Suppression of ICE and apoptosis in mammary epithelial cells by extracellular matrix

Mechanisms of integrin-mediated calcium signaling in MDCK cells: regulation of adhesion by IP3- and store-independent calcium influx

Peptides containing Arg-Gly-Asp (RGD) immobilized on beads bind to integrins and trigger biphasic, transient increases in intracellular free Ca2+ ([Ca2+]i) in Madin-Darby canine kidney epithelial cells. The [Ca2+]i increase participates in feedback regulation of integrin-mediated adhesion in these cells. We examined influx pathways and inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ store release as possible sources of the [Ca2+]i rise. The RGD-induced [Ca2+]i response requires external Ca2+ (threshold approximately 150 microM), and its magnitude is proportional to extracellular calcium. RGD-induced transients were attenuated by Ca2+ channel inhibitors (Ni2+ and carboxy-amidotriazole) or by plasma membrane depolarization, indicating that Ca2+ influx contributes to the response. Loading cells with heparin reduced the size of RGD-induced [Ca2+]i transients, indicating that IP3-mediated release of Ca2+ from stores may also contribute to the RGD response. Depletion of Ca2+ stores with thapsigargin activated Ni(2+)-sensitive Ca2+ influx that might also be expected to occur after IP3-mediated depletion of stored Ca2-. However, RGD elicited a Ni(2+)-sensitive Ca2+ influx even after pretreatment with thapsigargin, indicating that Ca2+ influx is controlled by a mechanism independent of IP3-mediated store depletion. We conclude that RGD-induced [Ca2+]i transients in Madin-Darby canine kidney cells result primarily from the combination of two distinct mechanisms: 1) IP3-mediated release of intracellular stores, and 2) activation of a Ca2+ influx pathway regulated independently of IP3 and Ca2+ store release. Because Ni2+ and carboxy-amidotriazole inhibited adhesion, whereas store depletion with thapsigargin had little effect, we suggest that the Ca2+ influx mechanism is most important for feedback regulation of integrin-mediated adhesion by increased [Ca2+]i.

ICE/CED-3 proteasesin apoptosis

An expanding family of cysteine proteases, of which interleukin-1beta-converting enzyme (ICE) is the prototype, has been shown to play a key role in mammalian cell apoptosis. ICE is both a structural and functional homologue of the nematode 'death gene' ced-3. Here, Moira Whyte discusses how functional characterization of these ICE-like proteases and identification of their substrates is helping to elucidate the biochemical processes underlying the stereotyped morphology of apoptosis and to identify potential targets for therapy.

Differential requirements for alpha4 integrins during fetal and adult hematopoiesis

Mice chimeric for the expression of alpha4 integrins were used to dissect the roles of these receptors in development and traffic of lymphoid and myeloid cells. During fetal life, T cell development is alpha4 independent, but after birth further production of T cells becomes alpha4 dependent. Precursors for both T and B cells require alpha4 integrins for normal development within the bone marrow. In contrast, monocytes and natural killer cells can develop normally without alpha4 integrins. Thus, there are lymphocyte-specific, developmentally regulated requirements for alpha4 integrins in hematopoiesis in the bone marrow. We also show that alpha4 integrins are essential for T cell homing to Peyer's patches, but not to other secondary lymphoid organs, including spleen, lymph nodes, and intestinal epithelium.

Developmental regulation of murine integrin beta 1 subunit- and Hsc73-encoding genes in mammary gland: sequence of a new mouse Hsc73 cDNA

A partial integrin beta 1 subunit-encoding cDNA (Itg beta 1) and a new heat-shock protein 70-like-encoding cDNA (Hsc73) homologous to rat Hsc73 were cloned by differential display and RT-PCR from mouse mammary gland. Their developmental regulation during pregnancy, lactation and involution is reported. The Itg beta 1 mRNA content was stable in the first half of gestation, decreased to a minimum during lactation and increased markedly in early involution. Hsc73 gene expression was high in the first half of gestation and decreased to a minimum during lactation. The possible significance of the two observed patterns of expression is discussed.

An SH3 domain-containing GTPase-activating protein for Rho and Cdc42 associates with focal adhesion kinase

The integrin family of cell surface receptors mediates cell adhesion to components of the extracellular matrix (ECM). Integrin engagement with the ECM initiates signaling cascades that regulate the organization of the actin-cytoskeleton and changes in gene expression. The Rho subfamily of Ras-related low-molecular-weight GTP-binding proteins and several protein tyrosine kinases have been implicated in mediating various aspects of integrin-dependent alterations in cell homeostasis. Focal adhesion kinase (FAK or pp125FAK) is one of the tyrosine kinases predicted to be a critical component of integrin signaling. To elucidate the mechanisms by which FAK participates in integrin-mediated signaling, we have used expression cloning to identify cDNAs that encode potential FAK-binding proteins. We report here the identification of a cDNA that encodes a new member of the GTPase-activating protein (GAP) family of GTPase regulators. This GAP, termed Graf (for GTPase regulator associated with FAK), binds to the C-terminal domain of FAK in an SH3 domain-dependent manner and preferentially stimulates the GTPase activity of the GTP-binding proteins RhoA and Cdc42. Subcellular localization studies using Graf-transfected chicken embryo cells indicates that Graf colocalizes with actin stress fibers, cortical actin structures, and focal adhesions. Graf mRNA is expressed in a variety of avian tissues and is particularly abundant in embryonic brain and liver. Graf represents the first example of a regulator of the Rho family of small GTP-binding proteins that exhibits binding to a protein tyrosine kinase. We suggest that Graf may function to mediate cross talk between the tyrosine kinases such as FAK and the Rho family GTPase that control steps in integrin-initiated signaling events.

Extracellular matrix and lung inflammation

Lung injury triggers an acute inflammatory response characterized by increased expression and deposition of extracellular matrix (ECM) components such as fibronectin and collagen. Although the function of newly deposited matrices in injured lungs is unknown, their ability to affect the migration, proliferation, differentiation, and activation state of cells in vitro suggests an important role in the initiation and maintenance of the inflammatory response in vivo. Interactions between immune and nonimmune cells with the lung ECM are mediated via cell surface receptors of the integrin family which link the ECM with intracellular molecules involved in signal transduction. Activation of integrin-mediated intracellular signals may promote inflammation by facilitating leukocyte recruitment and cytokine expression.

