Siah-1 binds and regulates the function of Numb.

The Drosophila Seven in absentia (Sina) gene product originally was described as a protein that controls cell fate decisions during eye development. Its mammalian homolog, Siah-1, recently was found to be involved in p53-dependent and -independent pathways of apoptosis and G(1) arrest. We report that Siah-1 interacts directly with and promotes the degradation of the cell fate regulator Numb. Siah-1-mediated Numb degradation leads to redistribution of endogenous cell-surface Notch to the cytoplasm and nucleus and to augmented Notch-regulated transcriptional activity. These data imply that through its ability to target Numb for degradation, Siah-1 can act as a key regulator of Numb-related activities, including Notch signaling.

SIAH-1 promotes apoptosis and tumor suppression through a network involving the regulation of protein folding, unfolding, and trafficking: identification of common effectors with p53 and p21(Waf1).

We have previously described biological model systems for studying tumor suppression in which, by using H-1 parvovirus as a selective agent, cells with a strongly suppressed malignant phenotype (KS or US) were derived from malignant cell lines (K562 or U937). By using cDNA display on the K562/KS cells, 15 cDNAs were now isolated, corresponding to genes differentially regulated in tumor suppression. Of these, TSAP9 corresponds to a TCP-1 chaperonin, TSAP13 to a regulatory proteasome subunit, and TSAP21 to syntaxin 11, a vesicular trafficking molecule. The 15 cDNAs were used as a molecular fingerprint in different tumor-suppression models. We found that a similar pattern of differential regulation is shared by activation of p53, p21(Waf1), and the human homologue of Drosophila seven in absentia, SIAH-1. Because SIAH-1 is differentially expressed in the various models, we characterized it at the protein and functional levels. The 32-kDa, mainly nuclear protein encoded by SIAH-1, can induce apoptosis and promote tumor suppression. These results suggest the existence of a common mechanism of tumor suppression and apoptosis shared by p53, p21(Waf1), and SIAH-1 and involving regulation of the cellular machinery responsible for protein folding, unfolding, and trafficking.

p53-independent apoptosis and p53-dependent block of DNA rereplication following mitotic spindle inhibition in human cells.

We have studied the response of human transformed cells to mitotic spindle inhibition. Two paired cell lines, K562 and its parvovirus-resistant KS derivative clone, respectively nonexpressing and expressing p53, were continuously exposed to nocodazole. Apoptotic cells were observed in both lines, indicating that mitotic spindle impairment induced p53-independent apoptosis. After a transient mitotic delay, both cell lines exited mitosis, as revealed by flow-cytometric determination of MPM2 antigen and cyclin B1 expression, coupled to cytogenetic analysis of sister centromere separation. Both cell lines exited mitosis without chromatid segregation. K562 p53-deficient cells further resumed DNA synthesis, giving rise to cells with a DNA content above 4C, and reentered a polyploid cycle. In contrast, KS cells underwent a subsequent G1 arrest in the tetraploid state. Thus, G1 arrest in tetraploid cells requires p53 function in the rereplication checkpoint which prevents the G1/S transition following aberrant mitosis; in contrast, p53 expression is dispensable for triggering the apoptotic response in the absence of mitotic spindle.

Inhibition of presenilin 1 expression is promoted by p53 and p21WAF-1 and results in apoptosis and tumor suppression.

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.

p21WAF-1 reorganizes the nucleus in tumor suppression.

Interphasic nuclear organization has a key function in genome biology. We demonstrate that p21WAF-1, by influencing gene expression and inducing chromosomal repositioning in tumor suppression, plays a major role as a nuclear organizer. Transfection of U937 tumor cells with p21WAF-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 fluorescence 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.

Varying sensitivity of human mammary carcinoma cells to the toxic effect of parvovirus H-1.

We have previously demonstrated lysis of non-established cultures of human mammary carcinoma cells by parvovirus H-1, which has little effect on the proliferation of corresponding normal cultures. In the present study, we examined this effect in a number of breast-tumour specimens and found them to differ as to the amplitude of their response to parvoviral attack. We first investigated whether the differences in cell sensitivity to parvovirus infection reflected the differentiation level of the initial tumour. Among the biochemical and anatomopathological indicators of original tumour differentiation, the presence of oestrogenic receptors (ER) was found to have a predictive value as to the sensitivity of derived cultures to the cytopathic effect of H-1 virus. The ER+ tumour-derived cultures showed an increased sensitivity to the lytic effect of H-1 virus compared with the ER-tumour-derived cultures, in spite of similar average proliferation rates for the two types of cultures. The proliferation rate was more heterogeneous among ER+ tumour-derived cultures and, in this group, the faster growing cultures were also the most sensitive. This observation was corroborated by the study of established cell lines retaining ER expression under in vitro culture conditions. Oestradiol was found to increase the sensitivity of these cells to the parvovirus in parallel with induction of proliferation. This effect appeared to be mediated by ER activation, since it was not observed in the ER-negative cell line MDA-MB-231. These data point to the importance of hormonal influences and cellular parameters, notably differentiation and proliferation, in determining the extent to which human cancer cells can be targets for the cytopathic effect of parvoviruses.

Constitutive activation of U937 promonocytic cell clones selected for their resistance to parvovirus H-1 infection.

