Previously, we cloned a cDNA fragment, TSIP 2 (tumor suppressor inhibited pathway clone 2), that detects by northern blot analysis of M1-LTR6 cells a 3-kb mRNA downregulated during p53-induced apoptosis. Cloning the full-length TSIP 2 cDNA showed that it corresponds to the presenilin 1 (PS1) gene, in which mutations have been reported in early-onset familial Alzheimer's disease. Here we demonstrate that PS1 is downregulated in a series of model systems for p53-dependent and p53-independent apoptosis and tumor suppression. To investigate the biological relevance of this downregulation, we stably transfected U937 cells with antisense PS1 cDNA. The downregulation of PS1 in these U937 transfectants results in reduced growth with an increased fraction of the cells in apoptosis. When injected into mice homozygous for severe combined immunodeficiency disease (scid/scid mice), these cells show a suppression of their malignant phenotype. Our results indicate that PS1, initially identified in a neurodegenerative disease, may also be involved in the regulation of cancer-related pathways.

We report the isolation of 10 differentially expressed cDNAs in the process of apoptosis induced by the p53 tumor suppressor. As a global analytical method, we performed a differential display of mRNA between mouse Ml myeloid leukemia cells and derived clone LTR6 cells, which contain a stably transfected temperature-sensitive mutant of p53. At 32°C wild-type p53 function is activated in LTR6 cells, resulting in programmed cell death.

Developmentally regulated genes in Drosophila, which are conserved through evolution, are potential candidates for key functions in biological processes such as cell cycle, programmed cell death, and cancer. We report cloning and characterization of the human homologue of the Drosophila seven in absentia gene (HUMSIAH), which codes for a 282 amino acids putative zinc finger protein. HUMSIAH is localized on human chromosome 16ql2-ql3. This gene is activated during the physiological program of cell death in the intestinal epithelium. Moreover, human cancer-derived cells selected for suppression of their tumorigenic phenotype exhibit constitutively elevated levels of HUMSIAH mRNA. A similar pattern of expression is also displayed by the p2lwan. These results suggest that mammalian seven in absentia gene, which is a target for activation by p53, may play a role in apoptosis and tumor suppression.

Interphasic nuclear organization has a key function in genome biology. We demonstrate that p21 WAF-1 , by inf luencing gene expression and inducing chromosomal repositioning in tumor suppression, plays a major role as a nuclear organizer. Transfection of U937 tumor cells with p21 WAF-1 resulted in expression of the HUMSIAH (human seven in absentia homologue), Rb, and Rbr-2 genes and strong suppression of the malignant phenotype. p21 WAF-1 drastically modified the compartmentalization of the nuclear genome. DNase I genome exposure and f luorescence in situ hybridization show, respectively, a displacement of the sensitive sites to the periphery of the nucleus and repositioning of chromosomes 13, 16, 17, and 21. These findings, addressing nuclear architecture modulations, provide potentially significant perspectives for the understanding of tumor suppression.

Protein kinase-C (PKC) is a ubiquitous eukaryotic kinase that plays a key role in transmembrane signaling and influences important cellular processes, such as proliferation. Increases in its activity and expression have been demonstrated in adenomatous human pituitaries, with protein expression being the highest in invasive tumors (1). Moreover, in these same invasive tumors, the mean increase in expression (8.9-fold) does not correlate with the mean increase in activity (2.6-fold), suggesting a dysfunction in PKC in these tumors. Here, we show that the PKC alpha-isoform (alpha PKC) is overexpressed in human pituitary tumors. The complete sequencing of the PKC cDNA from four invasive tumors has revealed a point mutation that is absent in the noninvasive tumors analyzed. The point mutation is located at position 294 of the protein, in the V3 region, leading to a substitution of a negatively charged aspartic acid by an apolar glycine. Thus, not only is alpha PKC overexpressed in human pituitary tumors, but it is also structurally altered in the invasive subpopulation of these tumors.