Expression of beta 1 integrin receptors in transformed mouse epidermal keratinocytes: upregulation of alpha 5 beta 1 in spindle carcinoma cells

Integrin Signaling as a Cancer Drug Target

Integrins are transmembrane receptors that mediate cell adhesion to neighboring cells and to the extracellular matrix. Here, the various modes in which integrin-mediated adhesion regulates intracellular signaling pathways impinging on cell survival, proliferation, and differentiation are considered. Subsequently, evidence that integrins also control crucial signaling cascades in cancer cells is discussed. Lastly, the important role of integrin signaling in tumor cells as well as in stromal cells that support cancer growth, metastasis, and therapy resistance indicates that integrin signaling may be an attractive target for (combined) cancer therapy strategies. Current approaches to target integrins in this context are reviewed.

Single-nucleotide polymorphisms of integrins are associated with the risk and lymph node metastasis of oral squamous cell carcinoma

Integrins mediate cell-extracellular matrix (ECM) interaction and play a critical role in cell proliferation, migration, survival, and invasion. Polymorphisms of the Integrin gene can alter integrin protein expression, which may further increase the risk of malignancies or cause poor clinical outcomes. The aim of this study was to investigate the potential associations of single-nucleotide polymorphisms (SNPs) of the integrin gene with the susceptibility and clinicopathologic parameters of oral squamous cell carcinoma (OSCC). Two hundred patients with OSCC and 200 healthy volunteers were recruited. Five SNPs of Integrin genes were analyzed by polymerase chain reaction-restriction fragment length polymorphism genotyping. Their correlations with OSCC risk and clinical tumor characteristics were evaluated. The AT genotype of rs3809865 and GC genotype of rs1190271 increased the risk of OSCC by 2.1-fold (95% confidence interval 1.323-3.829, P=0.004) and 2.85-fold (95% confidence interval 1.583-5.138, P<0.01), respectively. Cases with AT genotype of rs3809865 and GC genotype of rs11902171 were prone to have regional lymph nodes metastasis (P<0.01). The SNPs of rs3809865 and rs1190271 might contribute to the prediction of susceptibility and lymph node metastasis of OSCC, which should be thought as a diagnostic marker and a potential therapeutic target.

New roles for integrins in squamous-cell carcinoma

Although integrins are known to mediate invasion and metastasis, recent studies reveal new ways in which they contribute to squamous-cell carcinoma. Integrin mutation or upregulation can expand the tumour stem-cell compartment by inhibiting differentiation or apoptosis. Integrins that are expressed by differentiated cells can stimulate or inhibit the proliferation of neighbouring tumour stem cells. These findings provide a mechanistic basis for the well-established links between altered integrin expression and tumour prognosis.

Suprabasal alpha 5 beta1 integrin expression stimulates formation of epidermal squamous cell carcinomas without disrupting TGFbeta signaling or inducing spindle cell tumors

During epidermal chemical carcinogenesis benign papillomas convert to squamous cell carcinomas, some of which undergo epithelial-mesenchymal conversion to highly malignant spindle cell tumors. TGFbeta inhibits early stages of carcinogenesis but promotes the spindle cell phenotype in later stages. One hallmark of spindle cell tumors is upregulation of the alpha 5 beta 1 integrin fibronectin receptor. To examine the significance of altered alpha 5 beta1 integrin expression, we induced tumors in transgenic mice expressing alpha 5 beta1 in the suprabasal epidermal layers. Invalpha 5 beta1 mice developed threefold more papillomas and squamous cell carcinomas than wild-type (Wt) littermates; however, no spindle cell tumors or increased metastases were observed. Suprabasal expression of the alpha 6 beta 4 integrin increases squamous cell carcinoma formation and decreases TGFbeta sensitivity while alpha 3 beta1 may have the opposite effect. In contrast, nuclear phosphoSmad2 labeling in Invalpha 5 beta1 epidermis and tumors was indistinguishable from Wt, and suprabasal alpha 5 beta1 did not block TGFbeta-induced Smad2/3 translocation or growth inhibition in cultured keratinocytes. We conclude that upregulation of alpha 5 beta1 does not predispose the epidermis to undergo conversion to spindle cell tumors and that the mechanism by which alpha 5 beta1 influences susceptibility to carcinogenesis is independent of perturbed TGFbeta signaling.

