Genetics of growth arrest and cell death: key determinants of tissue homeostasis

Cell density-dependent reduction of dihydroceramide desaturase activity in neuroblastoma cells

We applied a metabolic approach to investigate the role of sphingolipids in cell density-induced growth arrest in neuroblastoma cells. Our data revealed that sphingolipid metabolism in neuroblastoma cells significantly differs depending on the cells' population context. At high cell density, cells exhibited G0/G1 cell-cycle arrest and reduced ceramide, monohexosylceramide, and sphingomyelin, whereas dihydroceramide was significantly increased. In addition, our metabolic-labeling experiments showed that neuroblastoma cells at high cell density preferentially synthesized very long chain (VLC) sphingolipids and dramatically decreased synthesis of sphingosine-1-phosphate (S1P). Moreover, densely populated neuroblastoma cells showed increased message levels of both anabolic and catabolic enzymes of the sphingolipid pathway. Notably, our metabolic-labeling experiments indicated reduced dihydroceramide desaturase activity at confluence, which was confirmed by direct measurement of dihydroceramide desaturase activity in situ and in vitro. Importantly, we could reduce dihydroceramide desaturase activity in low-density cells by applying conditional media from high-density cells, as well as by adding reducing agents, such as DTT and L-cysteine to the media. In conclusion, our data suggest a role of the sphingolipid pathway, dihydroceramides desaturase in particular, in confluence-induced growth arrest in neuroblastoma cells.

Human papillomavirus and vulval intra-epithelial neoplasia

The association between human papillomavirus vulval skin infection and vulval intra-epithelial neoplasia is strong. Vulval skin carcinogenesis is, however, multifactorial. Both human papillomavirus oncogenic subtype infection and p53 mutations are likely to contribute to the risk of malignant transformation of normal epithelium. The long-term cumulative risk of developing vulval squamous carcinoma following the diagnosis of high-grade vulval intra-epithelial neoplasia is thought to be approximately 6% per decade, but observational data supporting this are mostly non-population based and retrospective. Surgical treatment may reduce this risk, but the rates of recurrence and treatment-related morbidity are high. Surveillance should therefore be discussed as an alternative. New treatments being researched include photodynamic therapy, human papillomavirus vaccines, immunotherapy, immune modulators and gene therapy. The advantages of these new modalities over surgery is the potential to preserve body image and sexual functioning while targeting more generalized epithelial molecular dysfunction.

The patterns of expression of an apoptosis-related CK18 neoepitope, the bcl-2 proto-oncogene, and the Ki67 proliferation marker in normal, hyperplastic, and malignant endometrium

Expression of a neoepitope on cytokeratin 18, recognized by the monoclonal antibody M30, is an early indicator of apoptosis in epithelial cells. The aim of this study was to determine the equilibrium between apoptosis (M30), anti-apoptosis (bcl-2), and proliferation (Ki-67) in different endometrial conditions. Paraffin-embedded samples (n = 107), representing proliferative endometrium (18), secretory endometrium (19), postmenopausal endometrium (15), disordered proliferative endometrium (6), simple hyperplasia (12), complex hyperplasia (8), and endometrial adenocarcinoma (29), were evaluated immunohistochemically. The indirect streptavidin-biotin-horseradish peroxidase technique, with 3-amino-9-ethylcarbazole as the chromogen, was used to visualize the reactions. Proliferative endometrium showed high bcl-2 and Ki-67 expression levels with no M30. In the secretory phase, the balance was tipped in favor of M30 with a decrease of bcl-2 and Ki-67. Postmenopausal endometrium revealed high Ki-67 and bcl-2 expression levels and no M30. In complex hyperplasia, M30, bcl-2, and Ki-67 showed increased expression. In endometrial carcinoma, an increasing reactivity for M30 and Ki-67 was seen as the grade progressed. bcl-2 reacted weakly and only in grade 1 cancer. Immunohistochemistry facilitates the study of the expression of proteins related to cyclic endometrial activity. Interruption of these cyclic events is associated with specific disturbances in the expression patterns of these proteins.

