Apoptosis of adherent cells by recruitment of caspase-8 to unligated integrins.

Integrin-mediated adhesion promotes cell survival in vitro, whereas integrin antagonists induce apoptosis of adherent cells in vivo. Here, we demonstrate that cells adherent within a three-dimensional extracellular matrix undergo apoptosis due to expression of unligated integrins, the beta subunit cytoplasmic domain, or its membrane proximal sequence KLLITIHDRKEF. Integrin-mediated death requires initiator, but not stress, caspase activity and is distinct from anoikis, which is caused by the loss of adhesion per se. Surprisingly, unligated integrin or beta integrin tails recruit caspase-8 to the membrane, where it becomes activated in a death receptor-independent manner. Integrin ligation disrupts this integrin-caspase containing complex and increases survival, revealing an unexpected role for integrins in the regulation of apoptosis and tissue remodeling.

Matrix valency regulates integrin-mediated lymphoid adhesion via Syk kinase.

Lymphocytes accumulate within the extracellular matrix (ECM) of tumor, wound, or inflammatory tissues. These tissues are largely comprised of polymerized adhesion proteins such as fibrin and fibronectin or their fragments. Nonactivated lymphoid cells attach preferentially to polymerized ECM proteins yet are unable to attach to monomeric forms or fragments of these proteins without previous activation. This adhesion event depends on the appropriate spacing of integrin adhesion sites. Adhesion of nonactivated lymphoid cells to polymeric ECM components results in activation of the antigen receptor-associated Syk kinase that accumulates in adhesion-promoting podosomes. In fact, activation of Syk by antigen or agonists, as well as expression of an activated Syk mutant in lymphoid cells, facilitates their adhesion to monomeric ECM proteins or their fragments. These results reveal a cooperative interaction between signals emanating from integrins and antigen receptors that can serve to regulate stable lymphoid cell adhesion and retention within a remodeling ECM.

Intrinsic FAK activity and Y925 phosphorylation facilitate an angiogenic switch in tumors.

Elevated focal adhesion kinase (FAK) expression occurs in advanced cancers, yet a signaling role for FAK in tumor progression remains undefined. Here, we suppressed FAK activity in 4T1 breast carcinoma cells resulting in reduced FAK Y925 phosphorylation, Grb2 adaptor protein binding to FAK, and signaling to mitogen-activated protein (MAP) kinase (MAPK). Loss of a FAK-Grb2-MAPK linkage did not affect 4T1 cell proliferation or survival in culture, yet FAK inhibition reduced vascular endothelial growth factor (VEGF) expression and resulted in small avascular tumors in mice. This FAK-Grb2-MAPK linkage was essential in promoting angiogenesis as reconstitution experiments using Src-transformed FAK-null fibroblasts revealed that point mutations affecting FAK catalytic activity (R454) or Y925 phosphorylation (F925) disrupted the ability of FAK to promote MAPK- and VEGF-associated tumor growth. Notably, in both FAK-inhibited 4T1 and Src-transformed FAK-null cells, constitutively activated (CA) mitogen-activated protein kinase kinase 1 (MEK1) restored VEGF production and CA-MEK1 or added VEGF rescued tumor growth and angiogenesis. These studies provide the first biological support for Y925 FAK phosphorylation and define a novel role for FAK activity in promoting a MAPK-associated angiogenic switch during tumor progression.

Decreased angiogenesis and arthritic disease in rabbits treated with an alphavbeta3 antagonist.

Rheumatoid arthritis (RA) is an inflammatory disease associated with intense angiogenesis and vascular expression of integrin alphavbeta3. Intra-articular administration of a cyclic peptide antagonist of integrin alphavbeta3 to rabbits with antigen-induced arthritis early in disease resulted in inhibition of synovial angiogenesis and reduced synovial cell infiltrate, pannus formation, and cartilage erosions. These effects were not associated with lymphopenia or impairment of leukocyte function. Furthermore, when administered in chronic, preexisting disease, the alphavbeta3 antagonist effectively diminished arthritis severity and was associated with a quantitative increase in apoptosis of the angiogenic blood vessels. Therefore, angiogenesis appears to be a central factor in the initiation and persistence of arthritic disease, and antagonists of integrin alphavbeta3 may represent a novel therapeutic strategy for RA.