GnT-V, macrophage and cancer metastasis: a common link

GnT-V generated, beta1,6-branched polylactosamines are a common feature shared by normal granulocytes, monocytes, and a variety of malignant cells. Furthermore, activation of GnT-V in oncogenic transformation induces invasiveness and metastatic potential in mice as well as in humans. In view of the common expression of lymphocytic/monocytic trait, motility, and GnT-V by metastatic cancer cells, macrophage fusion hybrids were generated in vitro with Cloudman S91 mouse melanoma cells to test whether the parental traits are co-expressed in hybrids and how those are related to altered phenotypes in relation to metastasis. In fact, the fusion hybrids are highly metastatic in vivo, motile in vitro, and express macrophage-associated traits of increased GnT-V activity, beta1,6 branching, and polylactosamine content. A Spontaneously formed lung melanoma metastases have been identified and characterized as host x tumor hybrid containing higher DNA content than parental cells and increased GnT-V activity. The results, taken together, could reflect prior fusion of tumor-associated macrophages with cells of the primary tumor, and therefore establish a possible common link between elevated expression of GnT-V and malignant transformation, a well-known report. Moreover, the fusion hybrids with metastatic potential ranging from high to low offer a genetically matched model system, for identification and characterization of differentially expressed genes in association with metastasis, since the fusion partners are derived from the same species of mouse (DBA/2J).

Reduced focal adhesion kinase and paxillin phosphorylation in BCR-ABL-transfected cells

BACKGROUND

BCR-ABL formation is critical to oncogenic transformation in chronic myelogenous leukemia and has been implicated as a key event leading to alterations in cytoskeletal structures and adhesion in the leukemic cells. The authors therefore investigated the effect of p210(BCR-ABL) on actin polymerization as well as on the expression and phosphorylation state of the adhesion proteins paxillin and focal adhesion kinase (FAK).

METHODS

Transfection with BCR-ABL constructs abrogated the ability of NIH 3T3 fibroblasts to adhere and the cells underwent striking morphologic changes.

RESULTS

Scanning electron microscopy revealed that the cells lost their elongated appearance and became rounded. This alteration was associated with significantly reduced actin polymerization. In addition, steady-state levels of paxillin and FAK protein were increased. However, while the overall level of phosphotyrosines was also increased, the amount of tyrosine phosphorylated paxillin and FAK was reduced in the BCR-ABL-transfected cells as compared to the parental cells. Culture on extracellular fibronectin matrix partially reversed the morphologic changes and resulted in a return, albeit incomplete, of filamentous actin in BCR-ABL-transfected 3T3 fibroblasts. In addition, phosphorylation of paxillin and FAK in the BCR-ABL-transfected NIH 3T3 cells was restored.

CONCLUSIONS

The authors conclude that, in the current system, transfection of BCR-ABL attenuates FAK and paxillin phosphorylation and reduces actin polymerization, events accompanied by significant alterations in cellular morphology. The observation that exposure of the cells to fibronectin partially reverses all these changes suggests that the focal adhesion proteins and actin structures nevertheless remain responsive to signaling from the outside.

Adhesive mechanisms regulating invasion and metastasis in oral cancer

It is the relentless invasion and growth into surrounding tissue that characterize oral squamous cell carcinoma. Metastasis is perhaps the most challenging and important aspect of cancer progression, in that it generally signifies limited survival and ineffective therapy. Inherent in metastasis is invasion, the process by which cells infiltrate into adjacent tissues, degrading basement membranes and extracellular matrix and disrupting tissue architecture and sometimes organ function. The factors that regulate these processes are complex and likely involve loss of the controls that are normally in place in physiologic tissue modeling. Adhesion receptors and their ligands are important in modulating not only invasion of oral squamous cell carcinoma cells but also their survival and proliferation. Normal oral mucosal epithelial cells use integrins to maintain their anchorage to the basement membrane, whereas the formation of stratifying cell layers depends on the formation of intercellular adhesions mediated by cadherins. The process of squamous cell carcinoma invasion and dissemination requires active cell migration through the extracellular matrix with the simultaneous remodeling of intercellular adhesions. Integrins are clearly important in the invasive process, whereas intercellular adhesion receptors restrain invasion and promote a more differentiated phenotype.