Apoptosis after gamma irradiation. Is it an important cell death modality?

Apoptosis and necrosis are two different forms of cell death that can be induced by cytotoxic stress, such as ionizing radiation. We have studied the importance of apoptotic death induced after treatment with 6 Gy of gamma-irradiation in a panel of eight human tumour cell lines of different radiosensitivities. Three different techniques based on the detection of DNA fragmentation have been used, a qualitative one--DNA ladder formation --and two quantitative approaches--in situ tailing and comet assay. No statistically significant relationship between the two quantitative assays was found (r= 0.327, P = 0.159) so these methods seem to show different aspects of the process of cell death. The presence of the DNA ladder related well to the end-labelling method in that the least amount of end labelling was seen in samples in which necrotic degradation rather than apoptotic ladders were seen. However, as the results obtained by the comet assay are not in agreement with the DNA ladder experiments, we suggest that the distinction between the degraded DNA produced by apoptosis and necrosis may be difficult by this technique. Finally, although apoptosis has been proposed to be dependent on p53 functionality, and this may explain differences in cellular radiosensitivity, no statistically significant relationship was found between these parameters and apoptosis in the eight cell lines studied.

Molecular events in uterine cervical cancer

OBJECTIVE

To review the literature regarding the molecular events which occur in the development of uterine cervical cancer, with particular reference to human papillomavirus (HPV) infection.

METHODOLOGY

Bibliographic searches of Medline and the ISI citation databases using appropriate keywords, including the following: papillomavirus, cervix, pathology, cyclin, chromosome, heterozygosity, telomerase, smoking, hormones, HLA, immune response, HIV, HSV, EBV.

CONCLUSIONS

It has become clear that most cervical neoplasia, whether intraepithelial or invasive, is attributable in part to HPV infection. However, HPV infection alone is not sufficient, and, in a small proportion of cases, may not be necessary for malignant transformation. There is increasing evidence that HPV gene products interfere with cell cycle control leading to secondary accumulation of small and large scale genetic abnormalities. This may explain the association of viral persistence with lesion progression but, in many patients, secondary factors, such as smoking and immune response, are clearly important. However, the mechanisms involved in the interaction between HPV and host factors are poorly understood.

Upregulation of apoptosis with dietary restriction: implications for carcinogenesis and aging

The maintenance of cell number homeostasis in normal tissues reflects a highly regulated balance between the rates of cell proliferation and cell death. Under pathologic conditions such as exposure to cytotoxic, genotoxic, or nongenotoxic agents, an imbalance in these rates may indicate subsequent risk of carcinogenesis. Apoptotic cell death, as opposed to necrotic cell death, provides a protective mechanism by selective elimination of senescent, preneoplastic, or superfluous cells that could negatively affect normal function and/or promote cell transformation. The relative efficiency or dysfunction of the cell death program could therefore have a direct impact on the risk of degenerative or neoplastic disease. Dietary restriction of rodents is a noninvasive intervention that has been reproducibly shown to retard tumor development and most physiologic indices of aging relative to ad libitum-fed animals. As such, it provides a powerful model in which to study common mechanistic processes associated with both aging and cancer. In a recent study we established that chronic dietary restriction (DR) induces an increase in spontaneous apoptotic rate and a decrease in cell proliferation rate in hepatocytes of 12-month-old B6C3F1 DR mice relative to ad libitum (AL)-fed mice. This diet-induced shift in cell death/proliferation rates was associated with a marked reduction in subsequent development of spontaneous hepatoma and a marked increase in disease-free life span in DR relative to AL-fed mice. These results suggest that total caloric intake may modulate the rates of cell death and proliferation in a direction consistent with a cancer-protective effect in DR mice and a cancer-promoting effect in AL mice. To determine whether the increase in spontaneous apoptotic rate was maintained over the life span of DR mice, apoptotic rates were quantified in 12-, 18-, 24- and 30-month-old DR and AL mice. The rate of apoptosis was elevated with age in both diet groups; however, the rate of apoptosis was significantly and consistently higher in DR mice regardless of age. In double-labeling experiments, an age-associated increase in the glutathione S-transferase-II expression in putative preneoplastic hepatocytes in AL mice was rapidly reduced by apoptosis upon initiation of DR. Thus, intervention that promote a low-level increase in apoptotic cell death may be expected to protect genotypic and phenotypic stability with age. If during tumor promotion an adaptive increase in apoptosis effectively balances the dysregulated increase proliferation, the risk of permanent genetic error and carcinogenesis would be minimized.

