Caspase-mediated cleavage of the TIAM1 guanine nucleotide exchange factor during apoptosis.

Rho family GTPases Rac and Cdc42 are pivotal regulators of apoptosis in multiple cell types. However, little is known about the mechanism by which these GTPases are regulated in response to apoptotic stimuli. Here, we demonstrate that TIAM1, a Rac-specific guanine nucleotide exchange factor, is cleaved by caspases during apoptosis. TIAM1 cleavage occurs in multiple cell lines in response to diverse apoptotic stimuli such as ceramide, Fas, and serum deprivation. Processing occurs at residue 993 of TIAM1 and removes the NH(2)-terminal of TIAM's two pleckstrin homology domains, leaving a stable fragment containing the Dbl homology and COOH-terminal pleckstrin homology domains. This leads to functional inactivation of TIAM1, as determined by failure of the cleavage product to stimulate GTP loading of Rac in vivo. Furthermore, this product is defective in signaling to two independent Rac effectors, c-Jun NH(2)-terminal kinase and serum response factor. Finally, we demonstrate that in cells treated with ceramide, cleavage of TIAM1 coincided with the inactivation of endogenous Rac. These results reveal a novel mechanism for regulating guanine nucleotide exchange factor activity and GTPase-mediated signaling pathways.

Effects of cocaine context on NAcc dopamine and behavioral activity after repeated intravenous cocaine administration.

In two conditioning experiments, identical procedures (previously shown to produce place preferences for a cocaine-paired environment) were used to assess dopaminergic and behavioral activity correlates of cocaine reward conditioning and sensitization. In these experiments, animals received repeated injections of intravenous cocaine (4.2 mg/kgx6) or saline (0.2 mlx6) on alternating days. One group in each of these experiments ('Cocaine Cues') occupied a consistent distinctive environment during cocaine treatments and testing sessions. For the other conditioned group ('Novel'), all procedures were the same, except that the last cocaine injection was administered while animals were occupying a novel environment. During day 1 and day 6 of the cocaine treatment, behavioral activity was assessed in experiment 1 and in vivo microdialysis procedures were conducted in experiment 2. Over the course of the conditioning sessions, cocaine-induced behavioral activity (locomotion and rearing) increased significantly in the Cocaine Cues group, but not in the Novel group. In addition, cocaine-induced increases in NAcc dopamine levels were significantly greater when cocaine-experienced animals were tested in a cocaine-paired environment compared to equally experienced and cocaine-naive animals tested in a novel environment. Context-dependent behavioral sensitization is a well-documented phenomenon. The observation of a corresponding enhancement of dopamine efflux in lieu of a lengthy withdrawal period is uncommon, but can be attributed to methodological differences across studies. The present study uniquely demonstrates concurrent context-dependent potentiation of behavioral and dopaminergic responses to cocaine occurring in conjunction with cocaine reward.

Aflatoxins in animal and human health.

Aflatoxins remain as a threat to the health of livestock as well as humans by their continuing intermittent occurrence in both feeds and foods. The finding that aflatoxin-contaminated feeds, and eventually purified aflatoxins, were carcinogenic in rats and trout initiated a multitude of studies in search of the role of these toxins in human liver disease, especially cancer. Although aflatoxins have caused acute liver disease in humans, epidemiologic evidence of the involvement of aflatoxins in PLC has not been clarified. Earlier studies did not consider that the hepatitis B virus (HBV) may have contributed to the PLC in the selected populations. Although later studies that did include measurement of the HBV antigen in serum provided conflicting evidence for the role of aflatoxin in PLC in these populations, the latest and most comprehensive study found no association between aflatoxin exposure and PLC mortality. The technological advances and findings of the chemical, immunologic, and metabolic activities of aflatoxins such as binding to DNA and protein to form adducts, development of monoclonal antibodies, and mutational specificity of the genotoxic compounds will, it is hoped, help to clarify the role of aflatoxin as a risk factor, among many others, in the development of primary liver cancer in humans. Aflatoxicosis of animals is usually manifested by pathologic changes in the liver, but they have been found to be carcinogenic and teratogenic as well as causing impaired protein formation, coagulation, weight gains, and immunity. The importance of the carcinogenic effect in livestock is diminished because they are not fed contaminated diets for a sufficient time prior to marketing for slaughter. Animals are variably susceptible to aflatoxins, depending on such factors as age, species, breed, sex, nutrition, and certain stresses. Swine, cattle, and poultry are the domestic species of greatest economic concern in terms of aflatoxicosis. In all species, the evidence of disease is a general unthriftiness and reduction in weight gains, feed efficiency, immunity, and production. More conclusive evidence of aflatoxin involvement in disease includes acute to chronic liver disease with concomitant increases in specific liver enzymes in the serum. In cattle, milk production is affected, but of greater significance is that the aflatoxins in feeds can be rather efficiently converted to toxic metabolites in milk, with even small amounts being readily detectable. The poultry industry probably suffers greater economic loss than any of the livestock industries because of the greater susceptibility of their species to aflatoxins than other species.(ABSTRACT TRUNCATED AT 400 WORDS)

Assessment of the safety of drugs for animals.

In summary, the evaluation of the safety of drugs for animals should take advantage of the fact that toxicity can be evaluated in the target species. Acute studies are useful in providing information for dose setting and the possible relevant observations that can be incorporated in the protocol for a longer, more definitive study; however, they are of limited value in the final evaluation of a drug. Longer tests should be conducted to mimic the conditions of use as much as possible, using labeled and exaggerated doses. Flexible protocols should be individually designed for each study. Clinical studies, conducted in several locations at different times of the year if appropriate, should supplement the laboratory studies. New equipment and methodology facilitate measuring even more subtle toxic effects. It is easy to evaluate a compound that kills animals, that causes colic or other gastro-intestinal signs, or that results in serious neurological impairment, such as circling or blindness, shortly after treatment. But the line is blurred between subtle toxic effects and physiological change. When a rise in serum enzymes, an increase in smooth endoplasmic reticulum or a decrease in formed elements of the blood is demonstrated, the significance of the effects produced must be evaluated in the context of the clinical aspects of the proposed usage. A knowledge of medicine and/or drug action and interaction is necessary to make this appraisal. It is incumbent upon the sponsor to discuss thoroughly all aspects of the toxic effects detected, and it is incumbent upon the reviewer to understand them.