Involvement of integrins in cell survival

Propensity for macrophage apoptosis is related to the pattern of expression and function of integrin extracellular matrix receptors

Ligation of integrins to an extracellular matrix activates signal transduction systems which produce multiple responses in different cell types. Adhesion often provides a survival signal to cells; disruption of adhesion frequently results in apoptosis. Our laboratory has utilized apoptosis-sensitive and -resistant cell lines to investigate the role of integrin expression and function in regulation of apoptosis in macrophages. Chronic exposure of murine macrophage-like RAW264.7 cells to apoptosis-inducing agents (bacterial lipopolysaccharide and interferon-gamma) resulted in the generation of a derivative cell line (RES) resistant to apoptosis. Observation of RAW and RES cultures indicated a difference in adhesion between the two cell types. The two cell lines also exhibit significant differences in expression of integrins previously characterized to be important in apoptosis.

Chronic Lymphocytic Leukemic B Cells But Not Normal B Cells Are Rescued From Apoptosis by Contact With Normal Bone Marrow Stromal Cells

The leukemic B lymphocytes from chronic lymphocytic leukemic (CLL) patients have a long survival in vivo, although ex vivo they rapidly die by apoptosis. To further investigate the mechanism of this, we have studied the influence of bone marrow stromal cells from normal subjects on apoptosis of B-CLL cells and normal umbilical cord blood (UCB) B lymphocytes. After 48 hours of incubation in medium alone, leukemic and normal B cells showed, respectively, 22 ± 3% and 31 ± 5% of apoptosis. Cocultures with stromal cells reduced the percentage of leukemic cells undergoing apoptosis (8 ± 2%, P< .0005) and prevented the loss of bcl-2 protein expression. In contrast, stromal cells slightly increased normal B-cell apoptosis (37 ± 6%). Direct contact between leukemic cells and stromal cells was found to be essential for inhibition of leukemic cell apoptosis; indeed, separation of leukemic cells from stromal cells by microporous membrane increased spontaneous apoptosis, and comparable results were obtained with stromal cell conditioned medium. The difference in behavior observed between normal and leukemic B cells plated on stromal cells can be explained by the fact that only a few normal B cells adhere to stromal cells in comparison with B-CLL cells. B-CLL cell adhesion to stromal cells is mediated by β1 and β2 integrins acting simultaneously. Contact between B-CLL cells and bone marrow stromal cells seems to play a major role in the accumulation and survival of B-CLL cells in the bone marrow.

Chronic Lymphocytic Leukemic B Cells But Not Normal B Cells Are Rescued From Apoptosis by Contact With Normal Bone Marrow Stromal Cells

The leukemic B lymphocytes from chronic lymphocytic leukemic (CLL) patients have a long survival in vivo, although ex vivo they rapidly die by apoptosis. To further investigate the mechanism of this, we have studied the influence of bone marrow stromal cells from normal subjects on apoptosis of B-CLL cells and normal umbilical cord blood (UCB) B lymphocytes. After 48 hours of incubation in medium alone, leukemic and normal B cells showed, respectively, 22 ± 3% and 31 ± 5% of apoptosis. Cocultures with stromal cells reduced the percentage of leukemic cells undergoing apoptosis (8 ± 2%, P< .0005) and prevented the loss of bcl-2 protein expression. In contrast, stromal cells slightly increased normal B-cell apoptosis (37 ± 6%). Direct contact between leukemic cells and stromal cells was found to be essential for inhibition of leukemic cell apoptosis; indeed, separation of leukemic cells from stromal cells by microporous membrane increased spontaneous apoptosis, and comparable results were obtained with stromal cell conditioned medium. The difference in behavior observed between normal and leukemic B cells plated on stromal cells can be explained by the fact that only a few normal B cells adhere to stromal cells in comparison with B-CLL cells. B-CLL cell adhesion to stromal cells is mediated by β1 and β2 integrins acting simultaneously. Contact between B-CLL cells and bone marrow stromal cells seems to play a major role in the accumulation and survival of B-CLL cells in the bone marrow.

