The DNA-dependent protein kinase catalytic subunit (p460) is cleaved during Fas-mediated apoptosis in Jurkat cells.

The DNA-dependent protein kinase (DNA-PK) is a serine/threonine kinase linked to DNA repair and V(D)J recombination. It is composed of a 460-kDa catalytic subunit (DNA-PKcs) and a 70/86-kDa heterodimeric regulatory component that is identical with the human autoantigen Ku. The regulatory subunit targets the catalytic subunit to the free ends of dsDNA breaks. Since apoptosis is associated with internucleosomal chromatin fragmentation and creation of dsDNA breaks, we examined whether the biochemical amounts of either DNA-PKcs or Ku changed during apoptosis mediated by the cell surface receptor Fas. We found that the catalytic subunit was cleaved into several smaller polypeptides early in apoptosis. In contrast to DNA-PKcs, Ku was neither cleaved nor decreased in amount during apoptosis. We then extended our in vivo results to a cellfree system. Cytosolic extracts derived from apoptotic cells were able to cleave DNA-PKcs into polypeptides of sizes identical with those seen in vivo, and this cleavage was inhibited by the cysteine protease inhibitors iodoacetamide and N-ethylmaleimide. Furthermore, DNA-PKcs was cleaved in vitro by purified apopain (CPP32), but not IL-1beta-converting enzyme. Cleavage was also inhibited by the specific tetrapeptide DEVD (amino acids 2709-2712 of the DNA-PKcs sequence), suggesting a candidate position for protease action. Finally, we found that the catalytic activity of DNA-PKcs was decreased in apoptotic cells. We conclude that DNA-PKcs is subject to selective cleavage by proteases during apoptosis. Cleavage of DNA-PKcs may represent a mechanism for regulating the function of DNA-dependent kinase during programmed cell death.

Expression of a cut-related homeobox gene in developing and polycystic mouse kidney.

Cut is a diverged homeobox gene that is essential for normal development of the Malpighian tubules in Drosophila melanogaster. Homologues of Drosophila cut that encode transcriptional repressors have been identified in several mammalian species and cell lineages. We examined the expression of a murine cut homologue (named Cux-1) in the developing mouse using Northern blot analysis and in situ hybridization. At 12.5 d.p.c. and 13.5 d.p.c., Cux-1 was highly expressed in a subset of embryonic tissues, including the developing metanephros. Within the metanephros, Cux-1 was expressed in the nephrogenic zone including both mesenchymal cells (uninduced and condensed mesenchyme) and epithelial cells (ureteric buds, renal vesicles, S-shaped bodies). During later stages of nephrogenesis, Cux-1 was down-regulated such that there was minimal expression in mature glomeruli and tubules. In addition, Cux-1 was detected in the mesonephros, mesonephric duct, and bladder. Expression of Cux-1 was also examined in polycystic kidneys from C57BL/6J-cpk/ cpk mice. At 21 days of age, Cux-1 was highly expressed in cyst epithelium of polycystic kidneys but was minimally expressed in kidneys from phenotypically normal littermates. These results demonstrate that a cut-related homeobox gene is expressed in the developing kidney and urinary tract of the mouse. Expression of Cux-1 in the kidney is inversely related to degree of cellular differentiation. Cux-1 may encode a transcriptional repressor that inhibits terminally differentiated gene expression during early stages of nephrogenesis.

Correlation between urea reduction rates and serum albumin levels in patients on hemodialysis.

Monthly urea reduction rates and serum albumin levels were measured in 12 dialysis patients during a year when standard hemodialysis was replaced by high efficiency hemodialysis without changing treatment duration. Initially, mean urea reduction rates were 57-58% and mean serum albumin levels were 3.6-3.7 g/dl. Introduction of high efficiency hemodialysis gradually increased mean urea reduction rates to 68-70% (an increase in dialysis delivery of 40%). There was a parallel increase in mean albumin levels by 0.5 g/dl to 4.1-4.2 g/dl two months after the rise in urea reduction rate. Increased insulin administration was necessary to compensate for probable increased caloric intake in diabetic patients. Hemodialysis patients achieve improvement in nutritional status after dialysis delivery is increased using high efficiency hemodialysis without decreasing treatment duration.

