The protein encoded by a growth arrest-specific gene (gas6) is a new member of the vitamin K-dependent proteins related to protein S, a negative coregulator in the blood coagulation cascade.

A set of growth arrest-specific genes (gas) whose expression is negatively regulated after serum induction has previously been described (C. Schneider, R. M. King, and L. Philipson, Cell 54:787-793, 1988). The detailed analysis of one of them, gas6, is reported here, gas6 mRNA (2.6 kb) is abundantly expressed in serum-starved (48 h in 0.5% fetal calf serum) NIH 3T3 cells but decreases dramatically after fetal calf serum or basic fibroblast growth factor stimulation. The human homolog of gas6 was also cloned and sequenced, revealing a high degree of homology and a similar pattern of expression in IMR90 human fibroblasts. Computer analysis of the protein encoded by murine and human gas6 cDNAs showed significant homology (43 and 44% amino acid identity, respectively) to human protein S, a negative coregulator in the blood coagulation pathway. By using an anti-human Gas6 monospecific affinity-purified antibody, we show that the biosynthetic level of human Gas6 fully reflects mRNA expression in IMR90 human fibroblasts. This finding thus defines a new member of vitamin K-dependent proteins that is expressed in many human and mouse tissues and may be involved in the regulation of a protease cascade relevant in growth regulation.     

A link between apoptosis and degree of phosphorylation of high mobility group A1a protein in leukemic cells

Nuclear phosphoprotein HMGA1a, high mobility group A1a, (previously HMGI) has been investigated during apoptosis. A change in the degree of phosphorylation of HMGA1a has been observed during apoptosis induced in four leukemic cell lines (HL60, K562, NB4, and U937) by drugs (etoposide, camptothecin) or herpes simplex virus type-1. Both hyper-phosphorylation and de-phosphorylation of HMGA1a have been ascertained by liquid chromatography-mass spectrometry. Hyper-phosphorylation (at least five phosphate groups/HMGA1a molecule) occurs at the early apoptotic stages and is probably related to HMGA1a displacement from DNA and chromatin release from the nuclear scaffold. De-phosphorylation (one phosphate or no phosphate groups/HMGA1a molecule) accompanies the later formation of highly condensed chromatin in the apoptotic bodies. We report for the first time a direct link between the degree of phosphorylation of HMGA1a protein and apoptosis according to a process that involves the entire amount of HMGA1a present in the cells and, consequently, whole chromatin. At the same time we report that variously phosphorylated forms of HMGA1a protein are also mono-methylated.     

Expression and function of the homeodomain-containing protein Hex in thyroid cells

The homeodomain-containing protein Hex (also named Prh) is expressed in primitive endoderm (during the early phases of development), in some endoderm-derived tissues and in endothelial and hematopoietic precursors. Hex expression is extinguished during terminal differentiation of endothelial and hematopoietic cells as well as in adult lung. Previous investigations have demonstrated that Hex is expressed during early thyroid gland development. No information has been reported on Hex expression in adult thyroid gland or on the function of this protein in follicular thyroid cells. These issues represent the focus of the present study. We demonstrate that Hex mRNA is present in rat and human adult thyroid gland as well as in differentiated follicular thyroid cell lines. In FRTL-5 cells TSH reduces Hex expression. In thyroid cell lines transformed by several oncogenes Hex expression is completely abolished. By using co-transfection assays we demonstrate that Hex is a repressor of the thyroglobulin promoter and that it is able to abolish the activating effects of both TTF-1 and Pax8. These data would suggest that Hex may play an important role in thyroid cell differentiation. Protein–DNA interaction experiments indicate that Hex is able to bind sites of the thyroglobulin promoter containing either the core sequence 5′-TAAT-3′ or 5′-CAAG-3′. The DNA binding specificity of the Hex homeodomain, therefore, is more ‘relaxed’ than that observed in the majority of other homeodomains.     

Genomic organization and chromosome mapping of the human homeobox gene HHEX.

In the present study, we report the genomic reconstruction of the human homeobox-containing gene HHEX by the use of the data available in public databases. This analysis allowed characterization of the gene organization showing that it is very similar to the mouse gene. Moreover the gene was mapped using FISH to 10q24.     

Discovering high mobility group A molecular partners in tumour cells

DNA-based activities rely on an extremely coordinated sequence of events performed by several chromatin-associated proteins which act in concert. High Mobility Group A (HMGA) proteins are non-histone architectural nuclear factors that participate in the regulation of specific genes but they are also believed to have a more general role in chromatin dynamics. The peculiarity of these proteins is their flexibility, both in terms of DNA-binding and in protein-protein interactions. Since these proteins act as core elements in the assembly of multiprotein complexes called enhanceosomes, and have already displayed the ability to interact with several different proteins, we started a proteomic approach for the systematic identification of their molecular partners. By a combination of affinity chromatography, two-dimensional gel electrophoresis and mass spectrometry we have identified about twenty putative HMGA interactors which could be roughly assigned to three different classes: mRNA processing proteins, chromatin remodelling related factors and structural proteins. Direct HMGA interaction with some of these proteins was confirmed by glutathione-S-transferase pull-down assays and the HMGA domain involved was mapped. Blot-overlay experiments reveal that members of the HMGA family share most of their molecular partners but, interestingly, it seems that there are some cell-type specific partners. Taken together, these experimental data indicate that HMGA proteins are highly connected nodes in the chromatin protein network. Since these proteins are strongly implicated with cancer development, the identification of molecules able to perturb the HMGA molecular network could be a possible tool to interfere with their oncogenic activity.