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

A lethal perinatal cardiac phenotype resulting from altered integrin function in cardiomyocytes

BACKGROUND

Integrins are heterodimeric receptors that couple the extracellular matrix to intracellular signaling pathways and the cyoskeleton. Integrins are strain transducers and candidates for modulators or effectors of cardiac hypertrophy.

METHODS

To begin to probe this function, we have transgenically expressed a chimeric protein that alters integrin function in cardiomyocytes. The transgene (Tac-beta(1D)) consists of the biologically inert extracellular and transmembrane domain of the interleukin-2 receptor alpha subunit (Tac) fused to the cytoplasmic tail of the human beta(1D) integrin driven by the cardiac alpha-myosin heavy chain promoter. Transgene expression results in a severe, usually fatal, perinatal cardiac phenotype, characterized by initial electrocardiographic abnormalities followed by extensive myocyte loss, macrophage infiltration, and replacement fibrosis.

RESULTS

Expression of Tac-beta(1D) resulted in displacement of endogenous beta(1D) integrin from Z-lines and T-tubules, decreased expression of endogenous beta(1D), and disrupted the fibronectin pericellular matrix. These results are consistent with an essential role for beta(1) integrins in maintenance of cardiomyocyte viability and interaction with extracellular matrix.

CONCLUSION

The appearance of conduction abnormalities before morphologic changes suggests that integrins are important in the development or maintenance of the conducting system of the heart.

Integrin alpha(v)beta3 expression confers on tumor cells a greater propensity to metastasize to bone

The reasons why tumor cells metastasize to bone remain obscure. There is some evidence to support the theory that integrins (acting as cell surface adhesion receptors) play a role in mediating metastasis in certain organs. Here, we report that overexpression of a functionally active integrin alpha(v)b3 in Chinese hamster ovary (CHO) tumor cells drastically increased the incidence, number, and area of bone metastases in nude mice compared with those observed in mock-transfected CHO cells (CHO dhfr+) or in CHO cells expressing a functionally inactive integrin alpha(v)b3 (CHO beta3Delta744). Moreover, a breast cancer cell line (B02) established from bone metastases caused by MDA-MB-231 cells constitutively overexpressed integrin alpha(v)b3, whereas the cell surface expression level of other integrins remained unchanged. In vivo, the extent of bone metastases in B02-bearing mice was significantly increased compared with that of MDA-MB-231-bearing mice. In vitro, B02 cells and CHO cells expressing a functionally active integrin alpha(v)b3 exhibited substantially increased invasion of and adhesion to mineralized bone, bone sialoprotein, and collagen compared with those found with MDA-MB-231, CHO dhfr+, and CHO beta3Delta744 cells, respectively. Overall, our findings suggest that integrin alpha(v)b3 expression in tumor cells accelerates the development of osteolytic lesions, presumably through increased invasion of and adhesion to bone.

Tiny dancers: the integrin-growth factor nexus in angiogenic signaling

A vital step in growth factor-driven angiogenesis is the coordinated engagement of endothelial integrins with the extracellular matrix. The molecular mechanisms that partner growth factors and integrins are being elucidated, revealing an intricate interaction of surface receptors and their signaling pathways.

Loss of p53 compensates for alpha v-integrin function in retinal neovascularization

alpha(v)-Integrin antagonists block neovascularization in various species, whereas 20% of alpha(v)-integrin null mice are born with many normal looking blood vessels. Given that blockade of alpha(v)-integrins during angiogenesis induces p53 activity, we utilized p53 null mice to elucidate whether loss of p53 can compensate for alpha(v)-integrin function in neovascularization of the retina. Murine retinal vascularization was inhibited by systemic administration of an alpha(v)-integrin antagonist. In contrast, mice lacking p53 were refractory to this treatment, indicating that neovascularization in normal mice depends on alpha(v)-integrin-mediated suppression of p53. Blockade of alpha(v)-integrins during neovascularization resulted in an induction of p21(CIP1) in wild type and, surprisingly, in p53 null retinas, indicating that alpha(v)-integrin ligation regulates p21(CIP1) levels in a p53-independent manner. In conclusion, we demonstrate for the first time an in vivo intracellular mechanism for compensation of integrin function and that p53 and alpha(v)-integrins act in concert during retinal neovascularization.

Sensing the environment: a historical perspective on integrin signal transduction

Cell adhesion mediated by integrin receptors has a critical function in organizing cells in tissues and in guiding haematopoietic cells to their sites of action. However, integrin adhesion receptors have broader functions in regulating cell behaviour through their ability to transduce bi-directional signals into and out of the cell and to engage in reciprocal interactions with other cellular receptors. This historical perspective traces the key findings that have led to our current understanding of these important functions of integrins.