The human promonocytic cell line U937 is highly sensitive to the lytic effect of the autonomous parvovirus H-1. Rare cell variants that resisted H-1 virus infection could be isolated, of which four (RU1, RU2, RU3, and RU4) were further characterized. In contrast to parental cells, the RU clones sustained an abortive H-1 virus infection. Three of the clones showed a significant decrease in the accumulation levels of the c-Myc oncoprotein and in their capacity for forming tumors in immunodeficient mice. Surprisingly, all RU clones resisted the suppressing effect of 12-O-tetradecanoylphorbol-13-acetate (TPA) on c-myc oncogene expression and cell proliferation. In contrast, RU clones exhibited the TPA-induced changes in membrane surface antigens and nonspecific esterase activities that are characteristic of monocytic differentiation. Studies of the activation steady-state of RU cells demonstrated the constitutive production of significant amounts of nitric oxide (NO) and superoxide anion (O-.2). Inhibitors of NO and O-.2, production sensitized all RU cells to the killing effect of parvovirus H-1 and increased the production of infectious viral particles. These data argue for the participation of active oxygen species in macrophage defence mechanisms against parvovirus infection. Moreover, the use of parvovirus H-1 as a selective agent in a cell-colony formation assay allowed us to show that expression of defined markers of monocytic differentiation can be uncoupled from suppression of proliferation.

Activation of the human homologue of the Drosophila sina gene in apoptosis and tumor suppression.

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 16q12-q13. 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 p21waf1. 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.

Lack of a detectable effect of capsid proteins on the cell-dependent activity of parvovirus MVMp promoters.

Deletion of upstream elements from the early P4 promoter of parvovirus MVM (minute virus of mice, prototype strain MVMp) has a differential effect on its activity, depending on the host cell considered. This indicates that upstream motifs participate in the control of promoter P4 functioning and are responsive to factors which are, at least in part, cell-type specific. In contrast with other viral systems, the capsid proteins of MVMp had no detectable effect on gene expression driven by either the early P4 or late P38 promoter of MVMp in permissive and non-permissive cells.

Parvovirus H-1 inhibits growth of short-term tumor-derived but not normal mammary tissue cultures.

Infection with parvovirus H-1 strongly interfered with the proliferation of non-established tissue cultures derived from human breast tumors, but had little effect on the growth of corresponding normal human mammary cells. Even though tumor cells were always more sensitive to the virus than normal tissue from the same patient, appreciable quantitative differences were observed among tumor specimens. With time and sub-cultures, the killing effect of the virus on tumor cells became amplified. The impaired growth of infected tumor cells was due both to cytotoxic and to cytostatic action of H-1 virus and was associated with their greater capacity for virus-DNA amplification as compared with normal cells.

A model for tumor suppression using H-1 parvovirus.

A model system is proposed to investigate, at the molecular level, the pathways of tumor suppression. As a tool for the selection of cells with a suppressed phenotype, we used the H-1 parvovirus that preferentially kills various neoplastic cells. From the human K562 leukemia cells, we isolated a clone, KS, that is resistant to the cytopathic effect of the H-1 virus and displays a suppressed malignant phenotype. The suppressed malignancy and the cellular resistance to H-1 killing appear to depend on the activity of wild-type p53. Whereas the KS cells express wild-type p53, the protein is undetectable in the parental K562 cells. Experiments with p53 mutants suggest that wild-type p53, in its functionally intact state, contributes to the resistance against the cytopathic effect of H-1 parvovirus.

In vitro infection of normal human keratinocytes by human papillomavirus type 1 followed by amplification of the viral genome in reconstructed epidermis.

Primary cultures of normal human keratinocytes were inoculated in vitro with human papillomavirus type 1 (HPV-1), the agent responsible for deep plantar warts. Upon transfer to dead de-epidermized dermis and growth at the air-liquid interface, keratinocytes reconstituted a pseudoepidermis. Under these highly differentiating conditions, HPV-1 DNA amplification was found to take place in the reconstructed epidermis, being detectable from 7 days after the transfer and persisting for at least 10 days thereafter. The extent of keratinocyte differentiation may be insufficient to allow a complete HPV infectious cycle.

Dose-dependent induction of resistance to terminal differentiation in x-irradiated cultures of normal human keratinocytes.

Epidermal cell clones able to proliferate under conditions that cause normal human foreskin keratinocytes (NHK) to terminally differentiate were obtained in a dose-dependent fashion after repeated x-irradiation. No terminal differentiation-resistant (TDR) clones were detected unless the total x-ray dose was split in several fractions given at protracted intervals. The x-ray-induced TDR cells were aneuploid and differed from growing NHK with regard to expression of specific protein markers of differentiation. One of the isolated TDR clones escaped senescence but failed to form tumors in nude mice. Altogether, these data indicate that NHK cultures can be used to quantitate phenotypic changes associated with neoplastic transformation of normal human epithelial cells upon exposure to defined regimens of physicochemical treatments.

Sensitization of human keratinocytes to killing by parvovirus H-1 takes place during their malignant transformation but does not require them to be tumorigenic.

To investigate the antineoplastic activity of parvoviruses, proliferating normal human epidermal cells and a series of established keratinocyte cell lines derived from squamous cell carcinomas or transformed in vitro, were compared for the outcome of H-1 virus infection. All established keratinocyte cell lines were more sensitive to killing by H-1 virus than normal epidermal cells, although to varying extents. Using a step-wise procedure for malignant transformation in vitro, we found that sensitization of transformed epidermal cells to H-1 virus can be dissociated from the acquisition of a tumorigenic phenotype. Thus, spontaneously- or SV40-immortalized human keratinocytes were moderately and highly sensitive to H-1 virus, respectively, and could be made tumorigenic by Harvey-ras oncogene transfection without a major change in their susceptibility to the virus. The capacity of human keratinocytes for replicating and expressing H-1 virus DNA appears to be a revealer of cellular alterations that take place in at least some pathways to malignant transformation but that may be insufficient to confer a tumorigenic potential.