Regulation of a physiological apoptosis: mouse mammary involution

Continuous milk production during lactation is dependent on a complex interplay of lactogenic hormones and the suckling stimulus exerted by the young. Involution can be initiated in the mouse mammary gland at any stage of lactation by removing the pups; involution then remains reversible for about 30 to 36 h. Involution in the mouse mammary gland is characterized by a massive loss of secretory epithelial cells from programmed cell death. The nuclear activation of protein kinase A and transcription factor activator protein 1 precede the irreversible phase of involution that is characterized by internucleosomal DNA fragmentation. Activation of activator protein 1 and fragmentation of chromosomal DNA can be prevented by lactogenic hormone treatment in explant cultures derived from mammary tissue at lactation. The elevation in activator protein 1 coincides with the epithelial expression of sulfated glycoprotein 2, a potential target gene of activator protein 1. Programmed cell death in the mammary gland is associated with the expression of the growth arrest gene, gas-1, and the integrin-associated protein gene, IAP, which codes for a putative Ca2+ channel that is dependent on integrin. Their potential roles during involution are discussed.

Expression and function of matrix metalloproteinases and their inhibitors at the maternal-embryonic boundary during mouse embryo implantation

Gelatinase B, a matrix metalloproteinase (MMP) of high specific activity, is highly expressed and activated by mouse blastocysts in culture, and inhibition of this enzyme activity inhibits lysis of extracellular matrix (Behrendtsen, O., Alexander, C. M. and Werb, Z. (1992) Development 114, 447–456). Because gelatinase B expression is linked to invasive potential, we studied the expression of gelatinase B mRNA and protein in vivo, in implanting trophoblast giant cells, and found that it was expressed and activated during colonization of the maternal decidua. mRNAs for several other MMPs (stromelysin-1, stromelysin-3 and gelatinase A) and MMP inhibitors (TIMP-1 and TIMP-2) were expressed in the undifferentiated stroma toward the outside of the decidua, and TIMP-3 mRNA was expressed in primary and some mature decidual cells during their differentiation. Both mRNA and TIMP-3 protein were present at high concentrations transiently, and declined from 6.5 days post coitum onward, as the cells underwent apoptosis during the main period of gelatinase B expression and ectoplacental growth and expansion. To assess the function of MMPs during implantation and decidual development, we either injected a peptide hydroxamate MMP inhibitor into normal mice or studied transgenic mice overexpressing TIMP-1. In both cases, decidual length and overall size were reduced, and the embryo was displaced mesometrially. Embryo orientation was less strictly regulated in inhibitor-treated deciduae than in control deciduae. Morphogenesis and development of oil-induced deciduomas were also slowed in the presence of the inhibitor. We conclude that administration of MMP inhibitors retards decidual remodeling and growth, and we suggest that the MMPs expressed in precursor stromal cells promote their differentiation and expansion.

The contrasting roles of ICE family proteases and interleukin-1beta in apoptosis induced by trophic factor withdrawal and by copper/zinc superoxide dismutase down-regulation

We compare here the mechanisms of apoptotic death of PC12 cells induced by down-regulation of Cu2+,Zn2+ superoxide dismutase (SOD1) and withdrawal of trophic support (serum/nerve growth factor). Our previous results indicated that the initiating causes of death are different in each paradigm. However, bcl-2 rescues cells in either paradigm, suggesting common downstream elements to the cell death pathway. To determine whether the ICE [interleukin 1beta converting enzyme] family of proteases, which is required for apoptosis on trophic factor withdrawal, is also required for apoptosis induced by oxidative stress, we have developed a novel peptide inhibitor that mimics the common catalytic site of these enzymes and thereby blocks their access to substrates. This differs from the more usual pseudosubstrate approach to enzyme inhibition. Blockade of ICE family proteases by either this inhibitor or by a permeant competitive ICE family antagonist rescues PC12 cells from apoptotic death following apoptosis induced by down-regulation of SOD1, as well as from trophic factor/nerve growth factor deprivation. SOD1 down-regulation results in an increase in interleukin 1beta (IL- 1beta) production by the cells, and cell death under these conditions can be prevented by either blocking antibodies against IL-1beta or the IL-1 receptor antagonist (IL-1Ralpha). In contrast, trophic factor withdrawal does not increase IL-1beta secretion, and the blocking antibody failed to protect PC12 cells from trophic factor withdrawal, whereas the receptor antagonist was only partially protective at very high concentrations. There were substantial differences in the concentrations of pseudosubstrate inhibitors which rescued cells from SOD1 down-regulation and trophic factor deprivation. These results suggest the involvement of different members of the ICE family, different substrates, or both in the two different initiating causes of cell death.

The cytotoxic cell protease granzyme B initiates apoptosis in a cell-free system by proteolytic processing and activation of the ICE/CED-3 family protease, CPP32, via a novel two-step mechanism

The major mechanism of cytotoxic lymphocyte killing involves the directed release of granules containing perforin and a number of proteases onto the target cell membrane. One of these proteases, granzyme B, has an unusual substrate site preference for Asp residues, a property that it shares with members of the emerging interleukin-1beta-converting enzyme (ICE)/CED-3 family of proteases. Here we show that granzyme B is sufficient to reproduce rapidly all of the key features of apoptosis, including the degradation of several protein substrates, when introduced into Jurkat cell-free extracts. Granzyme B-induced apoptosis was neutralized by a tetrapeptide inhibitor of the ICE/CED-3 family protease, CPP32, whereas a similar inhibitor of ICE had no effect. Granzyme B was found to convert CPP32, but not ICE, to its active form by cleaving between the large and small subunits of the CPP32 proenzyme, resulting in removal of the prodomain via an autocatalytic step. The cowpox virus protein CrmA, a known inhibitor of ICE family proteases as well as granzyme B, inhibited granzyme B-mediated CPP32 processing and apoptosis. These data demonstrate that CPP32 activation is a key event during apoptosis initiated by granzyme B.