Involvement of Mac-1-mediated adherence and sphingosine 1-phosphate in survival of phorbol ester-treated U937 cells

Phorbol esters exert a dual function in human leukemia cells, induction of differentiation and activation of integrin-mediated functions. Here we have shown that the plastic adherence of phorbol ester-treated U937 cells is mediated by expression of integrin Mac-1 (CD11b/CD18) on the cell surface and that these adherent cells exhibit anoikis (apoptosis when adherent cells are detached or adherence is inhibited). We used U937-derived clones overexpressing either antisense RNAs antisense to CD11b and CD18 mRNAs or mRNA from a truncated mutant CD11b gene. We have also shown that apoptosis in non-adherent cells or anoikis was mediated by sphingosine and that survival of adherent cells was achieved by a shift of the dynamic balance between sphingosine and sphingosine 1-phosphate toward the latter by adherence-activated sphingosine 1-kinase.

Histomorphometric evidence for echistatin inhibition of bone resorption in mice with secondary hyperparathyroidism

Echistatin, an RGD-containing peptide, was shown to inhibit the acute calcemic response to exogenous PTH or PTH-related protein (PTH-rP) in thyroparathyroidectomized rats, suggesting that echistatin inhibits bone resorption. In this study: 1) we present histological evidence for echistatin inhibition of bone resorption in mice with secondary hyperparathyroidism, and show that 2) echistatin binds to osteoclasts in vivo, 3) increases osteoclast number, and 4) does not detectably alter osteoclast morphology. Infusion of echistatin (30 microg/kg x min) for 3 days prevented the 2.6-fold increase in tibial cancellous bone turnover and the 36% loss in bone volume, produced by a low calcium diet. At the light microscopy level, echistatin immunolocalized to osteoclasts and megakaryocytes. Echistatin treatment increased osteoclast-covered bone surface by about 50%. At the ultrastructural level, these osteoclasts appeared normal, and the fraction of cells containing ruffled borders and clear zones was similar to controls. Echistatin was found on the basolateral membrane and in intracellular vesicles of actively resorbing osteoclasts. Weak labeling was found in the ruffled border, and no immunoreactivity was detected at the clear zone/bone surface interface. These findings provide histological evidence for echistatin binding to osteoclasts and for inhibition of bone resorption in vivo, through reduced osteoclast efficacy, without apparent changes in osteoclast morphology.

Ligand-targeted liposomes

Liposomes have gained increased attention as systemic drug delivery vehicles following recent regulatory approvals of several vesicle-formulated drugs. These products have demonstrated improved therapeutic indices over their corresponding conventional drugs by avoiding sensitive tissues and/or increasing delivery to specific targets in vivo. They have achieved these improvements primarily through physical means: (1) by retaining drug within vesicles while in the circulation, thus avoiding or minimizing uptake by sensitive normal tissues; and (2) by selectively extravasating into target tissues, releasing active drug. In order to improve upon these therapies in the future, clinically active liposome delivery systems most likely will need to include site-directed surface ligands to further enhance their selective delivery. This may be crucial for the in vivo transport and delivery of macromolecules, including antisense, oligonucleotide aptamers, and genes, which-unlike most conventional drugs-do not circulate well and often require cellular uptake by fusion, endocytosis, or other processes to reach their active sites. This manuscript reviews technologies applicable to directing liposomes and their contents to selected in vivo targets using surface-bound, site-specific ligands. Presented are the biological barriers to be overcome, criteria for selecting the determinants to be targeted, various targeting ligands and overall delivery system design considerations. Several novel targets as well as novel ligand constructs for site-directed therapy are reviewed and discussed. Systemic liposome therapy, which currently must be administered by the intravenous route, is the principal focus of this analysis.