Integrins as a distinct subtype of dependence receptors.

In the absence of their cognate ligand, dependence receptors trigger programmed cell death. This function is the defining feature of dependence receptors, which include members of several different protein families. The integrins are a family of heterodimeric receptors for extracellular matrix (ECM) proteins, mediating cell anchorage and migration. Integrins share characteristics with dependence receptors, and integrin binding to substrate ECM ligands is essential for cell survival. Although integrins do not conform in all characteristics to the established definitions of dependence receptors, alterations in the expression of integrins and their ligands during physiological and pathological events, such as wound healing, angiogenesis and tumorigenesis, do regulate cell fate in a ligand-dependent manner. This biosensory function of integrins fits well with our current concept of dependence receptor action, and thus integrins may rightly be considered to comprise a distinct subclass of dependence receptor.

TIFA, an inflammatory signaling adaptor, is tumor suppressive for liver cancer.

TIFA (TNF receptor associated factor (TRAF)-interacting protein with a Forkhead-associated (FHA) domain), also called T2BP, was first identified using a yeast two-hybrid screening. TIFA contains a FHA domain, which directly binds phosphothreonine and phosphoserine, and a consensus TRAF6-binding motif. TIFA-mediated oligomerization and poly-ubiquitinylation of TRAF6 mediates signaling downstream of the Tumor necrosis factor alpha receptor 1 (TNFaR-I) and interleukin-1/Toll-like receptor 4 (TLR4) pathways. Examining TIFA expression in hepatocellular carcinoma (HCC) tissues microarrays, we noted marked decreases TIFA reactivity in tumor versus control samples. In agreement, we found that HCC cell lines show reduced TIFA expression levels versus normal liver controls. Reconstituting TIFA expression in HCC cell lines promoted two independent apoptosis signaling pathways: the induction of p53 and cell cycle arrest, and the activation of caspase-8 and caspase-3. In contrast, the expression of a non-oligomerizing mutant of TIFA impacted cells minimally, and suppression of TIFA expression protected cells from apoptosis. Mice bearing TIFA overexpression hepatocellular xenografts develop smaller tumors versus TIFA mutant tumors; terminal deoxynucleotidyl transferase dUTP nick end labeling staining demonstrates increased cell apoptosis, and decreased proliferation, reflecting cell cycle arrest. Interestingly, p53 has a greater role in decreased proliferation than cell death, as it appeared dispensable for TIFA-induced cell killing. The findings demonstrate a novel suppressive role of TIFA in HCC progression via promotion of cell death independent of p53.

Molecular signaling mechanisms of cell migration and invasion.

The ability of immune cells to migrate and invade the extracellular matrix (ECM) is a central process involved in immunologic surveillance as well as inflammation. We have shown that interaction of cells with adhesive proteins or growth factors (chemokines) present in the ECM control cell migration/invasion through activation of mitogen-activated protein kinases ERK1 and ERK2 and molecular coupling of the adapter proteins p130CAS and c-CrkII. During cell migration, ERK and CAS/Crk coupling operate as distinct signaling pathways that facilitate actin-myosin motor assembly and actin membrane ruffles, respectively. Furthermore, activation of these signaling pathways protects cells from apoptosis during invasion of the ECM, which is necessary as migratory cells colonize foreign sites in the body.

A role for angiogenesis in rheumatoid arthritis.