Young neurons from the adult subependymal zone proliferate and migrate along an astrocyte, extracellular matrix-rich pathway

The subependymal zone (SEZ) of the lateral ventricle of adult rodents has long been known to be mitotically active. There has been increased interest in the SEZ, since it has been demonstrated that neuroepithelial stem cells residing there generate neurons in addition to glia in vitro. In the present study, we have examined parasagittal sections of the adult mouse brain using immunocytochemistry for extracellular matrix (ECM) molecules (tenascin and chondroitin sulfate-containing proteoglycans), glial fibrillary acidic protein (GFAP, a cytoskeletal protein prominently expressed by immature and reactive astrocytes), RC-2 (a radial glial and immature astrocyte cytoskeletal marker), TuJ1 (a class III beta-tubulin isoform expressed solely by postmitotic and adult neurons), nestin (a cytoskeletal protein associated with stem cells), neuron-specific enolase, and bromodeoxyuridine (BrdU, which is taken up by dividing cells). Our results demonstrate that a population of young neurons reside within an ECM-rich, GFAP-positive astrocyte pathway from the rostral SEZ all the way into the olfactory bulb. Furthermore, BrdU labeling studies indicate that there is a high level of cell division along the entire length of this path, and double-labeling studies indicate that neurons committed to a neuronal lineage (i.e., TuJ1+) take up BrdU (suggesting they are in the DNA synthesis phase of the cell cycle), again along the entire length of the SEZ "migratory pathway." Thus, the SEZ appears to retain the ability to produce neurons and glia throughout the life of the animal, functioning as a type of "brain marrow." The implications of these findings are discussed in relation to the role that such a glial/ ECM-rich boundary (as seen in the embryonic cortical subplate and other developing areas) may play in: confining the migratory populations and maintaining them in a persistent state of immaturity; facilitating their migration to the olfactory bulb, where they are incorporated into established adult circuitries; and potentially altering SEZ cell cycle dynamics that eventually lead to cell death.

Extensive apoptosis in ductal carcinoma in situ of the breast

BACKGROUND

More than 50% of breast ductal carcinomas in situ (DCIS) contain significant histologic necrosis, an important prognostic factor for determining recurrence and progression to invasive breast cancer. We have examined whether the mechanism of this spontaneous cell death might be apoptosis, a genetically encoded suicide pathway that may be triggered by various events including dysregulated cell proliferation.

METHODS

Twenty-five untreated DCIS cases accessioned at our institution were examined for subtype, grade, and presence of apoptosis using two criteria: (1) cellular morphology (shrinkage, nuclear condensation, fragmentation, apoptotic bodies, and lack of inflammatory component); and (2) terminal transferase (TUNEL) staining of DNA fragmentation, a characteristic though less specific feature of apoptosis. Immunohistochemical staining was also carried out to assess whether wild-type p53, a regulator of apoptosis, was associated with this cell death.

RESULTS

In all 19 cases with intraductal necrosis, cellular morphology was consistent with apoptotic death, despite its presence within sheets of "geographic necrosis." Additionally, the identical regions were all strongly TUNEL-positive. No evidence of TUNEL staining was seen in 5 Grade I DCIS cases without intraductal necrosis. Immunohistochemical staining suggested that this apoptosis was independent of p53 mutational status.

CONCLUSIONS

Extensive intraductal necrosis in DCIS is likely to represent apoptosis. However, it is unlikely that this apoptosis is regulated by p53. The apparently abundant apoptosis identified here, particularly in high grade DCIS, may be important in explaining why spontaneous cell death in DCIS is associated with a worse prognosis.

Extracellular matrix remodeling and the regulation of epithelial-stromal interactions during differentiation and involution

An intact basement membrane is essential for the proper function, differentiation and morphology of many epithelial cells. The disruption or remodeling of the basement membrane occurs during normal development as well as in the disease state. To examine the importance of basement membrane during development in vivo, we altered the matrix metalloproteinase and tissue inhibitor of metalloproteinases balance in mammary gland. Inhibition of matrix metalloproteinase synthesis by glucocorticoids or implants or transgenic overexpression of tissue inhibitor of metalloproteinases -1 delays matrix degradation and the involution process after weaning. The mammary glands from transgenic mice that inappropriately express auto-activating isoforms of stromelysin-1 are both functionally and morphologically altered throughout development. Transgenic mammary glands have supernumerary branches, and show precocious development of alveoli that express beta-casein expression and undergo unscheduled apoptosis during pregnancy. This is accompanied by progressive development of an altered stroma, which becomes fibrotic after postweaning involution, and by development of neoplasias. These data suggest that metalloproteinases and disruption of the basement membrane may play key roles in branching morphogenesis of mammary gland, cell cycle, apoptosis, and stromal fibrosis as well as in induction and progression of breast cancer.

Inhibition of colony formation in agarose of metastatic human breast carcinoma and melanoma cells by synthetic glycoamine analogs

We studied the influence of 10 synthetic glycoamine analogs on colony formation in 0.3 and 0.9% agarose by metastatic human breast carcinoma (MDA-MB-435) and melanoma (TXM-13) cells. Nine synthetic analogs significantly inhibited the colony formation in 0.9% agarose of MDA-MB-435 human breast carcinoma cells; five compounds caused a 73-83% reduction of colony formation. Seven synthetic glycoamines caused a significant inhibition of colony formation in 0.9% agarose by TXM-13 melanoma cells with the inhibitory effect ranging from 71 to 87%. The 50% inhibition (I50) doses and relative activity rank of the compounds were similar for both breast carcinoma and melanoma cell lines. The murine B16 melanoma cell aggregation assay was employed to elucidate the potential mechanism(s) of the inhibitory activity of synthetic glycoamines. The relative activity ranks of the compounds based on the independently determined I50 doses for both cell aggregation and clonogenic growth assays were very similar for the four most active synthetic analogs and clearly indicated the importance of hydrophobic amino acid in mediating the bioactivity of synthetic glycoamines. In both experimental systems (clonogenic growth in agarose and cell aggregation assay) the leading compound was N-(1-deoxy-D-fructos-1-yl)-D-leucine (Fru-D-Leu) and the least active analog was N-(l-deoxy-D-fructos-1-yl)-glycine (Fru-Gly). These results show that synthetic glycoamines may act by competing for specific carbohydrate-lectin interactions, particularly those involving beta-galactoside-specific lectins expressed on metastatic cells.

