Caspase-8 as a regulator of tumor cell motility

The caspases are a family of ubiquitously expressed cysteine proteases best known for their roles in programmed cell death. However, caspases play a number of other roles in vertebrates. In the case of caspase-8, loss of expression is an embryonic lethal phenotype, and caspase-8 plays roles in suppressing cellular necrosis, promoting differentiation and immune signaling, regulating autophagy, and promoting cellular migration. Apoptosis and migration require localization of caspase-8 in the periphery of the cells, where caspase-8 acts as part of distinct biosensory complexes that either promote migration in appropriate cellular microenvironments, or cell death in inappropriate settings. In the cellular periphery, caspase-8 interacts with components of the focal adhesion complex in a tyrosine-kinase dependent manner, promoting both cell migration in vitro and metastasis in vivo. Mechanistically, caspase-8 interacts with components of both focal adhesions and early endosomes, enhancing focal adhesion turnover and promoting rapid integrin recycling to the cell surface. Clinically, this suggests that the expression of caspase-8 may not always be a positive prognostic sign, and that the role of caspase-8 in cancer progression is likely context-dependent.

Cell growth and matrix invasion of EBV-immortalized human B lymphocytes is regulated by expression of alpha(v) integrins

alpha(v) integrins have been shown to play an important role in epithelial-derived cell migration, cell growth and tumor invasion/metastasis, however their role on cells of hematopoietic origin is less clear. Epstein-Barr virus (EBV), a human herpesvirus associated with several lymphoproliferative disorders in man, induces expression of alpha(v) integrins on transformed B lymphocytes. In the studies reported here, we show that EBV infection increases alpha(v), beta3 and beta5 integrin subunit mRNAs as well as upregulates the expression of the alphavbeta3 integrin protein on human B cells. Among the nine different EBV proteins expressed in latently infected B cells (nuclear and plasma membrane-associated), only LMP1, LMP2A and EBNA2 were shown to selectively transactivate the alpha(v) integrin promoter. Treatment of EBV-transformed B cells with alpha(v) antisense oligonucleotides specifically reduced cell surface expression of alpha(v) integrins, inhibited cell growth in low serum, reduced cell invasion in matrigels and decreased expression of metalloprotease, MMP9. These studies indicate that alpha(v) integrins play a significant role in EBV-induced B-lymphocyte proliferation and invasion. Strategies to interfere with alphav integrin expression and/or function may therefore be of potential value in the treatment of EBV-associated lymphoproliferative disorders.

Adenovirus endocytosis requires actin cytoskeleton reorganization mediated by Rho family GTPases

Adenovirus (Ad) endocytosis via alphav integrins requires activation of the lipid kinase phosphatidylinositol-3-OH kinase (PI3K). Previous studies have linked PI3K activity to both the Ras and Rho signaling cascades, each of which has the capacity to alter the host cell actin cytoskeleton. Ad interaction with cells also stimulates reorganization of cortical actin filaments and the formation of membrane ruffles (lamellipodia). We demonstrate here that members of the Rho family of small GTP binding proteins, Rac and CDC42, act downstream of PI3K to promote Ad endocytosis. Ad internalization was significantly reduced in cells treated with Clostridium difficile toxin B and in cells expressing a dominant-negative Rac or CDC42 but not a H-Ras protein. Viral endocytosis was also inhibited by cytochalasin D as well as by expression of effector domain mutants of Rac or CDC42 that impair cytoskeletal function but not JNK/MAP kinase pathway activation. Thus, Ad endocytosis requires assembly of the actin cytoskeleton, an event initiated by activation of PI3K and, subsequently, Rac and CDC42.

Adenovirus endocytosis via alpha(v) integrins requires phosphoinositide-3-OH kinase

Integrins mediate cell adhesion and motility on the extracellular matrix, yet they also promote viral attachment and/or entry. Evidence is presented that adenovirus internalization by alpha(v) integrins requires activation of phosphoinositide-3-OH kinase (PI3K), whereas alpha(v) integrin-mediated cell motility depends on the ERK1/ERK2 mitogen-activated protein kinase pathway. Interaction of adenovirus with alpha(v), integrins induced activation of PI3K. Pharmacologic or genetic disruption of endogenous PI3K activity blocked adenovirus internalization and virus-mediated gene delivery yet had no effect on integrin-mediated cell adhesion or motility. Therefore, integrin ligation engages distinct signaling pathways that promote viral endocytosis or cell movement.

Growth arrest of Epstein-Barr virus immortalized B lymphocytes by adenovirus-delivered ribozymes

Epstein-Barr virus (EBV) infection is associated with several human diseases that involve unrestricted proliferation of B lymphocytes. EBV nuclear antigen 1 (EBNA-1) is expressed in all EBV-infected cells and plays an essential role in persistence of the EBV genome. EBNA-1 has also been reported to have oncogenic potential. As an approach for treating EBV infections, we examined the capacity of EBNA-1 ribozymes delivered by recombinant adenoviruses to suppress EBNA-1 expression and to block virus-induced B cell proliferation. In contrast to primary B cells, EBV-transformed B lymphoblastoid cell lines expressed alphav integrins, the adenovirus internalization receptors, and were also susceptible to adenovirus-mediated gene delivery. Adenovirus delivery of a specific ribozyme (RZ1) to lymphoblastoid cell lines, suppressed EBNA-1 mRNA and protein expression, significantly reduced the number of EBV genomes, and nearly abolished cell proliferation in low serum. Adenovirus delivery of RZ1 also prevented EBV infection of an established EBV-negative B cell line. These studies demonstrate the potential use of adenovirus-encoded ribozymes to treat EBV-induced lymphoproliferative disorders.

The interactions of gamma delta T cells with extracellular matrix: receptor expression and utilization patterns

Purified populations and clones of human gamma delta T cells were examined for their ability to interact with extracellular matrix (ECM) components. The stimulation of these cells with phorbol ester induced cellular adhesion for ECM. The adhesion structures for fibronectin and collagen were shown to be members of the CD29 integrin family. The expression patterns of beta 1, beta 2 and beta 3 integrins by these cells were examined. The receptor expression and utilization patterns suggest that alpha beta, gamma delta T cells and B cells have similar repertoires of adhesion structure.

Beta 1 integrin-mediated lymphocyte adherence to extracellular matrix is enhanced by phorbol ester treatment

The interaction of the human T cell leukemia, Jurkat, with the extracellular matrix components collagen and fibronectin was examined. These cells displayed constitutive binding to fibronectin and low levels of adherence to collagen which were enhanced following stimulation with phorbol esters. The relevant binding structures were identified as members of the CD29/beta 1 integrin family of adhesion molecules. Adherence to collagen and to fibronectin was mediated by alpha 2 beta 1 and alpha 5 beta 1, respectively. The enhancement of adherence by phorbol esters did not involve up-regulation of receptor expression but appeared to derive from the increased functionality of structures which were expressed on the cell surface.