous mice were viable, normal and fertile. However, homozygous Amd1(-/-) embryos died early in embryonic development, between E3.5 and E6.5 days post-coitus. Homozygous (Amd1(-/-)) blastocysts at E3.5 arrested cell proliferation immediately after the onset of cell culture, and this arrest was rescued by the addition of spermidine. Chromosomal DNA breakage did not occur in Amd1(-/-) blastocysts at E3.5, as determined by TUNEL assay.

CONCLUSIONS

These results indicate that AdoMetDC plays an essential role in embryonic development and that polyamines are required for cell proliferation in the embryo after E3.5.

Characterization of monocot and dicot plant S-adenosyl-l-methionine decarboxylase gene families including identification in the mRNA of a highly conserved pair of upstream overlapping open reading frames

S-Adenosyl-L-methionine decarboxylase (AdoMetDC; EC 4.1.1.50) is one of the key regulatory enzymes in the biosynthesis of polyamines. Isolation of genomic and cDNA sequences from rice and Arabidopsis had indicated that this enzyme is encoded by a small multigene family in monocot and dicot plants. Analysis of rice, maize and Arabidopsis AdoMetDC cDNA species revealed that the monocot enzyme possesses an extended C-terminus relative to dicot and human enzymes. Interestingly, we discovered that all expressed plant AdoMetDC mRNA 5' leader sequences contain a highly conserved pair of overlapping upstream open reading frames (uORFs) that overlap by one base. The 5' tiny uORF consists of two or three codons and the 3' small uORF encodes 50-54 residues. Sequences of the small uORFs are highly conserved between monocot, dicot and gymnosperm AdoMetDC mRNA species and the C-terminus of the plant small uORFs is conserved with the C-terminus of nematode AdoMetDC uORFs; such a conserved arrangement is strongly suggestive of a translational regulatory mechanism. No introns were found in the main AdoMetDC proenzyme ORF from any of the plant genes encoding AdoMetDC, whereas introns were found in conserved positions flanking the overlapping uORFs. The absence of the furthest 3' intron from the Arabidopsis gene encoding AdoMetDC2 suggests that this intron was lost recently. Reverse-transcriptase-mediated PCR analysis of the two Arabidopsis genes for AdoMetDC indicated that AdoMetDC1 is abundant and ubiquitous, whereas the gene for AdoMetDC2 is expressed preferentially in leaves and inflorescences. Investigation of recently released Arabidopsis genome sequences has revealed that in addition to the two genes encoding AdoMetDC isolated as part of the present work, four additional genes are present in Arabidopsis but they are probably not expressed.