Bax and Bcl-xs are induced at the onset of apoptosis in involuting mammary epithelial cells

Mammary gland involution is a physiological process in which the entire organ is remodeled through the process of apoptosis. Apoptosis of secretory alveolar cells is initiated at the time of weaning, followed by the collapse and disappearance of the entire lobuloalveolar compartment. While apoptotic figures were rare in mammary epithelium of lactating mice, their number increased after weaning and reached a maximum on day 3 of involution. Active cell death continued until day 5 after weaning and only little parenchyma remained on day 8, when remodeling of the gland was completed. Bax mRNA levels increased during the first day of involution independent of the presence or absence of p53. Bax protein was detected in an increasing number of cells after weaning, peaking at day 3 and decreasing thereafter. Low levels of bcl-x mRNA and protein were present during lactation, followed by a sharp increase during the first 2 days of involution. The bcl-xS splice variant of bcl-x can promote cell death, and bcl-xL has a protective function in cell culture. The ratio of bcl-xS versus bcl-xL remained stable in the virgin, pregnant and lactating gland. However, during the first 2 days of involution, bcl-xS expression increased six-fold more than bcl-xL. To further evaluate the role of Bcl-xS which was less abundant in the mammary cells than Bcl-xL, cotransfection studies were performed in cell culture. They confirmed that Bcl-xS protein can facilitate apoptosis even when Bcl-xL is present in excess. These findings point to a significant role for Bax and Bcl-xS in the regulation of apoptosis of secretory alveolar cells during involution.

Use of monoclonal antibodies to study interleukin-1 beta-converting enzyme expression: only precursor forms are detected in interleukin-1 beta-secreting cells

We have generated a series of monoclonal antibodies (mAb) using recombinant interleukin (IL)-1 beta-converting enzyme (ICE) p20 and p10 subunits as immunogens. The mAb have been selected for further study based on their reactivity with ICE in transfected COS cells and their lack of cross-reactivity with TX, the closest ICE homolog known to date. Two anti-p20 and one anti-p10 mAb have been used to study ICE expression by Western blotting and immunodetection. In ICE-transfected COS cells, the mAb recognize the p45 ICE precursor and the maturation products (p20 or p10 subunits) for which they are specific. In monocytes and cell lines expressing ICE, only precursor forms are detected and intracellular immunostaining followed by confocal microscopy shows that they are located in the cytoplasm. Quantification experiments show that THP1 cells express approximately 67,000 molecules of ICE precursor per cell, with an estimated precursor to mature ratio of at least 100. In these cells as well as in monocytes, lipopolysaccharide stimulation did not change the pattern of ICE expression, although efficient secretion of mature IL-1 beta was measured. However, upon cell disruption, precursor maturation was observed. Our results, therefore, show that ICE is present in cells as a large pool of intracytoplasmic precursor, and that very limited amounts of mature ICE protein are present, but nevertheless sufficient to allow efficient IL-1 beta cleavage. Altogether, these observations suggest that post-translational maturation of the precursor protein could represent a specific step in the regulation of ICE enzymatic activity.

Suppression of apoptosis by basement membrane requires three-dimensional tissue organization and withdrawal from the cell cycle

The basement membrane (BM) extracellular matrix induces differentiation and suppresses apoptosis in mammary epithelial cells, whereas cells lacking BM lose their differentiated phenotype and undergo apoptosis. Addition of purified BM components, which are known to induce beta-casein expression, did not prevent apoptosis, indicating that a more complex BM was necessary. A comparison of culture conditions where apoptosis would or would not occur allowed us to relate inhibition of apoptosis to a complete withdrawal from the cell cycle, which was observed only when cells acquired a three-dimensional alveolar structure in response to BM. In the absence of this morphology, both the GI cyclin kinase inhibitor p21/WAF-1 and positive proliferative signals including c-myc and cyclin DI were expressed and the retinoblastoma protein (Rb) continued to be hyperphosphorylated. When we overexpressed either c-myc in quiescent cells or p21 when cells were still cycling, apoptosis was induced. In the absence of three-dimensional alveolar structures, mammary epithelial cells secrete a number of factors including transforming growth factor alpha and tenascin, which when added exogenously to quiescent cells induced expression of c-myc and interleukin-beta1-converting enzyme (ICE) mRNA and led to apoptosis. These experiments demonstrate that a correct tissue architecture is crucial for long-range homeostasis, suppression of apoptosis, and maintenance of differentiated phenotype.

Growth arrest and non-apoptotic cell death associated with the suppression of c-myc expression in MCF-7 breast tumor cells following acute exposure to doxorubicin

In the MCF-7 human breast [correction of beast] adenocarcinoma cell line, acute exposure to 1 muM doxorubicin inhibited cell proliferation by approximately 75%. Analysis of cell cycle distribution indicated that within 24 hr, the G(2)/M fraction increased more than 3-fold and the S-phase population declined by >50%. In addition to growth arrest, there was an approximately 40% reduction in the viable cell population after 72 hr. Gel electrophoretic resolution of low molecular weight DNA immediately after exposure of cells to doxorubicin failed to demonstrate "laddered" oligonucleosomal profiles associated with apoptosis. The absence of intracellular DNA fragments or release of fragmented DNA into the incubation medium was confirmed by spectrofluorophotometry over a 72 hr interval following exposure of cells to 1 muM doxorubicin. In addition, there was no evidence of the morphological features associated with apoptosis during this period. Acute exposure to 1 muM doxorubicin also produced a transient increase in c-myc message expression (within the first hour) followed by a decline to 70% of control levels within 2-4 hr. The reduction in c-myc mRNA levels was concentration dependent and corresponded closely with growth arrest (as well as with inhibition of DNA synthesis). These findings (as well as similar reports demonstrating a correspondence between reduced c-myc expression and growth inhibition by VM-26 and m-AMSA in MCF-7 cells) suggest that the down-regulation of c-myc expression may reflect perturbations in regulatory processes contributing to growth arrest in MCF-7 cells exposed to topoisomerase II inhibitors.

