Morphological change is one of the cardinal features of the senescent phenotype; for example, senescent human diploid cells have a flat large shape. However, the mechanisms underlying such senescence-related morphological alterations have not been well studied. To investigate this situation, we characterized the senescence-dependent changes of cellular structural determinants in terms of their levels and activities. These determinants included integrins, focal adhesion complexes, and small Rho GTPases, and special emphasis was placed on their relationships with caveolin-1 status. We observed that the expression integrin β₁ and focal adhesion kinase (FAK) were increased and that the phosphorylations of FAK and paxillin, hallmarks of focal adhesion formation, were also increased in senescent human diploid fibroblast cells. Moreover, the Rho GTPases Rac1 and Cdc42 were found to be highly activated in senescent cells. In addition, focal adhesion complexes and Rho GTPases were up-regulated in the caveolin-rich membrane domain in the senescent cells. Activated Rac1 and Cdc42 directly interacted with caveolin-1 in senescent cells. Interestingly, caveolin-1 knock-out senescent cells, achieved by using small interfering RNA and antisense oligonucleotide, showed disrupted focal adhesion formation and actin stress fibers via the inactivation of FAK, which resulted in morphological adjustment to the young cell-like small spindle shape. Based on the results obtained, we propose that caveolin-1 plays an important role in senescence-associated morphological changes by regulating focal adhesion kinase activity and actin stress fiber formation in the senescent cells.

Engagement of cells with the extracellular matrix (ECM) proteins is crucial for various biological processes, including cell adhesion, spreading, proliferation, differentiation, migration, apoptosis, and gene induction, contributing to maintenance of tissue integrity, embryogenesis, wound healing, and the metastasis of tumor cells (Hynes, 2002b; Juliano, 2002). The engagement involves cell adhesion mediated by integrins, a large family of cell adhesion receptors that are transmembrane glycoproteins which bind to ECM or to counter-receptors on neighbor cells. In this review, the molecular basis of signaling mediated by integrins and their collaboration with growth factor receptors will be discussed, based on recent observations. Although other cell adhesion receptors including cadherins, selectins, syndecans, and the immunoglobulin superfamily of cell adhesion molecules (IgCAMs) can play important roles or be involved in these processes, we suggest readers refer to recent outstanding reviews on them (Barclay, 2003; Brummendorf and Lemmon 2001; Panicker et al. 2003).

Tumor angiogenesis is required for tumor development and is stimulated by angiogenic inducers like VEGF (vascular endothelial growth factor). Our previous study demonstrated that STAT3 (signal transducer and activator of transcription 3) up-regulates HIF-1α (hypoxia inducible factor-1α) protein stability and enhances HIF-1-mediated VEGF expression in hypoxic solid tumor cells, thus suggesting that the inhibition of STAT3 signaling may have clinical applications. In this study, we examined in vitro and in vivo, whether caffeic acid (CA) or its derivative CADPE [3-(3,4-dihydroxy-phenyl)-acrylic acid 2-(3,4-dihydroxy-phenyl)-ethyl ester] exert anticancer activity by targeting STAT3. It was found that CA or CADPE significantly inhibit STAT3 activity, and that this in turn down-regulates HIF-1α activity. Consequently, sequential blockade of STAT3 and HIF-1α resulted in the down-regulation of VEGF by inhibiting their recruitment to the VEGF promoter. In mice bearing a Caki-I carcinoma, both CA and CADPE retarded tumor growth and suppressed STAT3 phosphorylation, HIF-1α expression, vascularization and STAT3-inducible VEGF gene expression in tumors. Taken together, our results demonstrate that CA and CADPE are potential inhibitors of STAT3 and that they suppress tumor angiogenesis by inhibiting the activity of STAT3, the expression of HIF-1α and VEGF.

Renewal of the gastrointestinal epithelium involves a coordinated process of terminal differentiation and programmed cell death. Integrins have been implicated in the control of apoptotic processes in various cell types. Here we examine the role of integrins in the regulation of apoptosis in gastrointestinal epithelial cells with the use of a rat small intestinal epithelial cell line (RIE1) as a model. Overexpression of the integrin α5 subunit in RIE1 cells conferred protection against several proapoptotic stimuli. In contrast, overexpression of the integrin α2 subunit had no effect on cell survival. The antiapoptotic effect of the α5 subunit was partially retained by a mutated version that had a truncation of the cytoplasmic domain. The antiapoptotic effects of the full-length or truncated α5 subunit were reversed upon treatment with inhibitors of phosphatidylinositol 3-kinase (PI-3-kinase), suggesting that the α5β1 integrin might interact with the PI-3-kinase/Akt survival pathway. When cells overexpressing α5 were allowed to adhere to fibronectin, there was a moderate activation of protein kinase B (PKB)/Akt, whereas no such effect was seen in α2-overexpressing cells adhering to collagen. Furthermore, in cells overexpressing α5 and adhering to fibronectin, there was a dramatic enhancement of the ability of growth factors to stimulate PKB/Akt; again, this was not seen in cells overexpressing α2 subunit and adhering to collagen or fibronectin. Expression of a dominant negative version of PKB/Akt in RIE cells blocked to ability of α5 to enhance cell survival. Thus, the α5β1 integrin seems to protect intestinal epithelial cells against proapoptotic stimuli by selectively enhancing the activity of the PI-3-kinase/Akt survival pathway.