SDI1 is an inhibitor of DNA synthesis that we isolated by expression screening cDNAs prepared from senescent, terminally nondividing human cells. Other groups then cloned this gene as a cyclin-dependent kinase (cdk)-interacting protein (CIP1, p21) that inhibits cdks; the gene was also isolated by screening for genes transactivated by p53 (WAF1). p53 levels are low in senescent and quiescent contact-inhibited or serum-deprived normal human cells, which we have found express high levels of SDI1 mRNA. This indicates that alternate pathways for upregulation of message level of this gene may exist. We therefore proceeded with the study presented here, treating human cells with a variety of growth-arrest-inducing agents, including some that damaged DNA, and found that RNA levels of SDI1 were increased in all cases that resulted in growth inhibition. More important, with the exception of gamma-radiation, most of these agents were able to elevate SDI1 message levels in cells lacking wild-type p53. At least two distinct kinetic profiles for RNA induction were observed, one that implicated p53 transactivation and occurred early enough to cause arrest, and another that clearly was p53 independent and suggested a role for the SDI1 gene product in the maintenance rather than in the cause of inhibition of DNA synthesis.
A morphological and cytometric analysis of the adult fat body cells and oenocytes was made on sections of abdomens from immature, mature and senescent Drosophila melanogaster of both sexes. There are about 18,000 fat body cells in abdomens of female and mature male flies. Immature and senescent males have about 12,000 and 15,000 cells, respectively. The size of the cells is almost the same for immature flies of both sexes and increases about six-fold to approximately 2600 micron2, so that mature flies of both sexes have equivalent amounts of fat body tissue. The proportions of lipid, glycogen, and background cytoplasm of fat body cells also remain relatively constant throughout adult life, but dense, proteinaceous granules are observed in cells of senescent flies. The amounts of cellular components change dramatically due to change of cell size with age; the amount of lipid shows the greatest sexual difference with about 2x more in the females at all stages studied. The oenocytes number about 6,000 in the abdomens of all but immature male flies, which have approximately 4,000. Although the cells of both sexes triple in size to about 700 micron 2, the oenocytes of males reach maximum size earlier than those of females. The major features of oenocytes appear to be dense background cytoplasm, putative lipid droplets found only in mature flies, and pigmented granules first seen in the cells of mature flies which accumulate with age to 33% of the cytoplasm. The number of cells and their anticipated capacity for protein synthesis is discussed in relation to the production of yolk protein precursors.
The number of genomic replications of four separate tissues was determined in Drosophila melanogaster by summing the total number of nuclei and the maximum amount of DNA per nucleus in each tissue. It was found that for a given tissue the total number of nuclei was generally inversely proportional to the maximum amount of DNA per nucleus. In addition the sum of the number of divisions giving rise to the nuclei and the number of DNA replications occurring in the nucleus in each tissue never exceeded 20.
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