Rheumatoid arthritis (RA) is a chronic debilitating disease characterized by distinct autoimmune, inflammatory and fibrovascular components which lead to synovial proliferation and joint destruction. However, existing treatments specifically target only autoimmune and inflammatory components despite the fact that neovascularization of the inflamed synovium is a hallmark of rheumatoid arthritis. Angiogenesis may contribute to synovial growth, leukocyte recruitment and tissue remodeling, thus potentiating disease progression. Although no therapies currently target angiogenesis, several existing therapies have anti-angiogenic activity. Recent advances in anti-angiogenic strategies in oncology, including the identification of integrin alpha v beta 3 as a crucial effector of angiogenesis, suggest a means to assess the role of angiogenesis in rheumatoid arthritis. Synovial endothelial cells have been shown to express integrin alpha v beta 3, suggesting that these cells may be targeted for angiogenesis inhibition. Prior studies in rat arthritis models have shown benefit after the addition of broad spectrum integrin antagonists. However, formal assessment of integrin-targeted anti-angiogenic activity is now underway. These controlled studies will be important in assessing the efficacy of therapies which target angiogenesis in RA.

Triggering necroptosis in cisplatin and IAP antagonist-resistant ovarian carcinoma.

Ovarian cancer patients are typically treated with carboplatin and paclitaxel, but suffer a high rate of relapse with recalcitrant disease. This challenge has fostered the development of novel approaches to treatment, including antagonists of the 'inhibitor of apoptosis proteins' (IAPs), also called SMAC mimetics, as apoptosis-inducing agents whose action is opposed by caspase inhibitors. Surprisingly, IAP antagonist plus caspase inhibitor (IZ) treatment selectively induced a tumor necrosis factor-α (TNFα)-dependent death among several apoptosis-resistant cell lines and patient xenografts. The induction of necroptosis was common in ovarian cancer, with expression of catalytically active receptor-interacting protein kinase-3 (RIPK3) necessary for death, and in fact sufficient to compromise survival of RIPK3-negative, necroptosis-resistant ovarian cancer cells. The formation of a necrosome-like complex with a second critical effector, receptor-interacting serine-threonine kinase-1 (RIPK1), was observed. RIPK1, RIPK3 and TNFα were required for the induction of death, as agents that inhibit the function of any of these targets prevented cell death. Abundant RIPK3 transcript is common in serous ovarian cancers, suggesting that further evaluation and targeting of this RIPK3-dependent pathway may be of clinical benefit.

EGFR-dependent pancreatic carcinoma cell metastasis through Rap1 activation.

Tyrosine kinase receptors have an essential role in various aspects of tumor progression. In particular, epidermal growth factor receptor (EGFR) and its ligands have been implicated in the growth and dissemination of a wide array of human carcinomas. Here, we describe an EGFR-mediated signaling pathway that regulates human pancreatic carcinoma cell invasion and metastasis, yet does not influence the growth of primary tumors. In fact, ligation/activation of EGFR induces Src-dependent phosphorylation of two critical tyrosine residues of p130CAS, leading to the assembly of a Crk-associated substrate (CAS)/Nck1 complex that promotes Ras-associated protein-1 (Rap1) signaling. Importantly, GTP loading of Rap1 is specifically required for pancreatic carcinoma cell migration on vitronectin but not on collagen. Furthermore, Rap1 activation is required for EGFR-mediated metastasis in vivo without impacting primary tumor growth. These findings identify a molecular pathway that promotes the invasive/metastatic properties of human pancreatic carcinomas driven by EGFR.

Paclitaxel promotes a caspase 8-mediated apoptosis through death effector domain association with microtubules.

Microtubule-perturbing drugs have become front-line chemotherapeutics, inducing cell-cycle crisis as a major mechanism of action. However, these agents show pleiotropic effects on cells and can induce apoptosis through other means. Paclitaxel, a microtubule-stabilizing agent, induces a caspase-dependent apoptosis, although the precise mechanism(s) remain unclear. Here, we used genetic approaches to evaluate the role of caspase 8 in paclitaxel-mediated apoptosis. We observed that caspase 8-expressing cells are more sensitive to paclitaxel than caspase 8-deficient cells. Mechanistically, caspase 8 was found associated with microtubules, and this interaction increased after paclitaxel treatment. The prodomains death effector domains (DEDs) of caspase 8 were sufficient for interaction with microtubules, but the caspase 8 holoprotein was required for apoptosis. DED-only forms of caspase 8 were found in both primary and tumor cell lines, associating with perinuclear microtubules and the centrosome. Microtubule association, and paclitaxel sensitivity, depends on a critical lysine (K156) within a microtubule-binding motif (KLD) in DED-b of caspase 8. The results show an unexpected pathway of apoptosis mediated by caspase 8.

