Overexpression of growth hormone affects alternatively spliced IGF-I mRNA expression in oMt1a-oGH transgenic mice

Robustness during Aging-Molecular Biological and Physiological Aspects

Understanding the process of aging is still an important challenge to enable healthy aging and to prevent age-related diseases. Most studies in age research investigate the decline in organ functionality and gene activity with age. The focus on decline can even be considered a paradigm in that field. However, there are certain aspects that remain surprisingly stable and keep the organism robust. Here, we present and discuss various properties of robust behavior during human and animal aging, including physiological and molecular biological features, such as the hematocrit, body temperature, immunity against infectious diseases and others. We examine, in the context of robustness, the different theories of how aging occurs. We regard the role of aging in the light of evolution.

The Regulation of IGF-1 Gene Transcription and Splicing during Development and Aging

It is commonly known that the insulin-like growth factor-I gene contains six exons that can be differentially spliced to create multiple transcript variants. Further, there are two mutually exclusive leader exons each having multiple promoter sites that are variably used. The mature IGF-I protein derived from the multiplicity of transcripts does not differ suggesting a regulatory role for the various transcript isoforms. The variant forms possess different stabilities, binding partners, and activity indicating a pivotal role for the isoforms. Research has demonstrated differential expression of the IGF-I mRNA transcripts in response to steroids, growth hormone, and developmental cues. Many studies of different tissues have focused on assessing the presence, or putative action, of the transcript isoforms with little consideration of the transcriptional mechanisms that generate the variants or the translational use of the transcript isoforms. Control points for the latter include epigenetic regulation of splicing and promoter usage in response to development or injury, RNA binding proteins and microRNA effects on transcript stability, and preferential use of two leader exons by GH and other hormones. This review will detail the current knowledge of the mechanical, hormonal, and developmental stimuli regulating IGF-1 promoter usage and splicing machinery used to create the variants.

An exonic splicing enhancer in human IGF-I pre-mRNA mediates recognition of alternative exon 5 by the serine-arginine protein splicing factor-2/alternative splicing factor

The human IGF-I gene has six exons, four of which are alternatively spliced. Variations in splicing involving exon 5 may occur, depending on the tissue type and hormonal environment. To study the regulation of splicing to IGF-I exon 5, we established an in vitro splicing assay, using a model pre-mRNA containing IGF-I exons 4 and 5 and part of the intervening intron. Using a series of deletion mutants, we identified an 18-nucleotide purine-rich splicing enhancer in exon 5 that increases the splicing efficiency of the upstream intron from 6 to 35%. We show that the serine-arginine protein splicing factor-2/alternative splicing factor specifically promotes splicing in cultured cells and in vitro and is recruited to the spliceosome in an enhancer-specific manner. Our findings are consistent with a role for splicing factor-2/alternative splicing factor in the regulation of splicing of IGF-I alternative exon 5 via a purine-rich exonic splicing enhancer.

Differential mRNA expression of insulin-like growth factor-1 splice variants in patients with idiopathic pulmonary fibrosis and pulmonary sarcoidosis

Insulin-like growth factor-1 (IGF-1) is a highly mitogenic polypeptide detectable in human lung. Using competitive reverse transcriptase/polymerase chain reaction (RT-PCR), expression of four IGF-1 transcripts was examined in bronchoalveolar lavage cells (BALC) from normal subjects, idiopathic pulmonary fibrosis (IPF), stage I/II (no fibrosis), and stage III/IV (confirmed fibrosis) pulmonary sarcoidosis patients, and fibroblast strains isolated from normal and IPF lungs. Transcripts studied were Class 1 and Class 2 (exons 1 or 2, respectively) with IGF-1Eb or IGF-1Ea (exons 5 or 6, respectively). Total IGF-1 expression was downregulated in BALC of both patients with IPF (p < 0.01) and patients with sarcoidosis (p < 0.04) compared with healthy subjects. In contrast, both constitutive (p < 0.003) and transforming growth factor-beta (TGF-beta)- induced (p < 0.02) IGF-1 expression was higher in fibrotic, compared with normal, fibroblasts. These changes were associated with differential expression of IGF-1 splice variants. Healthy subjects and sarcoidosis patients without fibrosis showed similar expression of Class 1/Class 2 and IGF-1Ea/IGF-1Eb. However, patients with fibrosis demonstrated discordant, increased relative abundance of Class 1 transcripts (p < 0.01). In parallel, all fibrosis patients failed to express Class 2, IGF-1Eb forms and sarcoidosis patients with fibrosis did not express the Class 1, IGF-1Eb variant either. Fibrotic fibroblasts expressed higher constitutive levels of Class 1, IGF-1Ea transcripts compared with normal fibroblasts. Class 2, IGF-1Eb forms were moderately expressed by fibroblasts only after stimulation with TGF-beta, which also further increased levels of Class 1, IGF-1Ea transcripts. Our findings suggest that transition from a healthy to a fibrotic phenotype occurs in association with a changing pattern of IGF-1 mRNA heterogeneity and leads us to hypothesize a potential role for specific IGF-1 variants in fibrogenesis.

Stability of alternatively spliced IGF-I mRNA in growth plate chondrocytes

Insulin-like growth factor-I (IGF-I) enhances the proliferation and hypertrophy of growth plate chondrocytes leading to increased growth plate width thus promoting bone elongation. Differing quantities of the multiple IGF-I transcripts within the growth plate suggest differential regulation of IGF-I. To assess this, the relative stabilities of the 1Ea, 1Eb, and 2Ea IGF-I mRNA classes in 2-6 week old rat growth plates were evaluated. The mean estimated half-life of class 1Ea was 3.7+/-0.05 h, while classes 1Eb and 2Ea decayed gradually over the course of the experiment. Estimated half-lives for each IGF-I mRNA species were unchanged at all ages examined. Incubation with Act D enhanced the transcription of class 1Ea, 1Eb, and 2Ea mRNAs to varying degrees. This implies that the differential stability of alternative IGF-I mRNA classes may be an inherent regulatory component that is not influenced by an animal's developmental state, and the differential abundance of alternative IGF-I mRNA species in the growth plate throughout development is due to transcriptional differences.