Apoptosis amd metastasis: a superior resistance of metastatic cancer cells to programmed cell death

We studied the response to different external signals leading to apoptosis of several poorly and highly metastatic cell lines employing a murine B16 melanoma experimental metastasis model. We found that highly metastatic cells exhibit a superior survival ability and resistance to apoptosis compared to poorly metastatic cells which would give the former an obvious selective growth advantage during tumor progression. Our results indicate that there is a genetic link between aggressive metastatic phenotype and resistance to apoptosis.

Cleavage of actin by interleukin 1 beta-converting enzyme to reverse DNase I inhibition

Three of the predominant features of apoptosis are internucleosomal DNA fragmentation, plasma membrane bleb formation, and retraction of cell processes. We demonstrate that actin is a substrate for the proapoptotic cysteine protease interleukin 1beta-converting enzyme. Actin cleaved by interleukin 1beta-converting enzyme can neither inhibit DNase I nor polymerize to its filamentous form as effectively as intact actin. These findings suggest a mechanism for the coordination of the proteolytic, endonucleolytic, and morphogenetic aspects of apoptosis.

Cyclin D1 induced apoptosis maintains the integrity of the G1/S checkpoint following ionizing radiation irradiation

Cell cycle "checkpoints" help to ensure the integrity of normal cellular functions prior to replicative DNA synthesis and/or cell division. Cell kinetic abnormalities, particularly arrests at the G1/S and G2/M cell cycle checkpoints, are induced following exposure to ionizing radiation in vitro. Following irradiation, cellular signaling pathways may lead to G1 arrest and/or apoptosis at the G1/S cell cycle transition point. Transfection of cyclin D1, a G1/S cyclin, into a rat embryo cells (REC) results in cellular populations that overexpress cyclin D1, are transformed morphologically, demonstrate an increased incidence of apoptosis, and are tumorigenic in immune-deficient mice. Despite such phenotypic changes, transfected cell populations maintain the integrity of the G1 checkpoint following ionizing radiation. The transfected cells overexpressing Cyclin D1 have a statistically significant increase in the incidence of apoptosis as compared to parental REC strains or mock-transfected REC. The work provides further evidence of Cyclin D1 playing a critical role in maintaining the integrity of the G1/S checkpoint, via the activation of apoptotic pathways following exposure to ionizing radiation in vitro.

Requirement of basement membrane for the suppression of programmed cell death in mammary epithelium

Apoptosis is an active mechanism of cell death required for normal tissue homeostasis. Cells require survival signals to avoid the engagement of apoptosis. In the mammary gland, secretory epithelial cells are removed by apoptosis during involution. This cell loss coincides with matrix metalloproteinase activation and basement membrane degradation. In this paper we describe studies that confer a new role for basement membrane in the regulation of cell phenotype. We demonstrate that the first passage epithelial cells isolated from pregnant mouse mammary gland die by apoptosis in culture, but that cell death is suppressed by basement membrane. The correct type of extracellular matrix was required, since only a basement membrane, not plastic or a collagen I matrix, lowered the rate of apoptosis. Attachment to a matrix per se was not sufficient for survival, since apoptotic cells were observed when still attached to a collagen I substratum. Experiments with individually isolated cells confirmed the requirement of basement membrane for survival, and demonstrated that survival is enhanced by cell-cell contact. A function-blocking anti-beta1 integrin antibody doubled the rate of apoptosis in single cells cultured with basement membrane, indicating that integrin-mediated signals contributed to survival. We examined the cell death-associated genes bcl-2 and bax in mammary epithelia, and found that although the expression of Bcl-2 did not correlate with cell survival, increased levels of Bax were associated with apoptosis. We propose that basement membrane provides a survival stimulus for epithelial cells in vivo, and that loss of interaction with this type of matrix acts as a control point for cell deletions that occur at specific times during development, such as in mammary gland involution.

Suppression of interleukin-1 beta-converting enzyme-mediated cell death by insulin-like growth factor

COS cells are resistant to cell death induced either by interleukin-1beta-converting enzyme (*ICE) and ICE homolog (ICH-1L) overexpression or by serum deprivation. COS cells deprived of serum undergo apoptosis after transfection with an ICE expression construct, but not an ICH-1L construct. ICE-mediated apoptosis of COS cells in serum-free medium is suppressed by insulin-like growth factor (IGF)-1 and insulin. Viability of Rat-1 cell line (Rat-1/ICE) expressing low levels of ICE-LacZ fusion protein is lower than those of cell lines expressing either both Bcl-2 and ICE or mutant ICEGly-->Ser during serum deprivation. Enzymatic activation and processing of ICE are observed in cells induced to die by serum deprivation, which are suppressed by IGF-1. IGF-1 or insulin suppresses ICE-mediated cell death without affecting the expression levels of Bcl-2, Bcl-x, or Bax. Taken together, these results indicate that ICE is activated by growth factor deprivation, and IGF-1 is able to suppress ICE-mediated cell death through a mechanism independent of the expression of Bcl-2, Bcl-x, or Bax.

