Eukaryotic translation initiation begins with assembly of a 48S ribosomal complex at the 5' cap structure or at an internal ribosomal entry segment (IRES). In both cases, ribosomal positioning at the AUG codon requires a 5' untranslated region upstream from the initiation site. Here, we report that translation of the genomic RNA of human immunodeficiency virus type 2 takes place by attachment of the 48S ribosomal preinitiation complex to the coding region, with no need for an upstream 5' untranslated RNA sequence. This unusual mechanism is mediated by an RNA sequence that has features of an IRES with the unique ability to recruit ribosomes upstream from its core domain. A combination of translation assays and structural studies reveal that sequences located 50 nucleotides downstream of the AUG codon are crucial for IRES activity.

Both BTG1 and BTG2 are involved in cell-growth control. BTG2 expression is regulated by p53, and its inactivation in embryonic stem cells leads to the disruption of DNA damage-induced G 2 /M cell-cycle arrest. In order to investigate the mechanism underlying Btg-mediated functions, we looked for possible functional partners of Btg1 and Btg2. Using yeast two-hybrid screening, protein-binding assays, and transient transfection assays in HeLa cells, we demonstrated the physical in vitro and in vivo interaction of both Btg1 and Btg2 with the mouse protein mCaf1 (i.e. mouse CCR4-associated factor 1). mCaf1 was identified through its interaction with the CCR4 protein, a component of a general transcription multisubunit complex, which, in yeast, regulates the expression of different genes involved in cell-cycle regulation and progression. These data suggest that Btg proteins, through their association with mCaf1, may participate, either directly or indirectly, in the transcriptional regulation of the genes involved in the control of the cell cycle. Finally, we found that box B, one of two conserved domains which define the Btg family, plays a functional role, namely that it is essential to the Btg-mCaf1 interaction.

Aims/hypothesis: Autotaxin is a lysophospholipase D that is secreted by adipocytes and whose expression is substantially up-regulated in obese, diabetic db/db mice. The aim of the present study was to depict the physiopathological and cellular mechanisms involved in regulation of adipocyte autotaxin expression. Methods: Autotaxin mRNAs were quantified in adipose tissue from db/db mice (obese and highly diabetic type 2), gold-thioglucose-treated (GTG) mice (highly obese and moderately diabetic type 2), high-fat diet-fed (HFD) mice (obese and moderately diabetic type 2), streptozotocin-treated mice (thin and diabetic type 1), and massively obese humans with glucose intolerance. Results: When compared to non-obese controls, autotaxin expression in db/db mice was significantly increased, but not in GTG, HFD, or streptozotocin-treated mice. During db/db mice development, up-regulation of autotaxin occurred only 3 weeks after the emergence of hyperinsulinaemia, and simultaneously with the emergence of hyperglycaaemia. Adipocytes from db/db mice exhibited a stronger impairment of insulin-stimulated glucose uptake than non-obese and HFD-induced obese mice. Autotaxin expression was up-regulated by treatment with TNFα (insulin resistance-promoting cytokine), and down-regulated by rosiglitazone treatment (insulin-sensitising compound) in 3T3F442A adipocytes. Finally, adipose tissue autotaxin expression was significantly up-regulated in patients exhibiting both insulin resistance and impaired glucose tolerance. Conclusions/interpretation: The present work demonstrates the existence of a db/db-specific upregulation of adipocyte autotaxin expression, which could be related to the severe type 2 diabetes phenotype and adipocyte insulin resistance, rather than excess adiposity in itself. It also showed that type 2 diabetes in humans is also associated with up-regulation of adipocyte autotaxin expression.