Citotoxidad de la beta lapachona: una o-naftoquinona con posibles usos terapeuticos / Cytotoxicity of beta-lapachone, an naphthoquinone with possible therapeutic use

La Beta-lapachona (Beta-lap) es una o-naftoquinona extraída de la madera del lapacho. Las observaciones iniciales mostraron su acción inhibidora del crecimiento del sarcoma de Yoshida y del carcinosarcoma de Walker 256. La Beta-lap genera productos reactivos del oxígeno (ROS: anión superóxido, radical hidroxilo y peróxido de hidrógeno) a los que inicialmente se atribuyó su citotoxicidad. Beta-Lap resultó un potente inhibidor de la síntesis de ADN en T. cruzi, de la topoisomerasas I y II y de la poli(ADP-ribosa) polimerasa (PARP) de diferentes orígenes, enzimas responsables de la conservación del ADN. Se investigó la citotoxicidad de Beta-lap en células de cáncer epidermoide de laringe, melanoma, cáncer de ovario, de mama, de próstata, de pulmón, adenocarcinoma de colon y leucemia, aportando un mejor conocimiento de los mecanismos moleculares involucrados en la acción de Beta-lap y su relación con los procesos de apoptosis y de necrosis. Se comprobó la activación de la calpaina, proteasa cuya actividad depende de tioles, seguida por la activación de quinasas (c-JUN NH2 -quinasa terminal), caspasas y nucleasas, enzimas que degradan al ADN y a las proteínas celulares. Una reacción importante para la actividad de la Beta-lap es su reducción, especialmente por la diaforasa y la NAD(P)H-quinona reductasa, que inician la producción de ROS. La acción de Beta-lap sobre células tumorales resultaría de la inhibición directa de enzimas como las topoisomerasas, PARP y el factor TNF, sumada a la acción de radicales libres. Los efectos citostáticos de ß-lap han abierto interesantes perspectivas para la quimioterapia del cáncer.

The Levels of MDM2 Protein Are Decreased by a Proteasome-Mediated Proteolysis Prior to Caspase-3-Dependent pRb and PARP Cleavages

MDM2 is a substrate of caspase-3 in p53-mediated apoptosis. In addition, MDM2 mediates its own ubiquitination in a RING finger-dependent manner. Thus, we investigated whether MDM2 is degraded through a ubiquitin-dependent proteasome pathway in the absence of p53. When HL-60 cells, p53 null, were treated with etoposide, MDM2 was markedly decreased prior to caspase-3-dependent retinoblastoma tumor suppressor protein (pRb) and poly (ADP- ribose) polymerase (PARP) cleavages. Moreover, down-regulation of MDM2 level was not coupled with its mRNA down-regulation. However, the level of MDM2 was partially restored by proteasome inhibitors such as LLnL and lactacystin, even in the presence of etoposide. Our results suggest that, in the p53 null status, MDM2 protein level is decreased by proteasome-mediated proteolysis prior to caspase-3-dependent PARP and pRb cleavages.

-Melanocyte Stimulating Hormone (MSH) decreases cyclosporine A induced apoptosis in cultured human proximal tubular cells

The pathogenesis of chronic cyclosporine A (CsA) nephrotoxicity has not been elucidated, but apoptosis is thought to play an important role in CsA induced tubular atrophy. Recently Fas-Fas ligand system mediated apoptosis has been frequently reported in many epithelial cells as well as in T lymphocytes. We investigated the ability of CsA to induce apoptosis in cultured human proximal tubular epithelial cells and also the effect of -MSH on them. Fas, Fas ligand, and an intracellular adaptor protein, Fas-associating protein with death domain (FADD) expression, and poly-ADP ribose polymerase (PARP) cleavage were also studied. CsA induced apoptosis in cultured tubular epithelial cells demonstrated by increased number of TUNEL positive cells and it was accompanied by a significant increase in Fas mRNA and Fas ligand protein expressions. FADD and the cleavage product of PARP also increased, indicating the activation of caspase. In -MSH co-treated cells, apoptosis markedly decreased with downregulation of Fas, Fas ligand and FADD expressions and also the cleavage product of PARP. In conclusion, these data suggest that tubular cell apoptosis mediated by Fas system may play a role in tubular atrophy in chronic CsA nephrotoxicity and pretreatment of -MSH may have a some inhibitory effect on CsA induced tubular cell apoptosis.

Development of two novel nontoxic mutants of Escherichia coli heat-labile enterotoxin

Escherichia coli heat-labile enterotoxin (LT) is composed of catalytic A and non-catalytic homo-pentameric B subunits and causes diarrheal disease in human and animals. In order to produce a nontoxic LT for vaccine and adjuvant development, two novel derivatives of LT were constructed by a site-directed mutagenesis of A subunit; Ser63 to Tyr63 in LTS63Y and Glu110, Glu112 were deleted in LT delta 110/112. The purified mutant LTs (mLTs) showed a similar molecular structural complex as AB5 to that of wild LT. In contrast to wild-type LT, mLTs failed to induce either elongation activity, ADP-ribosyltransferase activity, cAMP synthesis in CHO cells or fluid accumulation in mouse small intestine in vivo. Mice immunized with mLTs either intragastrically or intranasally elicited high titers of LT-specific serum and mucosal antibodies comparable to those induced by wild-type LT. These results indicate that substitution of Ser63 to Tyr63 or deletion of Glu110 and Glu112 eliminate the toxicity of LT without a change of AB5 conformation, and both mutants are immunogenic to LT itself. Therefore, both mLTs may be used to develop novel anti-diarrheal vaccines against enterotoxigenic E. coli.