Networks and crosstalk: integrin signalling spreads

Multicellular organisms must coordinate signals from adhesion receptors with those from other signalling receptors (for example, growth factor receptors). Here, we briefly review paradigms of integrin-adhesion-receptor signalling. We discuss how adhesive signalling is coordinately regulated through intersecting networks. We also examine some examples of how some forms of integrin crosstalk may lead to unforeseen and potentially deleterious responses.

ECM remodeling regulates angiogenesis: endothelial integrins look for new ligands

The process of angiogenesis is a dynamic one. Vascular endothelial cells are changing at the same time the extracellular matrix is being remodeled. Stupack and Cheresh discuss how remodeling of the extracellular matrix (ECM) and changes in the endothelial cell protein production and integrin expression contribute to the complex process of new blood vessel growth from an existing vascular bed.

Role of integrins in cell invasion and migration

As cancer cells undergo metastasis--invasion and migration of a new tissue--they penetrate and attach to the target tissue's basal matrix. This allows the cancer cell to pull itself forward into the tissue. The attachment is mediated by cell-surface receptors known as integrins, which bind to components of the extracellular matrix. Integrins are crucial for cell invasion and migration, not only for physically tethering cells to the matrix, but also for sending and receiving molecular signals that regulate these processes.

Specialization of tumour vasculature

Tumour blood vessels express markers that are not present in resting blood vessels of normal tissues, but that can be shared by angiogenic vessels in non-malignant conditions. Many of these proteins are functionally important in the angiogenic process. Some tumours also contain lymphatic vessels, as well as channels that consist of cancer cells and their extracellular matrix. These special features of tumour vessels are good targets for cancer therapies.

Focal adhesions require catalytic activity of Src family kinases to mediate integrin-matrix adhesion

Members of the Src family of tyrosine kinases function to phosphorylate focal adhesion (FA) proteins. To explore the overlapping functions of Src kinases, we have targeted Csk, a negative regulator of the Src family, to FA structures. Expression of FA-targeted Csk (FA-Csk) effectively reduced the active form (nonphosphorylated at the C-terminal regulatory tyrosine) of Src members in the cell. We found that fibroblasts expressing FA-Csk lost integrin-mediated adhesion. Activated Src (SrcY529F) as well as activation of putative Src signaling mediators (Fak, Cas, Crk/CrkL, C3G, and Rap1) blocked the effect of FA-Csk in a manner dependent on Rap1. SrcY529F also inhibited activated Ras-induced cell detachment but failed to rescue detachment caused by an activated mutant of Raf1 (Raf-BXB) that Rap1 cannot inhibit. Although normal spreading onto fibronectin was restored by the beta(1) integrin affinity-activating antibody TS2/16 in cells expressing FA-Csk or Raf-BXB, FAs were lost in these cells. On the other hand, Rap1 activation could restore FAs in cells expressing FA-Csk. Activation of the executioner caspase, caspase 3, is essential for many forms of apoptosis. While a caspase 3 inhibitor (Z-DEVD-FMK) inhibited cell detachment triggered by activation of caspase 8, this inhibitor had no effect on cell detachment caused by FA-Csk. Likewise, overexpression of an activated Akt made cells resistant to the effect of caspase 8 activation, but not to the effect of FA-Csk. It is therefore likely that the primary cause of cell rounding and detachment induced by FA-Csk involves dysfunction of FAs rather than caspase-mediated apoptosis that may result from possible loss of survival signals mediated by Src family kinases. We suggest that endogenous Src family kinases are essential for FAs through activation of Rap1 in fibroblasts.

Integrin and growth factor receptor crosstalk

Crosstalk between integrins and growth factor receptors are an important signaling mechanism to provide specificity during normal development and pathological processes in vascular biology. Evidence from several model systems demonstrates the physiological importance of the coordination of signals from growth factors and the extracellular matrix to support cell proliferation, migration, and invasion in vivo. Several examples of crosstalk between these two important classes of receptors indicate that integrin ligation is required for growth factor-induced biological processes. Furthermore, integrins can directly associate with growth factor receptors, thereby regulating the capacity of integrin/growth factor receptor complexes to propagate downstream signaling. Recent data suggest that antagonists of alpha(v) integrins can provide a therapeutic benefit in human cancer patients, whereas knockout mice lacking specific integrins can provide an interesting insight into the role of integrins during development. This review will focus on the biological importance of integrin and growth factor receptor crosstalk that occurs during cell growth, migration, and invasion as well as in endothelial cells during angiogenesis.