Apoptosis: molecular regulation of cell death

The field of apoptosis is unusual in several respects. Firstly, its general importance has been widely recognised only in the past few years and its surprising significance is still being evaluated in a number of areas of biology. Secondly, although apoptosis is now accepted as a critical element in the repertoire of potential cellular responses, the picture of the intra-cellular processes involved is probably still incomplete, not just in its details, but also in the basic outline of the process as a whole. It is therefore a very interesting and active area at present and is likely to progress rapidly in the next two or three years. This review emphasises recent work on the molecular mechanisms of apoptosis and, in particular, on the intracellular interactions which control this process. This latter area is of crucial importance since dysfunction of the normal control machinery is likely to have serious pathological consequences, probably including oncogenesis, autoimmunity and degenerative disease. The genetic analysis of programmed cell death during the development of the nematode Caenorhabditis elegans has proved very useful in identifying important events in the cell death programme. Recently defined genetic connections between C. elegans cell death and mammalian apoptosis have emphasised the value of this system as a model for cell death in mammalian cells, which, inevitably, is more complex. The signals inducing apoptosis are very varied and the same signals can induce differentiation and proliferation in other situations. However, some pathways appear to be of particular significance in the control of cell death; recent analysis of the apoptosis induced through the cell-surface Fas receptor has been especially important for immunology. Two gene families are dealt with in particular detail because of their likely importance in apoptosis control. These are, first, the genes encoding the interleukin-1 beta-converting enzyme family of cysteine proteases and, second, those related to the proto-oncogene bcl-2. Both of these families are homologous to cell death genes in C. elegans. In mammalian cells the number of members of both families which have been identified is growing rapidly and considerable effort is being directed towards establishing the roles played by each member and the ways in which they interact to regulate apoptosis. Other genes with established roles in the regulation of proliferation and differentiation are also important in controlling apoptosis. Several of these are known proto-oncogenes, e.g. c-myc, or tumour suppressors, e.g. p53, an observation which is consistent with the importance of defective apoptosis in the development of cancer. Viral manipulation of the apoptosis of host cells frequently involves interactions with these cellular proteins. Finally, the biochemistry of the closely controlled cellular self-destruction which ensues when the apoptosis programme has been engaged is also very important. The biochemical changes involved in inducing phagocytosis of the apoptotic cell, for example, allow the process to be neatly integrated within the tissues, under physiological conditions. Molecular defects in this area too may have important pathological consequences.

Requirement of an ICE-like protease for induction of apoptosis and ceramide generation by REAPER

Genetic studies indicated that the Drosophila melanogaster protein REAPER (RPR) controls apoptosis during embryo development. Induction of RPR expression in Drosophila Schneider cells rapidly stimulated apoptosis. RPR-mediated apoptosis was blocked by N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD-fmk), which suggests that an interleukin-1 beta converting enzyme (ICE)-like protease is required for RPR function. RPR-induced apoptosis was associated with increased ceramide production that was also blocked by Z-VAD-fmk, which suggests that ceramide generation requires an ICE-like protease as well. Thus, the intracellular RPR protein uses cell death signaling pathways similar to those used by the vertebrate transmembrane receptors Fas (CD95) and tumor necrosis factor receptor type 1.

Cell renewal and functional morphology of human lactating breast: presentation of a new mode of cell death ('magentosis') characterized by nuclear periodic acid-Schiff reactivity

Cell kinetics and functional morphology of the human lactating breast were analyzed using 21 paraffin-embedded specimens of the lactating breast obtained by biopsy or surgery and two of the 'resting' breast. Four types (I-IV) of lactating lobules were categorized histologically, and were well correlated with their functional status demonstrated by immunohistochemistry for epithelial membrane antigen, beta-casein, lipase, lactoferrin, secretory component and IgA. Type I corresponded to a pre-lactating state, type II to an actively lactating state, type III to an early stage of regression, and type IV to an advanced stage of regression. Cell proliferation monitored by Ki-67 (MIB-1) immunostaining was at the highest level in type I lobules followed by type IV, while the labeling indices were low in types II and III. Apoptosis demonstrated by terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate (dUTP)-biotin nick end-labeling (TUNEL) method showed labeling profiles comparable with the proliferative activity. Holocrine-type desquamative cells with intact-appearing nuclei were frequently recognized in type II lobules. In type III, a peculiar form of cell death, designated 'magentosis', was demonstrated. These degenerative cells occasionally shedding into the acinar lumen possessed homogeneous pale, TUNEL-negative nuclei with diastase-resistant periodic acid-Schiff (PAS) reactivity. 'Magentosis' specifically identified in an early phase of involution of the human lactating breast might represent a unique mode of cell death distinguishable from apoptosis and necrosis.

Apoptosis--molecular mechanisms and biomedical implications

Apoptosis is a distinct form of cell death of importance in tissue development and homeostasis and in several diseases. This review summarizes current knowledge about the regulation and molecular mechanisms of apoptosis and discusses the potential role of disregulated apoptosis in several major diseases. Finally, we speculate that modulation of apoptosis may be a target in future drug therapy.

ICE-related proteases in apoptosis

Apoptotic execution involves numerous enzymatic pathways, all of which appear to be triggered by the activation of one or more ICE-related proteases (IRPs). Considerable effort is currently being expended in the identification and functional characterization of the rapidly expanding superfamily of IRPs. Important questions that remain unsolved include the identity of the vertebrate IRP that triggers the apoptotic cascade and the identities of the crucial substrates whose cleavage results in the dramatic morphological changes during apoptosis.

T-lymphocyte interactions with endothelium and extracellular matrix

T-lymphocyte movement out of the bloodstream and into tissue is critical to the success of these cells in their role in immunosurveillance. This process involves interactions of the T-cell with endothelium as well as with extracellular matrix. Central to these interactions are a number of T-cell adhesion molecules and their endothelial and extracellular matrix ligands. The identification and functional characterization of adhesion molecules have been the subject of intensive research in recent years. We highlight here the latest developments in this rapidly expanding field as they pertain to T-cell interactions with endothelial cells and extracellular matrix components, including: (1) identification of adhesion molecule families, including the selectins, mucins, integrins, immunoglobulin superfamily members, and cadherins; (2) elucidation of the multi-step adhesion cascade that mediates the rolling, arrest, and eventual diapedesis of T-cells through the vascular endothelium into the surrounding tissue; (3) the changes in adhesion molecule expression that accompany T-cell maturation and activation, and the impact of those changes on T-cell migration; (4) the functional relevance of the extracellular matrix for T-cell function; and (5) the clinical relevance of adhesion molecules and the potential for targeting these molecules for the amelioration of immune-mediated diseases.