Osteopontin involvement in integrin-mediated cell signaling and regulation of expression of alkaline phosphatase during early differentiation of UMR cells

To clarify the function of osteopontin in osteoblast differentiation, we have examined the signal transduction pathway in an osteoblastic cell line (UMR106-6) bound to osteopontin, fibronectin, vitronectin and collagen type I surfaces. This was done by investigating the production and autophosphorylation of focal adhesion kinase (FAK) and the expression of alkaline phosphatase (ALP) at the transcription level. Results suggest that osteopontin was not only responsible for the autophosphorylation of FAK but regulated the expression of ALP, which was strongly correlated with FAK activity. These results suggest that osteopontin might act as a trigger in the early differentiation of osteoblasts.

Relationship between expression of integrins and granulosa cell apoptosis in ovarian follicles of the marmoset (Callithrix jacchus)

DNA fragmentation (apoptosis) was studied during the follicular, periovulatory and luteal phase in the marmoset monkey ovary by means of terminal transferase mediated in situ nick end labeling, and correlated with immunohistochemical localization of integrins (beta 1, alpha 2 and alpha 6 subunits). For this purpose a double-labeling technique was developed. During all phases, apoptosis of granulosa cells was exclusively restricted to tertiary follicles displaying advanced stages of atresia (as morphologically determined). During early stages of atresia in tertiary follicles, indicated by widened intercellular spaces, no apoptosis was seen. Staining intensities for integrins beta 1 and alpha 6 were strong in intact primordial/primary, secondary and tertiary follicles. Integrin expression of granulosa cells was weak in atretic tertiary follicles but not in atretic primary or secondary follicles. Double labeling revealed that DNA fragmentation was solely found in granulosa cells of tertiary follicles displaying faint or absent staining for both integrin subunits. During the periovulatory and the luteal phase, granulosa cells of atretic tertiary follicles bordering on the basal membrane, which were referred to as luteinizing cells, expressed the beta 1 subunit as well as the alpha 2-integrin subunit whereas granulosa cells neighboring to the antrum were apoptotic and negative for integrin immunoreactivity. In summary, early atresia of tertiary follicles is first characterized by morphological alterations as wide intercellular gaps, without any signs of granulosa cell apoptosis. Advanced stages of atresia, in tertiary follicles however, are accompanied by apoptosis of granulosa cells and a faint or absent staining for integrin subunits beta 1 and alpha 6. According to recent in vitro findings, our results point to a possible relation between granulosa cell apoptosis of tertiary follicles and integrin expression in ovarian marmoset follicles.

Integrin signaling

Integrins provide dynamic links between cells and extracellular matrix molecules. Although integrins were originally viewed as relatively simple adhesion molecules, it soon became clear that intracellular signal transduction initiated by integrins is centrally involved in many cellular processes. In fact, a remarkable number of classical signaling pathways are now known to be activated or modified by the interactions of cells with matrix proteins via integrins. These integrin signaling responses can also involve many other extracellular and intracellular molecules. The following mini-reviews were solicited from some of the future leaders in the field of integrin signaling. They examine selected important portions of this field, provide conceptual syntheses from a large and confusing literature, and then propose novel testable ideas. These ideas should encourage dialogue and open new avenues of research in this rapidly expanding, exciting field.

Msx2 is a transcriptional regulator in the BMP4-mediated programmed cell death pathway

