Biochemical adaptations of mammalian hibernation: exploring squirrels as a perspective model for naturally induced reversible insulin resistance

An important disease among human metabolic disorders is type 2 diabetes mellitus. This disorder involves multiple physiological defects that result from high blood glucose content and eventually lead to the onset of insulin resistance. The combination of insulin resistance, increased glucose production, and decreased insulin secretion creates a diabetic metabolic environment that leads to a lifetime of management. Appropriate models are critical for the success of research. As such, a unique model providing insight into the mechanisms of reversible insulin resistance is mammalian hibernation. Hibernators, such as ground squirrels and bats, are excellent examples of animals exhibiting reversible insulin resistance, for which a rapid increase in body weight is required prior to entry into dormancy. Hibernator studies have shown differential regulation of specific molecular pathways involved in reversible resistance to insulin. The present review focuses on this growing area of research and the molecular mechanisms that regulate glucose homeostasis, and explores the roles of the Akt signaling pathway during hibernation. Here, we propose a link between hibernation, a well-documented response to periods of environmental stress, and reversible insulin resistance, potentially facilitated by key alterations in the Akt signaling network, PPAR-γ/PGC-1α regulation, and non-coding RNA expression. Coincidentally, many of the same pathways are frequently found to be dysregulated during insulin resistance in human type 2 diabetes. Hence, the molecular networks that may regulate reversible insulin resistance in hibernating mammals represent a novel approach by providing insight into medical treatment of insulin resistance in humans.

Regulation of protein synthesis and the role of eIF3 in cancer

Maintenance of cell homeostasis and regulation of cell proliferation depend importantly on regulating the process of protein synthesis. Many disease states arise when disregulation of protein synthesis occurs. This review focuses on mechanisms of translational control and how disregulation results in cell malignancy. Most translational controls occur during the initiation phase of protein synthesis, with the initiation factors being the major target of regulation through their phosphorylation. In particular, the recruitment of mRNAs through the m7G-cap structure and the binding of the initiator methionyl-tRNAi are frequent targets. However, translation, especially of specific mRNAs, may also be regulated by sequestration into processing bodies or stress granules, by trans-acting proteins or by microRNAs. When the process of protein synthesis is hyper-activated, weak mRNAs are translated relatively more efficiently, leading to an imbalance of cellular proteins that promotes cell proliferation and malignant transformation. This occurs, for example, when the cap-binding protein, eIF4E, is overexpressed, or when the methionyl-tRNAi-binding factor, eIF2, is too active. In addition, enhanced activity of eIF3 contributes to oncogenesis. The importance of the translation initiation factors as regulators of protein synthesis and cell proliferation makes them potential therapeutic targets for the treatment of cancer.

Dengue virus-induced regulation of the host cell translational machinery

Dengue virus (DV)-induced changes in the host cell protein synthesis machinery are not well understood. We investigated the transcriptional changes related to initiation of protein synthesis. The human hepatoma cell line, HepG2, was infected with DV serotype 2 for 1 h at a multiplicity of infection of one. RNA was extracted after 6, 24 and 48 h. Microarray results showed that 36.5 percent of the translation factors related to initiation of protein synthesis had significant differential expression (Z-score ≥ ±2.0). Confirmation was obtained by quantitative real-time reverse transcription-PCR. Of the genes involved in the activation of mRNA for cap-dependent translation (eIF4 factors), eIF4A, eIF4G1 and eIF4B were up-regulated while the negative regulator of translation eIF4E-BP3 was down-regulated. This activation was transient since at 24 h post-infection levels were not significantly different from control cells. However, at 48 h post-infection, eIF4A, eIF4E, eIF4G1, eIF4G3, eIF4B, and eIF4E-BP3 were down-regulated, suggesting that cap-dependent translation could be inhibited during the progression of infection. To test this hypothesis, phosphorylation of p70S6K and 4E-BP1, which induce cap-dependent protein synthesis, was assayed. Both proteins remained phosphorylated when assayed at 6 h after infection, while infection induced dephosphorylation of p70S6K and 4E-BP1 at 24 and 48 h of infection, respectively. Taken together, these results provide biological evidence suggesting that in HepG2 cells DV sustains activation of the cap-dependent machinery at early stages of infection, but progression of infection switches protein synthesis to a cap-independent process.

Translational control of ceruloplasmin gene expression: Beyond the IRE

Translational control is a common regulatory mechanism for the expression of iron-related proteins. For example, three enzymes involved in erythrocyte development are regulated by three different control mechanisms: globin synthesis is modulated by heme-regulated translational inhibitor; erythroid 5-aminolevulinate synthase translation is inhibited by binding of the iron regulatory protein to the iron response element in the 5'-untranslated region (UTR); and 15-lipoxygenase is regulated by specific proteins binding to the 3'-UTR. Ceruloplasmin (Cp) is a multi-functional, copper protein made primarily by the liver and by activated macrophages. Cp has important roles in iron homeostasis and in inflammation. Its role in iron metabolism was originally proposed because of its ferroxidase activity and because of its ability to stimulate iron loading into apo-transferrin and iron efflux from liver. We have shown that Cp mRNA is induced by interferon (IFN)-ã in U937 monocytic cells, but synthesis of Cp protein is halted by translational silencing. The silencing mechanism requires binding of a cytosolic inhibitor complex, IFN-Gamma-Activated Inhibitor of Translation (GAIT), to a specific GAIT element in the Cp 3'-UTR. Here, we describe our studies that define and characterize the GAIT element and elucidate the specific trans-acting proteins that bind the GAIT element. Our experiments describe a new mechanism of translational control of an iron-related protein and may shed light on the role that macrophage-derived Cp plays at the intersection of iron homeostasis and inflammation.

Molecular heterogeneity of beta-thalassemia in Thailand.

beta-Globin genes in 294 chromosomes of beta-thalassemia homozygotes and patients of beta-thalassemia/HbE in the northeast, the middle and the south of Thailand were analyzed by the PCR related techniques: dot blot hybridization, direct restriction assay, direct cloning and direct sequencing of the amplified DNA fragments. Twelve different mutations were detected at various frequencies. They are an A-G at-28, codon 19 (AAC-AGC), a G-T at IVS-1 nt1,a G-C at IVS-1 nt5, a C-T at IVS-2 nt654, a G addition in codons 8/9, a C deletion in codon 41, a 4 bp deletion in codons 41/42, an A addition in codons 71/72, an AAG-TAG in codon 17, a CAG-TAG in codon 26, a TAC-TAA in codon 35 and a 8 bp deletion in codons 123-125. We also developed allele specific-polymerase chain reaction to facilitate non-radioactive detection of the mutation. Origins and spread of mutations are speculated based on the results of determination of haplotypes and frameworks that are linked to the thalassemia alleles.