The cell cycle and regulation of cancer cell growth

There are two points (brake-points) through which the cell must pass before it can enter cell division. Progress through each brake-point requires the presence of an active cyclin-dependent kinase (Cdk). There are specific cyclins to activate the Cdk's at different parts of the cell cycle. Activation of the cyclin-Cdk complex is tightly regulated by the phosphorylation state of the Cdk. Exogenous growth stimulators (hormones, growth factors, and cytokines) all work through an intracellular kinase cascade that drives the production and activation of early nuclear proteins that, in turn, induce transcription of the genes for cyclins, Cdk's, and other cell cycle regulators. Retinoblastoma protein regulates cell division by inactivating specific growth-promoting proteins. Therefore, mutation of the Rb gene can lead to uncontrolled cell division and thus promotion of transformed cells. p53 protein will prevent replication of cells with damaged DNA. Hence, transformed cells can only readily progress to tumors if the p53 gene is mutated in a manner that inactivates the protein product. Members of the bcl-2 family act, in homodimers and heterodimers, to shunt cells either into cell division or into apoptosis. Understanding the mechanisms by which the balance of cell cycle: apoptosis can be manipulated will lead to new ways of controlling abnormal cellular growth. Most aspects of cellular function reflect changes in phosphorylation of critical serine, threonine, and tyrosine residues on the relevant regulatory proteins. The kinases the phosphatases involved are themselves under tight control.

From targets to genes: a brief history of radiosensitivity

The biological work of Douglas Lea spanned the period from 1934 to his early death in 1947, and during this short period he made important contributions to the theory of radiation action. He interpreted experimental data relating to the effects of radiation on viruses, bacteria, bean roots, etc in terms of the inactivation of discrete targets, which he identified with cellular genes. He thus laid the foundation of much subsequent research. It is now well recognized that mammalian cells differ substantially in radiosensitivity, especially in the low-dose region of the survival curve. The dependence of radiosensitivity on dose rate has been widely studied; this has practical significance for clinical radiotherapy as well as mechanistic implications. Since Lea's time there have been a number of efforts to describe models that can relate cell killing to radiation dose, dose rate, and track structure. So far these have not led to a comprehensive and widely accepted picture. Microdosimetric considerations lead to the concept of differing severity of lesions induced in DNA. Much is known about the sequence of processes that subsequently lead to cell inactivation: this can be divided into phases of induction, processing, and manifestation. Chromosomal events are currently attracting much attention, as they did in Lea's time. Considerable progress has also been made in identifying genes that control the repair of radiation damage. It has been found that mutation is frequently associated with the loss of a large segment of the genome around the damage site and this will have important implications for interactive processes between particle tracks.

Coupling between gamma irradiation, p53 induction and the apoptotic response depends upon cell type in vivo

The accumulation of p53 protein following whole body irradiation of adult mice was studied using a new polyclonal antibody to mouse p53. While dramatic accumulation of the protein was apparent in splenocytes, thymocytes and osteocytes no p53 protein accumulation was detected in the hepatocytes of the irradiated mouse. Thus, the upstream initiating signals that control the induction of p53 are controlled in a tissue specific manner. While massive apoptosis accompanies p53 induction in thymocytes and splenocytes it is not seen in the osteocytes. Thus the downstream consequences of p53 induction are also tightly controlled. These results have profound significance for an understanding of the role of the p53 tumour suppression pathway in different tissues.