Regulation of angiogenesis: apoptotic cues from the ECM.

The extracellular matrix (ECM) acts both as a physical scaffold for cells and as a repository for growth factors. Moreover, ECM structure and physical-chemical properties convey precise information to cells that profoundly influences their biology by interactions with cell surface receptors termed integrins. During angiogenesis, the perivascular ECM plays a critical role in determining the proliferative, invasive and survival responses of the local vascular cells to the angiogenic growth factors. Dynamic changes in both the ECM and the local vascular cells act in concert to regulate new blood vessel growth. The digestion of ECM components by proteolysis is critical for the invasive capacity of endothelial cells, but also creates ECM fragments, which antagonize the mechanosensory function of integrins, and can be apoptogenic. Here, we discuss the roles of integrins in modulating cellular responses to a changing ECM, in particular the regulation of survival and invasion among invasive endothelial cells.

Distinct FAK-Src activation events promote alpha5beta1 and alpha4beta1 integrin-stimulated neuroblastoma cell motility.

Signals from fibronectin-binding integrins promote neural crest cell motility during development in part through protein-tyrosine kinase (PTK) activation. Neuroblastoma (NB) is a neural crest malignancy with high metastatic potential. We find that alpha4 and alpha5 integrins are present in late-stage NB tumors and cell lines derived thereof. To determine the signaling connections promoting either alpha4beta1- or alpha5beta1-initiated NB cell motility, pharmacological, dominant negative and short-hairpin RNA (shRNA) inhibitory approaches were undertaken. shRNA knockdown revealed that alpha5beta1-stimulated NB motility is dependent upon focal adhesion kinase (FAK) PTK, Src PTK and p130Cas adapter protein expression. Cell reconstitution showed that FAK catalytic activity is required for alpha5beta1-stimulated Src activation in part through direct FAK phosphorylation of Src at Tyr-418. Alternatively, alpha4beta1-stimulated NB cell motility is dependent upon Src and p130Cas but FAK is not essential. Catalytically inactive receptor protein-tyrosine phosphatase-alpha overexpression inhibited alpha4beta1-stimulated NB motility and Src activation consistent with alpha4-regulated Src activity occurring through Src Tyr-529 dephosphorylation. In alpha4 shRNA-expressing NB cells, alpha4beta1-stimulated Src activation and NB cell motility were rescued by wild type but not cytoplasmic domain-truncated alpha4 re-expression. These studies, supported by results using reconstituted fibroblasts, reveal that alpha4beta1-mediated Src activation is mechanistically distinct from FAK-mediated Src activation during alpha5beta1-mediated NB migration and support the evaluation of inhibitors to alpha4, Src and FAK in the control of NB tumor progression.

Integrins and angiogenesis.

The growth of new blood vessels is a dynamic yet highly regulated process that depends on coordinated signaling by growth factor and cell adhesion receptors. As part of the molecular program regulating angiogenesis, endothelial cells acquire a proliferative and invasive phenotype but also show increased susceptibility to apoptotic stimuli. Integrins are the principle adhesion receptors used by endothelial cells to interact with their extracellular microenvironment, and integrin-mediated interactions play a critical role in regulating cell proliferation, migration, and survival. Alterations in the repertoire and?or activity of integrins, as well as the availability and structural property of their ligands, regulate the vascular cell during the growth or repair of blood vessels.

