Alterations in pancreatic and hepatic ultrastructure following acute cycloheximide intoxication

Molecular epidemiology of human cancer

A challenging goal of molecular epidemiology is to identify an individual's risk of cancer. Molecular epidemiology integrates molecular biology, in vitro and in vivo laboratory models, biochemistry and epidemiology to infer individual cancer risk. Molecular dosimetry of carcinogen exposure is an important facet of molecular epidemiology and cancer risk assessment. Carcinogen macromolecular adduct levels, cytogenetic alterations and somatic cell mutations can be measured to determine the biologically effective doses of carcinogens. Molecular epidemiology also explores host cancer susceptibilities, such as carcinogen metabolism, DNA repair, and epigenetic and genetic alterations in tumor suppressor genes. p53 is a prototype tumor suppressor gene and is well suited for analysis of mutational spectrum in human cancer. The analyses of germ line and somatic mutation spectra of the p53 tumor suppressor gene provide important clues for cancer risk assessment in molecular epidemiology. For example, characteristic p53 mutation spectra have been associated with: dietary aflatoxin B1 exposure and hepatocellular carcinoma; sunlight exposure and skin carcinoma; and cigarette smoking and lung cancer. The mutation spectrum also reveals those p53 mutants that provide cells with a selective clonal expansion advantage during the multistep process of carcinogenesis. The p53 gene encodes a multifunctional protein involved in the cellular response to stress including DNA damage and hypoxia. Certain p53 mutants lose tumor suppressor activity and gain oncogenic activity, which is one explanation for the commonality of p53 mutations in human cancer. Molecular epidemiological results can be evaluated for causation by inference of the Bradford-Hill criteria, i.e., strength of association (consistency, specificity and temporality) and biological plausibility, which utilizes the "weight of the evidence principle."

p53 tumor-suppressor gene: clues to molecular carcinogenesis

The tumor-suppressor gene product p53 is clearly a component in several biochemical pathways, including transcription, DNA repair, genomic stability, cell-cycle control and apoptosis, that are central to human carcinogenesis. The p53 is functionally inactivated by mutational, viral, and cellular mechanisms in the majority of human cancers. Analysis of the spectrum of p53 mutations provides clues to the etiology and molecular pathogenesis of cancer. Recent insight into the p53-mediated biochemical pathways of cell-cycle arrest and apoptosis has provided further understanding of the mechanisms related to p53-mediated tumor suppression. This insight in turn may provide the potential molecular targets for the development of rational multimodality cancer therapy, including chemo-, immuno-, and gene-therapeutic strategies. The convergence of previously parallel lines of basic, clinical, and epidemiologic investigation may provide an opportunity to transfer research findings rapidly from the laboratory to the clinic.

The XPB and XPD DNA helicases are components of the p53-mediated apoptosis pathway

The molecular pathway of p53-dependent apoptosis (programmed cell death) is poorly understood. Because p53 binds to the basal transcription-repair complex TFIIH and modulates its DNA helicase activities, we hypothesized that TFIIH DNA helicases XPB and XPD are members of the p53-mediated apoptotic pathway. Whereas transfer of a wild-type p53 expression vector by microinjection or retroviral infection into primary normal human fibroblasts resulted in apoptosis, primary fibroblasts from individuals with xeroderma pigmentosum (XP), who are deficient in DNA repair and have germ-line mutations in the XPB or XPD gene, but not in the XPA or XPC gene, have a deficiency in the apoptotic response. This deficiency can be rescued by transferring the wild-type XPB or XPD gene into the corresponding mutant cells. XP-D lymphocytes also have a decreased apoptotic response to DNA damage by adriamycin, indicating a physiologically relevant deficiency. The XP-B or XP-D mutant cells undergo a normal apoptotic response when microinjected with the Ich-L, and ICE genes. Analyses of p53 mutants and the effects of microinjected anti-p53 antibody, Pab421, indicate that the carboxyl terminus of p53 may be required for apoptosis. Direct microinjection of the p53 carboxy-terminal-derived peptide (amino acid residues 319-393) resulted in apoptosis of primary normal human fibroblasts. These results disclose a novel pathway of p53-induced apoptosis.

An electron microscopic study of apoptosis induced by cycloheximide in rat liver

A histological and ultrastructural study, coupled with transmission and scanning electron microscopy of the early changes in the liver following a single administration of cycloheximide (CHX), was carried out in male Wistar rats. At the histological level, apoptosis was already present in the liver 2 h after treatment. By scanning electron microscopy, the following sequential changes were observed: brightness and progressive detachment of hepatocytes from neighbouring cells, formation of surface infolds with multiple blebs and, finally, release of several membrane-bounded apoptotic bodies (ABs) in the extracellular space and into the sinusoidal lumen. Three hours after CHX administration, the apoptotic cycle was completed, as shown by the presence of phagocytosed ABs inside the cytoplasm of intact liver cells. Light microscopic examination of the liver 6 h after CHX administration showed ABs mainly located in the cytoplasm of intact hepatocytes and inside activated Kupffer cells. By transmission electron microscopy, it was possible to demonstrate that cells undergoing apoptosis were hepatocytes. At 24 h, the livers of treated animals appeared normal, with no evidence of apoptosis.