The importance of the microenvironment in breast cancer progression: recapitulation of mammary tumorigenesis using a unique human mammary epithelial cell model and a three-dimensional culture assay

The extracellular matrix (ECM) is a dominant regulator of tissue development and homeostasis. "Designer microenvironments" in culture and in vivo model systems have shown that the ECM regulates growth, differentiation, and apoptosis in murine and human mammary epithelial cells (MEC) through a hierarchy of transcriptional events involving the intricate interplay between soluble and physical signaling pathways. Furthermore, these studies have shown that these pathways direct and in turn are influenced by the tissue structure. Tissue structure is directed by the cooperative interactions of the cell-cell and cell-ECM pathways and can be modified by stromal factors. Not surprisingly then, loss of tissue structure and alterations in ECM components are associated with the appearance and dissemination of breast tumors, and malignancy is associated with perturbations in cell adhesion, changes in adhesion molecules, and a stromal reaction. Several lines of evidence now support the contention that the pathogenesis of breast cancer is determined (at least in part) by the dynamic interplay between the ductal epithelial cells, the microenvironment, and the tissue structure (acini). Thus, to understand the mechanisms involved in carcinogenesis, the role of the microenvironment (ECM as well as the stromal cells) with respect to tissue structure should be considered and studied. Towards this goal, we have established a unique human MEC model of tumorigenesis, which in concert with a three-dimensional assay, recapitulates many of the genetic and morphological changes observed in breast in cancer in vivo. We are currently using this system to understand the role of the microenvironment and tissue structure in breast cancer progression.

Two distinct phases of apoptosis in mammary gland involution: proteinase-independent and -dependent pathways

Postlactational involution of the mammary gland is characterized by two distinct physiological events: apoptosis of the secretory, epithelial cells undergoing programmed cell death, and proteolytic degradation of the mammary gland basement membrane. We examined the spatial and temporal patterns of apoptotic cells in relation to those of proteinases during involution of the BALB/c mouse mammary gland. Apoptosis was almost absent during lactation but became evident at day 2 of involution, when beta-casein gene expression was still high. Apoptotic cells were then seen at least up to day 8 of involution, when beta-casein gene expression was being extinguished. Expression of sulfated glycoprotein-2 (SGP-2), interleukin-1 beta converting enzyme (ICE) and tissue inhibitor of metalloproteinases-1 was upregulated at day 2, when apoptotic cells were seen initially. Expression of the matrix metalloproteinases gelatinase A and stromelysin-1 and the serine proteinase urokinase-type plasminogen activator, which was low during lactation, was strongly upregulated in parallel starting at day 4 after weaning, coinciding with start of the collapse of the lobulo-alveolar structures and the intensive tissue remodeling in involution. The major sites of mRNA synthesis for these proteinases were fibroblast-like cells in the periductal stroma and stromal cells surrounding the collapsed alveoli, suggesting that the degradative phase of involution is due to a specialized mesenchymal-epithelial interaction. To elucidate the functional role of these proteinases during involution, at the onset of weaning we treated mice systemically with the glucocorticoid hydrocortisone, which is known to inhibit mammary gland involution. Although the initial wave of apoptotic cells appeared in the lumina of the gland, the dramatic regression and tissue remodeling usually evident by day 5 was substantially inhibited by systemic treatment with hydrocortisone. mRNA and protein for gelatinase A, stromelysin-1 and uPA were weakly induced, if at all, in hydrocortisone-treated mice. Furthermore, mRNA for membrane-type matrix metalloproteinase decreased after hydrocortisone treatment and paralleled the almost complete inhibition of activation of latent gelatinase A. Concomitantly, the gland filled with an overabundance of milk. Our data support the hypothesis that there are at least two distinct phases of involution: an initial phase, characterized by induction of the apoptosis-associated genes SGP-2 and ICE and apoptosis of fully differentiated mammary epithelial cells without visible degradation of the extracellular matrix, and a second phase, characterized by extracellular matrix remodeling and altered mesenchymal-epithelial interactions, followed by apoptosis of cells that are losing differentiated functions.

Extracellular matrix signaling from the cellular membrane skeleton to the nuclear skeleton: a model of gene regulation

It is well established that cells must interact with their microenvironment and that such interaction is crucial for coordinated function and homeostasis. However, how cells receive and integrate external signals leading to gene regulation is far from understood. It is now appreciated that two classes of cooperative signals are implicated: a soluble class including hormones and growth factors and a class of insoluble signals emanating from the extracellular matrix (ECM) directly through contact with the cell surface. Using 3-dimensional culture systems and transgenic mice, we have been able to identify some of the elements of this ECM-signaling pathway responsible for gene regulation in rodent mammary gland differentiation and involution. Our major observations are 1) the requirement for a laminin-rich basement membrane; 2) the existence of a cooperative signaling pathway between basement membrane and the lactogenic hormone prolactin (PRL);3) the importance of beta 1-integrins and bHLH transcription factor(s) and the presence of DNA response elements (exemplified by BCE-1, located on a milk protein gene, beta-casein); and 4) the induction of mammary epithelial cell programmed cell death following degradation of basement membrane. We hypothesize that this cooperative signaling between ECM and PRL may be achieved through integrin- and laminin-directed restructuring of the cytoskeleton leading to profound changes in nuclear architecture and transcription factor localization. We postulate that the latter changes allow the prolactin signal to activate transcription of the beta-casein gene. To further understand the molecular mechanisms underlying ECM and hormonal cooperative signaling, we are currently investigating ECM regulation of a "solid-state" signaling pathway including ECM fiber proteins, plasma membrane receptors, cytoskeleton, nuclear matrix and chromatin. We further postulate that disruption of such a pathway may be implicated in cell disorders including transformation and carcinogenesis.

