Adaptive cellular mechanisms in response to glutamine-starvation.

Glutamine (Gln) utilising cells suffer from Gln-starvation during critical illness when plasma Gln levels are decreased. This study investigates whether such cells activate adaptive mechanisms. Monocytic U937 cells were cultured at 0.6 and 0.2 mM Gln for up to four days. Within the first day a decrease of ATP (78% of control), intracellular free Gln (13%), Hsp70 (74%) and proliferation rate (79%) was observed. A prolonged culture at 0.6 mM Gln for additional three days led to a recovery of ATP (97%), Hsp70 (91%) and proliferation (92%). The intracellular free Gln increased only to 41%. At 0.2 mM Gln, however, all levels remained decreased. The activation of the metabolic sensor AMP activated kinase (AMPK) increased immediately in Gln-starving cells but regained normal values only in cells cultured at 0.6 mM. A proteomic analysis identified 23 proteins, which were affected by Gln starvation including metabolic enzymes, proteins involved in synthesis and degradation of RNA and proteins, and stress proteins. These data show that Gln-utilising cells activate adaptive mechanisms in response to Gln-starvation, which enable them to overcome a Gln shortage. At very low Gln concentrations, these adaptive mechanisms are not sufficient to countervail the lack of the amino acid.

SNEV is an evolutionarily conserved splicing factor whose oligomerization is necessary for spliceosome assembly.

We have isolated the human protein SNEV as downregulated in replicatively senescent cells. Sequence homology to the yeast splicing factor Prp19 suggested that SNEV might be the orthologue of Prp19 and therefore might also be involved in pre-mRNA splicing. We have used various approaches including gene complementation studies in yeast using a temperature sensitive mutant with a pleiotropic phenotype and SNEV immunodepletion from human HeLa nuclear extracts to determine its function. A human-yeast chimera was indeed capable of restoring the wild-type phenotype of the yeast mutant strain. In addition, immunodepletion of SNEV from human nuclear extracts resulted in a decrease of in vitro pre-mRNA splicing efficiency. Furthermore, as part of our analysis of protein-protein interactions within the CDC5L complex, we found that SNEV interacts with itself. The self-interaction domain was mapped to amino acids 56-74 in the protein's sequence and synthetic peptides derived from this region inhibit in vitro splicing by surprisingly interfering with spliceosome formation and stability. These results indicate that SNEV is the human orthologue of yeast PRP19, functions in splicing and that homo-oligomerization of SNEV in HeLa nuclear extract is essential for spliceosome assembly and that it might also be important for spliceosome stability.