Homeobox-containing genes play an important role in patterning processes that occur during embryogenesis. Programmed cell death is a key process during pattern formation. The mechanisms by which programmed cell death is spatially regulated are not well characterized. Msx1 and Msx2 are two closely related homeobox-containing genes that are expressed at sites where cellular proliferation and programmed cell death occur, including the developing limb and the cephalic neural crest. Tissue interactions are necessary for the maintenance of Msx1 and Msx2 expression and programmed cell death. It has been demonstrated that BMP4 can regulate cell death at these same sites as well as induce Msx expression. These observations lead to the hypothesis that Msx2 is a key regulator of cell death in the BMP-mediated pathway. Embryonic stem cell lines will undergo processes typical of early embryogenesis upon aggregation and have recently been shown to provide a model system for programmed cell death. In contrast to ES cells, we see that P19 cells do not undergo pronounced cell death upon aggregation; however, constitutive ectopic Msx2 expression in P19 cells results in a marked increase in apoptosis induced upon aggregation but has no effect when cells are grown as a monolayer. If aggregates are allowed to interact with a substrate, the process of programmed cell death is completely inhibited. Addition of BMP4 to aggregated P19 cells also results in cell death; however, BMP4 does not increase levels of cell death in Msx2-expressing cells. Addition of BMP4 to P19 cells results in an induction of Msx2 transcription consistent with its proposed role in cell death in the embryo. Our data support a model by which BMP4 induces programmed cell death via an Msx2-mediated pathway and provide direct functional evidence that Msx2 expression is a regulator of this process.

Comparison of integrin alpha chain expression in benign and malignant salivary gland tumors

OBJECTIVE

This study investigates the distribution of the alpha chain of the integrin family of extracellular matrix receptors in a series of adenomas and carcinomas of salivary gland origin to determine if the malignant phenotype is associated with modification of the expression of these receptors.

STUDY DESIGN

Cryostat sections of 36 tumor specimens were stained by a standard streptavidin-biotin-peroxidase technique using primary monoclonal antibodies against alpha 1-6 and alpha v integrin chains. The immunohistochemical reaction was scored using a three-point scale and the results were analyzed using Fisher's exact test.

RESULTS

In salivary adenomas, alpha 2, alpha 3, alpha 4, alpha 6, and alpha v chains were widely expressed in most of the cases studied. The alpha 1 subunit was prominently expressed by the epithelial cells of Warthin's tumor, whereas a minority of pleomorphic adenomas showed immunoreactivity for this antigen. We observed alpha 5 subunit expression only in the mesenchymal-like component of pleomorphic adenomas. In salivary carcinomas, integrin alpha chain expression was heterogeneous, varying greatly between different histotypes and within the same histotype. The distribution of the antigens was similar to that of adenomas, except for the alpha 6 chain, which localized not only at the interface between cell and matrix, but also at sites of cell-cell contact. When the immunohistochemical levels of integrin alpha chain expression were compared in adenomas and carcinomas, expression significantly decreased for the alpha 6 and alpha v chains (p = 0.0007; p = 0.002, respectively).

CONCLUSIONS

Loss of alpha 6 and alpha v integrin subunits occurring in salivary gland carcinomas could modify the adhesive properties of malignant cells, contributing to the invasive potential of these tumors.

Formation of spheroid structures in a human colon carcinoma cell line involves a complex series of intercellular rearrangements

The structural remodelling of tissues that occurs in vivo during animal morphogenesis can often prove difficult to study. Here we investigate the organizational processes of the LIM 1863 colon carcinoma cell line as it transforms from a single-cell stage into multicellular spherical structures called 'organoids'. The organoids can be dissociated into a viable single-cell suspension when cultured in calcium-depleted medium, and then induced to reform the organoid structure by the readdition of calcium. Previous studies have shown that initial cell attachment under these conditions is characterized by a novel mechanism of cell engulfment termed 'clutching'. This investigation reveals the subsequent appearance of junctional complexes between groups of 'clutched' cells prior to lumen formation, and the ultimate 'declutching' of entrapped cells as a means of cell rearrangement. Intact actin filaments but not microtubules were required for the initial clutching events, while inhibition of microtubule polymerization resulted in aberrant apical protein polarization, but did not affect the development of a luminal space within the spheroids. Single cells exhibited pools of intracellular microvilli contained in vacuolar apical compartments, which were resistant to the effects of cytoskeleton-disrupting drugs. However, these structures did not seem to be responsible for the swift development of the luminal surface observed in these cells. Two other cell lines, MDCK and DU 4475, were found to exhibit similar clutching conformations when induced to form three-dimensional structures, suggesting that this may be a widespread mechanism of cell rearrangement that reflects the process of organ morphogenesis in vivo.