Induction of alpha v beta 3 integrin-mediated attachment to extracellular matrix in beta 1 integrin (CD29)-negative B cell lines.

beta 1 integrin containing complexes have been implicated as the primary adhesion structures in many lymphocyte extracellular matrix (ECM) interactions. However, many B lymphocytes lack surface expression of the beta 1 subunit, implying that this subpopulation of lymphoid cells must employ alternate adhesion structures if they are to maintain an interactive capacity with ECM. An examination of the adherence properties of the beta 1 integrin-negative B cell line JY indicated that these cells exhibit little or no basal adherence to any of the ECM components examined. However, these cells could be induced to adhere to the ECM components fibronectin, laminin, and vitronectin following treatment with PMA. Blocking studies with monoclonal antibodies indicated the alpha v beta 3 integrin complex was involved in the attachment to each of these ligands. However, the adherence to fibronectin displayed a complex pattern of inhibition suggesting the involvement of other ECM receptors. The utilization of the alpha v beta 3 complex was not unique to the JY cell line. Other B cell lines were observed to employ alpha v beta 3, and these lines similarly lacked expression of beta 1 integrin. These results indicate that alpha v beta 3 can act as a lymphoid ECM-adhesion structure which may provide an alternative means for lymphocytes to interact with ECM. Furthermore, these studies provide evidence for the presence of lymphoid-associated alpha v beta 3 integrins with regulatable activity, which contrasts with the constitutive adhesive potential of these complexes when present on other cell types.

Control of lymphocyte integrin function: evidence for multiple contributing factors.

The control of lymphocyte adhesion is critical for proper cellular functions. The alpha 2 beta 1 integrin complex serves as a receptor for collagen on lymphocytes. The T cell leukemia Jurkat expresses alpha 2 beta 1 in a latent form on the cell surface. Three types of stimuli, antibody to the alpha 2 chain (JBS2), antibody to the beta 1 chain (JB1B), and phorbol ester (PMA), each induce activation of alpha 2 beta 1-dependent Jurkat binding to collagen. A comparison of the JBS2, JB1B, and PMA induction requirements indicated that the JB1B and the JBS2 effects are not sensitive to staurosporine while the PMA response is completely inhibited. Combinations of functionally saturating concentrations of the stimuli displayed an additive effect. Collectively these results suggest that several factors contribute to the generation of full integrin functionality and cellular adhesion, thus providing a possible basis for incrementally controlling the adhesive potential of lymphoid cells.

B lymphocyte fibronectin receptors: expression and utilization.

Lymphocytes adhere to fibronectin (FN) via multiple receptors of the VLA (beta 1, CD29 integrin) family. The cellular requirement for the variety of FN receptors (FNR) which have been described is unclear, but they may be associated with differential signalling processes, cooperative effects which may stabilize cellular attachment, or cell homing and retention processes. The present study was undertaken to examine the FN adherence properties and receptor utilization patterns of human B cells. Of ten B-cell culture lines which were studied, six demonstrated a significant adherence to FN. Among these, four employed alpha 4 beta 1, (CD49d/29) and two employed alpha 4 beta 1/alpha 5 beta 1, (CD49d/29, CD49e/29). There was no apparent correlation between the differentiation status of the lines and their FNR utilization patterns. Furthermore, expression of FNR alone was not sufficient to confer FN binding potential. Freshly isolated tonsil B cells did not display significant adherence to FN. Following stimulation, a marked increase in VLA antigens was observed, and the capacity to attach to FN-coated surfaces was co-acquired. Analysis of the induced bulk B-cell population demonstrated that both alpha 4 beta 1 and alpha 5 beta 1 were used for adherence. These results clearly indicate that activated B cells, similar to T cells, may express and utilize alpha 5 beta 1 as a FNR.

Control of beta1 integrin function. Localization of stimulatory epitopes.

