In multicellular organisms, homeostasis is maintained through a balance between cell proliferation and cell death. Although much is known about the control of cell proliferation, less is known about the control of cell death. Physiologic cell death occurs primarily through an evolutionarily conserved form of cell suicide termed apoptosis. The decision of a cell to undergo apoptosis can be influenced by a wide variety of regulatory stimuli. Recent evidence suggests that alterations in cell survival contribute to the pathogenesis of a number of human diseases, including cancer, viral infections, autoimmune diseases, neurodegenerative disorders, and AIDS (acquired immunodeficiency syndrome). Treatments designed to specifically alter the apoptotic threshold may have the potential to change the natural progression of some of these diseases.

https://science.sciencemag.org/content/sci/267/5203/1456.full.pdf

Apoptosis in the pathogenesis and treatment of disease

Endostatin: an endogenous inhibitor of angiogenesis and tumor growth.

Angiostatin: A novel angiogenesis inhibitor that mediates the suppression of metastases by a lewis lung carcinoma

Apoptosis is a distinct mode of cell death that is responsible for deletion of cells in normal tissues; it also occurs in specific pathologic contexts. Morphologically, it involves rapid condensation and budding of the cell, with the formation of membrane-enclosed apoptotic bodies containing well-preserved organelles, which are phagocytosed and digested by nearby resident cells. There is no associated inflammation. A characteristic biochemical feature of the process is double-strand cleavage of nuclear DNA at the linker regions between nucleosomes leading to the production of oligonucleosomal fragments. In many, although not all of the circumstances in which apoptosis occurs, it is suppressed by inhibitors of messenger RNA and protein synthesis. Apoptosis occurs spontaneously in malignant tumors, often markedly retarding their growth, and it is increased in tumors responding to irradiation, cytotoxic chemotherapy, heating and hormone ablation. However, much of the current interest in the process stems from the discovery that it can be regulated by certain proto-oncogenes and the p53 tumor suppressor gene. Thus, c-myc expression has been shown to be involved in the initiation of apoptosis in some situations, and bcl-2 has emerged as a new type of proto-oncogene that inhibits apoptosis, rather than stimulating mitosis. In p53-negative tumor-derived cell lines transfected with wild-type p53, induction of the gene has, in rare cases, been found to cause extensive apoptosis, instead of growth arrest. Finally, the demonstration that antibodies against a cell-surface protein designated APO-1 or Fas can enhance apoptosis in some human lymphoid cell lines may have therapeutic implications.

Apoptosis. Its significance in cancer and cancer Therapy

In cancer patients, dormant micrometastases are often asymptomatic and clinically undetectable, for months or years, until relapse. We have studied dormant lung metastases under angiogenesis suppression in mice. The metastases exhibited rapid growth when the inhibition of angiogenesis was removed. Tumour cell proliferation, as measured by bromodeoxyuridine incorporation and immunohistochemical staining proliferating cell nuclear antigen, was not significantly different in dormant and growing metastases. However, tumour cells of dormant metastases exhibited a more than threefold higher incidence of apoptosis. These data show that metastases remain dormant when tumour cell proliferation is balanced by an equivalent rate of cell death and suggest that angiogenesis inhibitors control metastatic growth by indirectly increasing apoptosis in tumour cells.

Dormancy of micrometastases: balanced proliferation and apoptosis in the presence of angiogenesis suppression.

Apoptosis is a form of cell death involved in the regulation of cell number in various organs and tumors. Quantitative determination of cell loss through apoptosis in histological sections requires, in addition to counts of apoptotic cells, information on the duration of the histologically visible stages of apoptosis. Here we describe a method to determine the duration of apoptosis in (i) normal and (ii) putative preneoplastic tissue of the liver. (i) Female rats were treated with high doses of the hepatomitogen cyproterone acetate (CPA) to induce liver hyperplasia. After stopping CPA treatment, the hyperplasia partially regressed and excessive hepatocytes were eliminated by apoptosis. CPA was given to block the initiation of apoptosis, and thereafter the time course of elimination of apoptotic cell residues (apoptotic bodies, ABs) from the liver was studied. The mean duration of the histological stages of apoptosis was found to be approximately 3 h. (ii) Phenotypically altered cell foci in rat liver were produced by a single dose of N-nitrosomorpholine and subsequent promotion for 39 weeks with phenobarbital (PB). PB was withdrawn to stop foci growth and to stimulate apoptosis. Then rats were retreated with PB to block initiation of apoptosis in foci. The results indicate that the majority of apoptotic bodies in foci disappeared within 4 h after PB, suggesting that the stages of apoptosis are as short in foci as in normal liver. Finally a simple formula is given to calculate the cell loss rate by apoptosis. The method presented may provide data for quantitative cancer risk assessment from mathematical models of carcinogenesis.

Determination of the length of the histological stages of apoptosis in normal liver and in altered hepatic foci of rats.

Numerous drugs, hormones and environmental pollutants induce liver growth by hypertrophy and/or hyperplasia, and promote preferential growth of putative preneoplastic foci in the liver. In the present study the regression of hyperplasia after cessation of inducer/promoter treatment was studied in normal liver and in liver foci. High doses of cyproterone acetate (CPA), a synthetic sex steroid, were administered to rats and produced a doubling of liver size; after cessation of treatment liver size declined, and 27% of the total liver DNA disappeared within 6 days. In histological sections from the involuting liver no necroses, but numerous apoptotic bodies (ABs) were found; retreatment with CPA interrupted the formation of ABs. These findings suggest that elimination of excess liver DNA after cessation of CPA treatment is due to controlled cell death by apoptosis. In a further series of experiments putative preneoplastic foci were produced by a single dose of N-nitrosomorpholine and subsequently stimulated to grow by 10 or 28 weeks of phenobarbital (PB) treatment. After withdrawal of PB numerous ABs were present in normal liver and in the foci; in both, retreatment with PB decreased the appearance of ABs. It appears that inhibition of cell death by PB may contribute to tumour promotion. Under all conditions tested more ABs were found in the foci than in non-focal parts of the liver, suggesting an enhanced cell turnover in foci. The apparent sensitivity of foci to mechanisms controlling cell death might eventually provide a means for elimination of preneoplastic lesions.

G. Barthel

Papers

Determination of the length of the histological stages of apoptosis in normal liver and in altered hepatic foci of rats.

Controlled death (apoptosis) of normal and putative preneoplastic cells in rat liver following withdrawal of tumor promoters.