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

The expression of the cytoskeletal focal adhesion protein paxillin in breast cancer correlates with HER2 overexpression and may help predict response to chemotherapy: a retrospective immunohistochemical study

Paxillin, a cytoskeletal focal adhesion adaptor protein, has been shown to be transcriptionally up-regulated and phosphorylated by human epidermal growth factor receptor-2 (HER2) signaling in vitro. Paxillin expression may also correlate with HER2 amplification in breast cancer patients. In the current study, we sought to explore the relationship further between paxillin expression and clinicopathologic features and clinical outcome in breast cancer. A total of 314 primary invasive breast carcinomas were assessed for paxillin expression via immunohistochemistry. Paxillin immunoreactivity was compared with estrogen receptor/progesterone receptor status, HER2 status by silver in situ hybridization, age, tumor size, stage, Bloom-Richardson grade, nodal status, disease-free survival (DFS), and overall survival (OS). Paxillin expression was identified in 27.7% of breast carcinomas as diffuse cytoplasmic staining and the expression correlated with HER2 overexpression (p < 0.001). The influence of paxillin on clinical outcome, in particular the response to chemotherapy, appeared to differ depending on the HER2 status of the tumor. For the subset of HER2 nonamplified cases treated with chemotherapy, patients whose tumor showed a loss of paxillin expression demonstrated a significantly lengthened DFS and OS. In contrast, loss of paxillin expression in the HER2 amplified subset of patients who received chemotherapy correlated with a significantly worse outcome. These data suggest that paxillin up-regulation may be a part of the HER2 pathway in some breast cancers and, furthermore, paxillin expression may also influence the clinical response to chemotherapy, depending upon the HER2 status of a given patient's tumor. Further study of a role for paxillin expression in predicting response to cytotoxic regimens or targeted treatments is warranted.

RNF5, a RING finger protein that regulates cell motility by targeting paxillin ubiquitination and altered localization

RNF5 is a RING finger protein found to be important in the growth and development of Caenorhabditis elegans. The search for RNF5-associated proteins via a yeast two-hybrid screen identified a LIM-containing protein in C. elegans which shows homology with human paxillin. Here we demonstrate that the human homologue of RNF5 associates with the amino-terminal domain of paxillin, resulting in its ubiquitination. RNF5 requires intact RING and C-terminal domains to mediate paxillin ubiquitination. Whereas RNF5 mediates efficient ubiquitination of paxillin in vivo, protein extracts were required for in vitro ubiquitination, suggesting that additional modifications and/or an associated E3 ligase assist RNF5 targeting of paxillin ubiquitination. Mutant Ubc13 efficiently inhibits RNF5 ubiquitination, suggesting that RNF5 generates polychain ubiquitin of the K63 topology. Expression of RNF5 increases the cytoplasmic distribution of paxillin while decreasing its localization within focal adhesions, where it is primarily seen under normal growth. Concomitantly, RNF5 expression results in inhibition of cell motility. Via targeting of paxillin ubiquitination, which alters its localization, RNF5 emerges as a novel regulator of cell motility.

S100A7 (psoriasin) interacts with epidermal fatty acid binding protein and localizes in focal adhesion-like structures in cultured keratinocytes

S100 proteins are calcium-responsive signaling proteins that are overexpressed in cancer and inflammatory diseases. They act by forming complexes with target proteins to modify target protein function. Identifying S100 intracellular distribution, site of action, and protein targets are important goals. S100A7 (psoriasin) is an important member of this family that is markedly overexpressed in psoriatic keratinocytes; however, its role in disease progression is poorly understood. In this study, we express S100A7 in normal keratinocytes as a means to study S100A7 function. We show that S100A7 is present in the cytosol and in BiP/GRP78-positive (endoplasmic reticulum) tubular structures. When cells are challenged with elevated intracellular calcium, cytoplasmic S100A7 redistributes to alpha-actinin- and paxillin-positive peripheral structures that contact the substrate surface. Epidermal fatty acid binding protein is also overexpressed in psoriasis, and is a putative target of S100A7 in keratinocytes. To study this interaction, we coexpressed S100A7 and epidermal fatty acid binding protein. These studies indicate that S100A7 and epidermal fatty acid binding protein colocalize in the cytoplasm in untreated cultures, and localize in peripheral structures in response to calcium challenge. In addition, S100A7 expression appears to stabilize epidermal fatty acid binding protein level, and vice versa. Moreover, the proteins can be coprecipitated in the presence of bifunctional cross-linker, suggesting that they are part of a common complex. The colocalization with alpha-actinin and paxillin suggests that S100A7 and epidermal fatty acid binding protein colocalize in focal adhesion-like structures following calcium treatment.

Focal adhesion kinase as a potential target in oncology

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that plays a pivotal role in signal transduction at integrin-linked cellular adhesions, which mediate cell contact with the extracellular matrix. It has been shown to play a role in the survival of anchorage-dependent cells and to be essential for integrin-linked cell migration - processes that are likely to play important roles in the development of malignancies. FAK is upregulated in a wide variety of human epithelial cancers, with expression being closely correlated to invasive potential. Recently, evidence has emerged directly linking FAK expression to tumour development in vivo, raising the possibility that intervention strategies to block FAK function may potentially provide an opportunity for the development of anticancer therapeutics.

Loss of the EP2 prostaglandin E2 receptor in immortalized human keratinocytes results in increased invasiveness and decreased paxillin expression

Prostaglandin E(2) (PGE(2)) receptor subtype EP(2), which is coupled to cAMP metabolism, is known to mediate proliferation of primary human keratinocytes in vitro. The effect of gain or loss of EP(2) receptors in immortalized human keratinocytes (HaCat cells) was examined. HaCat keratinocytes were transfected with sense or anti-sense constructs of the EP(2) receptor. Loss or gain of EP(2) expression was documented by immunoblot and associated changes in agonist-stimulated cAMP production. Loss or gain of EP(2) receptor expression correlated with alterations in plating efficiencies but with modest affects on growth. When cell lines were studied in an organ culture model, anti-sense clones were highly invasive compared with vector controls and sense transfectants. A marked increase in prostaglandin production is commonly seen in malignant lesions. Because prostaglandin receptors are known to undergo ligand-induced receptor down-regulation, we sought to determine whether EP(2) receptor down-regulation results in increased invasiveness. In vector controls, invasiveness was reproduced by ligand-dependent EP(2) receptor down-regulation as assessed by immunohistochemistry. In addition, loss of EP(2) receptor expression was associated with decreased paxillin expression, a critical component of focal adhesion assembly. Thus, down-regulation of EP(2) receptors represents a potential mechanism for neoplastic progression to an invasive phenotype.