Alterations in focal adhesion kinase activity and associated proteins during malignant conversion of mouse keratinocytes

Focal adhesion kinase (pp125FAK) has well-established functions in the attachment and growth of cells in culture and has been implicated as a marker of malignant progression in human tumors. To evaluate its role in the metastatic conversion of mouse skin tumors, pp125FAK activity and protein expression were examined in normal and transformed keratinocyte cell lines. Malignant mouse keratinocyte lines exhibited a reproducible increase in the specific activity of pp125FAK compared with that of nontransformed control cells. An increase in pp125FAK activity was not observed in papilloma-derived keratinocytes, indicating that this response correlated with malignant progression of cells and not cell transformation per se. Immune complex kinase assays and metabolic labeling with [32P]orthophosphate also revealed the specific loss of pp125FAK-associated proteins in the metastatic keratinocytes. Furthermore, immunocytochemical examination revealed an altered distribution of pp125FAK in the cells with malignant potential compared with normal and papilloma-inducing keratinocytes. The cells with malignant potential also exhibited reduced levels of paxillin and integrin beta1 as well as altered distribution of paxillin, reinforcing the notion that specific changes in the composition of focal adhesions contribute to the malignant conversion of mouse keratinocytes.

Induction of mutual stabilization and retardation of tumor growth by coexpression of plakoglobin and E-cadherin in mouse skin spindle carcinoma cells

The influence of plakoglobin on the phenotype and tumorigenicity of murine spindle carcinoma cells was analyzed by stable transfection of plakoglobin cDNA in the presence or absence of E-cadherin expression. In either situation, overexpression of plakoglobin was unable to modify the fibroblastic phenotype or to completely suppress the tumorigenic behavior of the spindle cells, but a moderate reduction in the growth rate of the tumors was induced by plakoglobin and was further enhanced by E-cadherin. Coexpression of E-cadherin and plakoglobin induced a mutual stabilization, increasing the half-life of both molecules in the double transfectants more than 5- and 30-fold, respectively, with a turnover rate similar to that observed in control keratinocytes. The stabilization of E-cadherin, as well as that of plakoglobin, was maintained in the tumors induced by the double transfectants, in contrast to the unstable expression of E-cadherin observed in tumors induced in plakoglobin-deficient cells. The E-cadherin/catenin complexes present in the double transfectants were functional in calcium-dependent aggregation assays and similar in composition to those of control keratinocytes. However, most of the components of the complexes of the transfectants were solubilized by non-ionic detergents, indicating a weak interaction with the actin cytoskeleton. These results indicated that restoration of E-cadherin/catenin complexes was not sufficient to induce the transition of the fibroblastic cells to an epithelial phenotype or to completely suppress the tumorigenicity of mouse skin spindle carcinoma cells.

Expression pattern of the cell adhesion molecules. E-cadherin, P-cadherin and alpha 6 beta 4 intergrin is altered in pre-malignant skin tumors of p53-deficient mice

Expression of the cell adhesion molecules E-cadherin, P-cadherin and alpha 6 beta 4 integrin and of the keratin K13 has been analyzed in chemically induced benign skin papillomas with genetically pre-determined risks for malignant conversion. It has been previously shown that papillomas induced in mice lacking both alleles of the p53 gene have a much higher rate of malignant conversion than those induced in wild-type and heterozygous p53 mice. Alterations in the expression pattern of the E-cadherin molecule, including focal loss at cell-cell contacts and heterogeneous distribution in the differentiated layers, were found in about 70% of the p53 null papillomas. In contrast, all of the wild-type and over 85% of the heterozygous p53 papillomas exhibited an expression pattern of E-cadherin indistinguishable from that of normal epidermis. Alterations in P-cadherin expression were also detected in the p53 null papillomas: aberrant suprabasal localization and heterogeneous distribution were observed more frequently than in heterozygous and wild-type p53 papillomas. The alpha 6 beta 4 integrin showed suprabasal expression in more than 70% of the papillomas derived from either wild-type, heterozygous or homozygous p53 null mice. Surprisingly, the extent of the suprabasal localization of alpha 6 beta 4 decreased in the p53 null papillomas. Aberrant keratin K13 expression was also detected in the majority of cases of all p53 genotypes, but again there was a clear decrease in expression levels in the p53 null papillomas. These alterations were also associated with keratinocytic atypia, which increased significantly in the p53 null papillomas. Changes in these parameters were particularly evident during malignant conversion in invasive regions of one progressing p53 null papilloma. Our results indicate the existence of dynamic changes in the expression pattern of the 3 cell adhesion molecules analyzed and identify down-regulation of E-cadherin as an early step in malignant conversion.