[Abstract unavailable

In response to various environmental stresses, eukaryotic cells down-regulate protein synthesis by phosphorylation of the ␣ subunit of eukaryotic translation initiation factor 2 (eIF-2␣). In mammals, the phosphorylation was shown to be carried out by eIF-2␣ kinases PKR and HRI. We report the identification and characterization of a cDNA from rat pancreatic islet cells that encodes a new related kinase, which we term pancreatic eIF-2␣ kinase, or PEK. In addition to a catalytic domain with sequence and structural features conserved among eIF-2␣ kinases, PEK contains a distinctive amino-terminal region 550 residues in length. Using recombinant PEK produced in Escherichia coli or Sf-9 insect cells, we demonstrate that PEK is autophosphorylated on both serine and threonine residues and that the recombinant enzyme can specifically phosphorylate eIF-2␣ on serine-51. Northern blot analyses indicate that PEK mRNA is expressed in all tissues examined, with highest levels in pancreas cells. Consistent with our mRNA assays, PEK activity was predominantly detected in pancreas and pancreatic islet cells. The regulatory role of PEK in protein synthesis was demonstrated both in vitro and in vivo. The addition of recombinant PEK to reticulocyte lysates caused a dose-dependent inhibition of translation. In the Saccharomyces model system, PEK functionally substituted for the endogenous yeast eIF-2␣ kinase, GCN2, by a process requiring the serine-51 phosphorylation site in eIF-2␣. We also identified PEK homologs from both Caenorhabditis elegans and the puffer fish Fugu rubripes, suggesting that this eIF-2␣ kinase plays an important role in translational control from nematodes to mammals.

Oxidative stress causes mitochondrial dysfunction and metabolic complications through unknown mechanisms. Cardiolipin (CL) is a key mitochondrial phospholipid required for oxidative phosphorylation. Oxidative damage to CL from pathological remodeling is implicated in the etiology of mitochondrial dysfunction commonly associated with diabetes, obesity, and other metabolic diseases. Here we show that ALCAT1, a lyso-CL acyltransferase up-regulated by oxidative stress and diet-induced obesity (DIO), catalyzes the synthesis of CL species which are highly sensitive to oxidative damage, leading to mitochondrial dysfunction, ROS production, and insulin resistance. These metabolic disorders were reminiscent of those observed in type 2 diabetes, and were reversed by rosiglitazone treatment. Consequently, ALCAT1 deficiency prevented the onset of DIO and significantly improved mitochondrial complex I activity, lipid oxidation, and insulin signaling in ALCAT1−/− mice. Collectively, these findings identify a key role of ALCAT1 in regulating CL remodeling, mitochondrial dysfunction, and susceptibility to DIO.

Cardiolipin is a major membrane polyglycerophospholipid that is required for the reconstituted activity of a number of key mitochondrial enzymes involved in energy metabolism. Cardiolipin is subjected to remodeling subsequent to its de novo biosynthesis to attain appropriate acyl composition for its biological functions. Yet, the enzyme(s) involved in the remodeling process have not been identified. We report here the identification and characterization of a murine gene that encodes an acyl-CoA:lysocardiolipin acyltransferase 1 (ALCAT1). Expression of the ALCAT1 cDNA in either insect or mammalian cells led to a significant increase in acylCoA:monolysocardiolipin acyltransferase and acyl-CoA: dilysocardiolipin acyltransferase activities that exhibited a dependence upon ALCAT1 enzyme levels. The recombinant ALCAT1 enzyme recognizes both monolysocardiolipin and dilysocardiolipin as substrates with a preference for linoleoyl-CoA and oleoyl-CoA as acyl donors. In contrast, no significant increases in acyltransferase activities by the recombinant ALCAT1 were detected against either glycerol-3-phosphate or a variety of other lysophospholipids as substrates, including lysophosphatidylcholine, lysophosphatidylethanolamine, and lysophosphatidylserine. Immunocytohistochemical analysis showed that the ALCAT1 enzyme is localized in the endoplasmic reticulum, which is supported by a significant ALCAT activity in isolated liver and heart microsomes. Northern blot analysis indicates that the mouse ALCAT1 is widely distributed, with the highest expression in heart and liver. In support of a role for ALCAT1 in maintaining heart function, the ALCAT1 gene is conserved among different species of vertebrates, but not in non-atrium organisms. ALCAT1 represents the first identified cardiolipin-remodeling enzyme from any living organism; its identification implies a novel role for the endoplasmic reticulum in cardiolipin metabolism.