Massive programmed cell death in intestinal epithelial cells induced by three-dimensional growth conditions: suppression by mutant c-H-ras oncogene expression

Deregulation of molecular pathways controlling cell survival and death, including programmed cell death, are thought to be important factors in tumor formation, disease progression, and response to therapy. Studies devoted to analyzing the role of programmed cell death in cancer have been carried out primarily using conventional monolayer cell culture systems. However the majority of cancers grow as three-dimensional solid tumors. Because gene expression, and possibly function, can be significantly altered under such conditions, we decided to analyze the control and characteristics of cell death using a compatible three-dimensional tissue culture system (multicellular spheroids) and compare the results obtained to those using two-dimensional monolayer cell culture. To do so we selected for study an immortalized, but nontumorigenic line of rat intestinal epithelial cells, called IEC-18, and several tumorigenic variants of IEC-18 obtained by transfection with a mutant (activated) c-H-ras oncogene. The rationale for choosing these cell lines was based in part on the fact that intestinal epithelial cells grow in vivo in a monolayer-like manner and form solid tumors only after sustaining certain genetic mutations, including those involving the ras gene family. We found that the IEC-18 cells, which grow readily and survive in monolayer cell culture, undergo massive cell death within 48-72 h when cultured as multicellular spheroids on a nonadhesive surface. This process was accompanied by a number of features associated with programmed cell death including chromatin condensation (Hoechst 33258 staining) apoptotic morphology, DNA degradation, and a virtual complete loss of colony forming (clonogenic) ability in the absence of apparent membrane damage as well as accumulation of lipid containing vacuoles in the cytoplasm. Moreover, enforced over-expression of a transfected bcl-2 gene could prevent this cell death process from taking place. In marked contrast, three different stably transfected ras clones of IEC-18 survived when grown as multicellular spheroids. In addition, an IEC cell line (called clone 25) carrying its mutant transfected ras under a glucocorticoid inducible promoter survived in three-dimensional culture only when the cells were exposed to dexamethasone. If exposure to dexamethasone was delayed for as long as 48 h the cells nevertheless survived, whereas the cells became irreversibly committed to programmed cell death (PCD) if exposed to dexamethasone after 72 h. These results suggest that intestinal epithelial cells may be programmed to activate a PCD pathway upon detachment from a physiologic two-dimensional monolayer configuration, and that this process of adhesion regulated programmed cell death (ARPCD) can be substantially suppressed by expression of a mutant ras oncogene.(ABSTRACT TRUNCATED AT 400 WORDS)

Glucocorticoid and progesterone inhibit involution and programmed cell death in the mouse mammary gland

Milk production during lactation is a consequence of the suckling stimulus and the presence of glucocorticoids, prolactin, and insulin. After weaning the glucocorticoid hormone level drops, secretory mammary epithelial cells die by programmed cell death and the gland is prepared for a new pregnancy. We studied the role of steroid hormones and prolactin on the mammary gland structure, milk protein synthesis, and on programmed cell death. Slow-release plastic pellets containing individual hormones were implanted into a single mammary gland at lactation. At the same time the pups were removed and the consequences of the release of hormones were investigated histologically and biochemically. We found a local inhibition of involution in the vicinity of deoxycorticosterone- and progesterone-release pellets while prolactin-release pellets were ineffective. Dexamethasone, a very stable and potent glucocorticoid hormone analogue, inhibited involution and programmed cell death in all the mammary glands. It led to an accumulation of milk in the glands and was accompanied by an induction of protein kinase A, AP-1 DNA binding activity and elevated c-fos, junB, and junD mRNA levels. Several potential target genes of AP-1 such as stromelysin-1, c-jun, and SGP-2 that are induced during normal involution were strongly inhibited in dexamethasone-treated animals. Our results suggest that the cross-talk between steroid hormone receptors and AP-1 previously described in cells in culture leads to an impairment of AP-1 activity and to an inhibition of involution in the mammary gland implying that programmed cell death in the postlactational mammary gland depends on functional AP-1.

Beta 3-endonexin, a novel polypeptide that interacts specifically with the cytoplasmic tail of the integrin beta 3 subunit

The adhesive and signaling functions of integrins are regulated through their cytoplasmic domains. We identified a novel 111 residue polypeptide, designated beta 3-endonexin, that interacted with the cytoplasmic tail of the beta 3 integrin subunit in a yeast two-hybrid system. This interaction is structurally specific, since it was reduced by 64% by a point mutation in the beta 3 cytoplasmic tail (S752-->P) that disrupts integrin signaling. Moreover, this interaction is integrin subunit specific since it was not observed with the cytoplasmic tails of the alpha IIb, beta 1, or beta 2 subunits. beta 3-Endonexin fusion proteins bound selectively to detergent-solubilized beta 3 from platelets and human umbilical vein endothelial cells, and beta 3-endonexin mRNA and protein were detected in platelets and other tissues. A related mRNA encoded a larger polypeptide that failed to bind to beta integrin tails. The apparent specificity of beta 3-endonexin for the beta 3 integrin subunit suggests potential mechanisms for selective modulation of integrin functions.

Loss of MDCK cell alpha 2 beta 1 integrin expression results in reduced cyst formation, failure of hepatocyte growth factor/scatter factor-induced branching morphogenesis, and increased apoptosis

Cellular interactions with collagen in a model of kidney tubulogenesis were investigated using Madin-Darby canine kidney (MDCK) cells in an in vitro morphogenetic system. MDCK cells adhered to collagen types I and IV in a Mg(2+)-dependent manner, typical of the alpha 2 beta 1 integrin. Collagen-Sepharose affinity chromatography and immunoblotting demonstrated the presence and collagen binding activity of the alpha 2 beta 1 integrin on MDCK cells. To assess the function of alpha 2 beta 1 integrin, MDCK cells were transfected with a plasmid pRSV alpha 2′ which allowed the expression of alpha 2-integrin subunit antisense RNA. Three G418-resistant clones showing reduced adhesion to collagen, stable genomic integration of the antisense construct, decreased alpha 2-integrin subunit mRNA and decreased alpha 2-integrin subunit protein expression were selected for analysis in morphogenetic experiments. MDCK cells and plasmid-only control transfectants, cultured in three-dimensional collagen type I gels, showed normal cyst formation, whereas the antisense RNA transfectants showed increased apoptosis and formed small rudimentary cysts. Stimulation with hepatocyte growth factor/scatter factor-containing 3T3 fibroblast-conditioned medium or recombinant hepatocyte growth factor/scatter factor resulted in extensive branching of the preformed control cysts whereas the surviving small cysts formed by antisense expressing cells increased in size but failed to elongate and branch upon stimulation. We conclude that alpha 2 beta 1 integrin collagen interactions play a crucial role in the hepatocyte growth factor/scatter factor-induced tubulogenesis and branching morphogenesis of MDCK cells in collagen gels as well as an important role in cell survival.