Coordinated expression of matrilysin during TPA-induced apoptosis of LNCaP cells: two parallel processes affected by TPA

Matrix metalloproteinases (MMPs) are upregulated by growth factors and 12-O-tetradecanoyl-phorbol-13-acetate (TPA). TPA (10 nM) induced apoptosis in LNCaP cells grown in serum-free medium at high seeding density, and increased mRNA and secreted protein levels for the MMP matrilysin. While the TPA-augmented increase in matrilysin mRNA was seen at 4 h, secreted matrilysin protein levels at 8 h, TPA-induced DNA ladder formation was seen only at 10 h and the TPA-induced apoptosis was detected at 12 h. The sequence of events suggested a functional role for matrilysin in apoptosis. However, when the MMP inhibitor BB-2516 was used (25 microM, with IC50 of 20 nM for matrilysin), there was no effect of BB-2516 on TPA-induced apoptosis in LNCaP cells (P = 0.2072). This observation indicates that MMPs including matrilysin do not play a primary role in TPA-induced apoptosis in LNCaP cells. We conclude that the TPA-induced apoptosis and the regulation of matrilysin (a TPA-response element (TRE)-containing gene), are independent and parallel processes.

Regulation and significance of apoptosis in the stem cells of the gastrointestinal epithelium

In rapidly proliferating tissues the stringent control of cell proliferation and cell death by apoptosis is central to the maintenance of tissue homeostasis. In the gastrointestinal tract most work studying the control of tissue cell number has traditionally focused on the growth factor control of proliferation, and the changes that occur during carcinogenesis. However, in recent years it has become increasingly apparent that the control of apoptosis is also crucial. Apoptosis is an important mechanism for eliminating both excess normal cells and those cells which have sustained damage; therefore maintaining a tissue, i.e., stem cells with preserved DNA integrity. In this review the incidence of apoptosis in the stem cells of both the small and large intestine will be discussed in relation to the expression of a number of apoptosis regulating genes (e.g. p53, Bcl-2, bax) within these cells. The importance of apoptosis as a means of controlling stem cell number (and therefore cellular output) will be addressed, as will the mechanisms by which any alterations to this process may contribute to malignancy.

Contiguous four-guanosine sequence in c-myc antisense phosphorothioate oligonucleotides inhibits cell growth on human lung cancer cells: possible involvement of cell adhesion inhibition

A contiguous four-guanosine (4G) sequence in c-myc antisense phosphorothioate oligonucleotides caused an antiproliferative effect in smooth muscle cells. To investigate the antiproliferative effect of c-myc antisense oligonucleotides on human lung cancer cell lines, we synthesized oligonucleotides of various lengths and sequences, focusing on the contiguous four-guanosine (4G) sequence. While a c-myc antisense oligonucleotide (20AS1 (4G)) targeted to the translation initiation codon of c-myc mRNA inhibited cell growth of A549 cells by 69% at 10 microM, a scrambled oligonucleotide (20SCR1 (4G)) containing the contiguous four-guanosine (4G) sequence also inhibited cell growth by 72% at the same dose. Although treatment with either 20AS1 (4G) or 20SCR1 (4G) inhibited cell adhesion by 70% at 10 microM, expression of c-myc protein was significantly suppressed only by 20AS1 (4G) (62%), and was only weakly inhibited by 20SCR1 (4G) (32%). Furthermore, a small cell lung carcinoma cell line, Lu65, which can grow in suspension form, was highly resistant to 20AS1 (4G) treatment (IC50>20 microM). These results suggest that the cell growth inhibition by c-myc antisense oligonucleotides containing the contiguous four-guanosine (4G) sequence was possibly correlated with inhibition of cell adhesion, but not with inhibition of c-myc protein expression, via a sequence-specific non-antisense mechanism.