The beta1 integrins can be expressed on the surface of cells in a latent form, which is activated by a variety of stimuli. As an approach to examining the transition to an active receptor, a panel of stimulatory antibodies to beta1 were produced and characterized. These antibodies induced adherence of the T-leukemic cell line Jurkat to collagen and fibronectin. Competitive antibody binding assays indicated the existence of at least three distinct epitope clusters A (B3B11, JB1B, 21C8), B (B44, 13B9), and C(N29) defined by the indicated antibodies. Two antibodies to the A site, JB1B and B3B11, were shown to localize to positions 671-703 and 657-670, respectively, of the beta1. This region is located in an area encompassing a predicted disulfide bond between linearly distant cysteines in beta1 (Cys415-Cys671). The homologous region of the beta3 integrin (490 690 and 602 690) has been shown to be one of the sites recognized by stimulatory antibodies to ligand-induced binding sites. The present results indicate the existence of multiple stimulatory regions and suggest considerable homology between the locations of beta1 and beta3 regulatory sites.

Perturbation of rat renal tubule transport of the organic cation amantadine in recent onset streptozotocin-induced diabetes and in uninephrectomy.

The effects of early-stage diabetes mellitus and uninephrectomy on the renal tubule transport of amantadine were investigated. Kidney tubules were isolated from streptozotocin-induced diabetic (+/- insulin treatment) uninephrectomized, and control male Sprague-Dawley rats. There were no differences in the Km of amantadine uptake in renal proximal and distal tubules for the imposed treatments compared with control values. Vmax for amantadine uptake in the proximal tubules of diabetic and uninephrectomized rats was higher than the respective control (P < 0.05). Vmax for insulin-treated diabetic rats was similar to control values but was lower than that for untreated diabetic rats (P < 0.05). Vmax for distal tubule uptake was not altered by any treatment. Structure-activity studies demonstrated that bicarbonate-dependent amantadine uptake was inhibited by glycolate and lactate, but not by propionate or alpha-, beta-, or gamma-hydroxybutyrate. Early stage streptozotocin-induced diabetes mellitus and uninephrectomy induced changes in the kidney that resulted in a similar selective increase in proximal tubule amantadine uptake. These data represent the first description that experimentally induced diabetes mellitus and uninephrectomy modulate the function of the renal tubule organic cation (amantadine) transport system. Both interventions represent potential models in which phenotypic modulation of the renal elimination of organic cationic drugs may be achieved and studied.

Heterogeneity among beta-adrenoreceptor blockers in the modulation of energy-dependent uptake of the organic cation amantadine by rat renal tubules.

Eight representative beta-adrenoreceptor blocking drugs, acebutolol, atenolol, labetalol, metoprolol, nadolol, pindolol, propranolol, and timolol, were studied in vitro with respect to their potential to block energy-dependent uptake of [3H]amantadine into proximal and distal rat renal tubule fragments in the presence and absence of bicarbonate. Five of the eight beta-adrenoreceptor blockers showed a dose-dependent inhibition of renal tubule accumulation of amantadine: labetalol, metoprolol, pindolol, propranolol, and timolol. Labetalol was the only beta-adrenoreceptor blocker with greater inhibitory potency in phosphate-based buffer than in bicarbonate-based buffer. Propranolol was the most potent inhibitor of renal tubule amantadine accumulation with IC50 values of 15 +/- 10 and 31 +/- 11 microM for proximal and distal tubule fragments, respectively, in a bicarbonate-based buffer environment. Inhibition in a phosphate-based buffer was less potent only in proximal tubules, with an IC50 of 76 +/- 30 microM. Kinetic studies of propranolol inhibition of amantadine uptake were consistent with both uncompetitive and competitive inhibition mechanisms in bicarbonate-based buffer in both proximal and distal renal tubule segments. There was no chiral preference between the R and S forms of propranolol for the inhibitory effects observed. These data suggest that there is potential for selection among the beta-adrenoreceptor blocking drugs to minimize or restrict the inhibition of amantadine energy-dependent uptake at the organic cation ion uptake sites characterized by amantadine in the presence and absence of bicarbonate.