Expression of two structurally identical viral superantigens results in thymic elimination at distinct developmental stages.

Mouse mammary tumor virus proviral integrants encode superantigens. Developing thymocytes bearing TCRs with particular V beta elements encounter these endogenous viral superantigens as self molecules in the thymus and are consequently clonally eliminated. To study this mechanism of tolerance induction, we have bred B10.BR-Mtv-1 and B10.BR-Mtv-6 mice, which carry either Mtv-1 or Mtv-6 proviruses but are otherwise genetically identical. The protein products of these mouse mammary tumor virus integrants, vSAG1 and vSAG6, both interact with V beta 3+ T cells and have identical amino acid sequences. Interestingly, vSAG6 expression results in the complete deletion of V beta 3+ peripheral T cells, whereas vSAG1 expression results in only partial deletion. Flow cytometric analyses indicate that B10.BR-Mtv-6 mice delete V beta 3+ thymocytes at the immature CD4+8+ stage, whereas B10.BR-Mtv-1 mice delete only mature CD4+ or CD8+ cells. In addition, the two strains exhibit different time courses of thymic deletion: neonatal B10.BR-Mtv-6 mice eliminate V beta 3+ T cells by day 2, in contrast to B10.BR-Mtv-1 mice in which deletion does not occur until day 15. RNase protection assays demonstrate that B10.BR-Mtv-6 mice have significantly greater thymic vSAG6 mRNA expression levels than vSAG1 levels in B10.BR-Mtv-1 animals, correlating with a more complete deletion of reactive thymocytes at an earlier point in the maturational sequence.

Effects of inhibitors on anion exchangers in rabbit renal brush border membrane vesicles.

Cl-formate, Cl-oxalate, and SO4-CO3 exchange participate in Cl and organic anion transport across the brush border membrane of the rabbit proximal tubule. To determine the functional similarity of these transporters to each other and to band 3, we characterized, in isolated membrane vesicles, the inhibition of these transporters by compounds known to inhibit erythrocyte band 3. 4,4'-Dinitro-2,2'-disulfonic stilbene (DNDS), diphenylamine-2-carboxylate (DPC), flufenamate, and 4-aceto-4'-isothiocyano-2,2'-disulfonic stilbene (SITS) were effective inhibitors of Cl-oxalate and SO4-CO3 exchange, suggesting at least some common structural motifs between these exchangers and band 3. Cl-formate exchange was relatively insensitive to DNDS and DPC but sensitive to flufenamate (IC50 = 43 microM). Sensitivity to DNDS but not 4-amino-4'-amino-2,2'-disulfonic stilbene, a feature of band 3, was seen only for the SO4-CO3 exchanger. None of the exchangers had significant affinity for dipyridamole, furosemide, or probenecid. Finally, the presence of DPC or flufenamate increased the IC50 for reversible inhibition by DNDS, consistent with at least a partial overlap between the disulfonic stilbene and diphenylamine carboxylate binding sites of the Cl-oxalate exchanger. We next examined the effect of irreversible SITS binding. The Cl-oxalate exchanger was inhibited 90%, consistent with its high affinity for reversible inhibition by stilbenes. SITS pretreatment caused 50% inhibition of the Cl-formate exchanger, consistent with the reduced affinity of this exchanger for reversible binding of stilbenes. Despite the high sensitivity of the SO4-CO3 exchanger to reversible inhibition by stilbenes, SITS pretreatment caused < 20% irreversible inhibition of this exchanger. Finally, we characterized the stilbene inhibition of the Cl-oxalate exchanger in more detail. The presence of oxalate increased the IC50 for reversible inhibition by DNDS or SITS, implying that oxalate can directly compete at the reversible stilbene binding site of the Cl-oxalate exchanger. However, oxalate could not protect against covalent inactivation of the Cl-oxalate exchanger by SITS, indicating the presence of a separate site for irreversible binding of disulfonic stilbenes. These results suggest a dissociation between the sensitivities of proximal tubule anion exchangers to reversible and irreversible inhibition by disulfonic stilbenes. In contrast to band 3, the Cl-oxalate exchanger must possess separate sites for reversible and irreversible interaction with stilbenes, with only the former overlapping the substrate binding site.