Ultrastructural changes of chick bursal epithelial cells experimentally infected with Cryptosporidium sp

The ultrastructural changes in the bursal epithelial cells of chickens infected with Cryptosporidium sp. were investigated. The principal features were as follows: deep invaginations or indentations of the nucleus with separate lobulations in some instances, nucleolar hypertrophy with formation of reticular or open nucleolonema, increase or swelling of mitochondria, increase in coated vesicles, ribosomes and well-developed rough endoplasmic reticulum (RER), increase in mitochondrial-RER association, appearance of small, smooth-membraned vacuoles in the apical cytoplasm, and formation of electron dense fibres. In addition, nuclear bodies, thin-shelled or ring-shaped nucleoli, nucleolar segregation, nucleolar margination, ribo-some-lamella complex, microtubuloreticular complex, disaggregation of polysomes and vesiculation of RER were rarely observed. Most of the ultrastructural changes observed here seemed to be related to a heightened metabolic activity, namely active or increased protein synthesis, in response to the parasitism.

Scattering of the silver-stained proteins of nucleolar organizer regions in Ishikawa cells by actinomycin D

Scattering of the silver-stained proteins of nucleolar organizer regions (Ag-NOR proteins) was produced by actinomycin D in Ishikawa cells. Scattering of Ag-NOR proteins was found only in cells treated with actinomycin D and various other agents had no effect. Scattering was dose-dependent up to 10(-2) micrograms/ml of actinomycin D, but it was not found at higher concentrations that caused marked inhibition of total DNA and RNA synthesis. Actinomycin D (10(-2) micrograms/ml) caused the following changes: (i) nucleolar segregation and (ii) emergence of dense fibrillar bodies in the nucleoplasm. Ag-NOR proteins were observed on the fibrillar centers and surrounding fibrillar components in control nucleoli, on the fibrillar and amorphous zones in segregated nucleoli, and on the dense fibrillar bodies emerging in the nucleoplasm. The scattering of Ag-NOR proteins was due to the argyrophilic nature of the dense fibrillar bodies. Actinomycin D (10(-1) micrograms/ml) also caused similar morphological alterations in the nucleolus and nucleoplasm, but Ag-NOR proteins were observed only on nucleolar remnants.

Effect of modeccin on rat liver ribosomes in vivo

1. Rat liver microsomes isolated at 6 and 12 h of poisoning with 3 x LD50 (0.3 microgram/100 g body wt.) of modeccin, the toxin of Adenia digitata, have a decreased capacity of protein synthesis in vitro. 2. A similar decrease of protein synthesis is observed with polysomes at 6 h of poisoning. Experiments with recombined ribosomal subunits demonstrate that this is due to inactivation of the 60 S ribosomal subunit. 3. At 6 h of poisoning there is a marked vesiculation and degranulation of the hepatocyte rough endoplasmic reticulum, which is completely fragmented at 24 h of poisoning. Hepatocyte mitochondria are swollen at 6 h and shrunk at 24 h of poisoning. 4. It is concluded that modeccin penetrates inside hepatocytes in vivo, and damages ribosomes in the same manner as it does in vitro. However, mitochondrial damage indicates that ribosomes may not be the only target of modeccin in vivo.

Effects of puromycin and actinomycin D on fertilized rabbit eggs cultured in vitro

Rabbit zygotes were cultured in media containing varying concentrations of puromycin and actinomycin D in order to determine what effect these agents have on events of fertilization and early development. Concentrations of puromycin greater than 10(-5) M inhibited mitosis, cytokinesis of fertilized eggs and leucine incorporation. In approximately 50% of the zygotes treated with puromycin (10(-5) M or greater) the female pronucleus remained at its site of formation, the cortex, and did not migrate centrad. In these instances the male pronucleus moved from its site of development to the female pronucleus. Concentrations of actinomycin D greater than 10(-7) M also inhibited mitosis and cytokinesis of fertilized eggs. Breakdown of the nuclear envelopes and association of the paternally- and maternally-derived chromatin did not occur in either puromycin- or actinomycin D-treated embryos. Actinomycin D at all of the concentrations employed (10(-9) to 10(-4) M) had little effect upon the structure of nucleolus-like bodies within male and female pronuclei. Male and female pronuclei of puromycin (10(-4) M) and actinomycin D (10(-5) M) treated zygotes, incubated with tritiated thymidine and prepared for scintillation counting and autoradiography, synthesized DNA at a reduced level when compared to control specimens. Although "side effects" of puromycin and actinomycin D treatment are not excluded, these results suggest that nascent proteins may be required for events of fertilization.

4-Hyroxyaminoquinoline-1-oxide-induced ultrastructural changes in the guinea-pig exocrine pancreas

4-hydroxyaminoquinoline, a carcinogen was injected into guinea-pigs intravenously in a single dose of 22-5 mg/kg. The sequential morphological changes in the exocrine cells of the pancreas were studied by electron microscopy at selected intervals between 18 and 60 hr. The earliest alterations in the acinar cells comprised nucleolar segregation into granular and fibrillar components and minimal dilatation of endoplasmic reticulum. At later intervals all the cytoplasmic organelles appeared altered. The endoplasmic reticular channels were markedly dilated with depletion of membrane bound and free ribosomes. The mitochondria were swollen and irregular with disruption of limiting membranes and cristae. The zymogen granules were markedly reduced or completely absent. The hyaloplasm contained morphologically variable cytoplasmic degenerative bodies. In addition to the acinar cells, centriacinar and ductal epithelial cells showed similar changes.