The hidden role of paxillin: localization to nucleus promotes tumor angiogenesis.

Paxillin (PXN), a key component of the focal adhesion complex, has been associated with cancer progression, but the underlying mechanisms are poorly understood. The purpose of this study was to elucidate mechanisms by which PXN affects cancer growth and progression, which we addressed using cancer patient data, cell lines, and orthotopic mouse models. We demonstrated a previously unrecognized mechanism whereby nuclear PXN enhances angiogenesis by transcriptionally regulating SRC expression. SRC, in turn, increases PLAT expression through NF-ĸB activation; PLAT promotes angiogenesis via LRP1 in endothelial cells. PXN silencing in ovarian cancer mouse models reduced angiogenesis, tumor growth, and metastasis. These findings provide a new understanding of the role of PXN in regulating tumor angiogenesis and growth.

Disruption of the dynamic sub-cellular localization of the Xenopus tumorhead protein causes embryonic lethality at the early gastrula transition.

The Xenopus laevis tumorhead (TH) protein, a positive regulator of cell proliferation during embryogenesis, shuttles from the cell periphery into the nucleus during embryogenesis. In these studies, we performed a detailed analysis of TH's subcellular localization pattern to characterize its dynamic behavior. We found that TH exhibits distinct patterns of localization in different germ layers. At the blastula stage, TH is present in the apical cell periphery of prospective mesodermal and ectodermal cells. At the gastrula stage, TH is distributed throughout the entire cytoplasm of prospective mesodermal and ectodermal cells, whereas it shows nuclear localization in presumptive endodermal cells. TH moves into the nucleus of mesodermal and ectodermal cells during the neurula and early tailbud stages. To understand if TH is regulated by changes in its subcellular localization, we used a TH mutant containing signals for farnesylation and palmitoylation to tether the protein to the plasma membrane. Ubiquitous overexpression of this mutant causes embryonic lethality at the early gastrula transition. Further examination using TUNEL assays indicated that wild-type TH overexpression induces apoptosis during gastrulation, and that this effect is exacerbated by the overexpression of the membrane-bound TH mutant. Taken together, our results suggest that changes in the sub-cellular localization of the TH protein are important for its function because blocking the nuclear translocation of overexpressed TH increases apoptosis and causes embryos to die. Our data also suggest that TH plays a role outside the nucleus when it is present at the cell periphery.

Identification of TH-B, a new oligomeric variant of the Xenopus morphogenetic factor, tumorhead.

The Xenopus laevis gene tumorhead (TH) is a regulator of cell proliferation of the ectodermal germ layer during embryonic development. TH overexpression results in increased cell proliferation within the developing ectoderm, causing an expansion of the neural plate. Conversely, loss of TH function results in inhibition of proliferation of ectodermal cells. Embryos with altered levels of TH protein are unable to express neural differentiation markers, indicating that the effect of TH in proliferation is linked with differentiation in the nervous system. To date, the molecular mechanism by which TH affects cell proliferation during embryogenesis is unknown. We have utilized the yeast two-hybrid system to identify protein partners of TH that could lead us to define the mechanism or pathway through which TH functions. Using this assay we have identified a new variant of TH designated TH-B, as a potential protein partner of the original TH, now referred to as TH-A. The sequence for TH-B was found to be 85% identical at the amino acid level to the TH-A sequence. Further characterization of the TH-B variant using RT-PCR indicates that it is expressed ubiquitously throughout development from early cleavage stages until at least the tadpole stage. TH-B association with TH-A was confirmed in co-immnoprecipitation studies in Xenopus, indicating that the two variants may function as an oligomer in vivo. These studies reveal the presence of an isoform of TH that may possess novel functional capabilities.

Differential Effects of EGFL6 on Tumor versus Wound Angiogenesis.

Angiogenesis inhibitors are important for cancer therapy, but clinically approved anti-angiogenic agents have shown only modest efficacy and can compromise wound healing. This necessitates the development of novel anti-angiogenesis therapies. Here, we show significantly increased EGFL6 expression in tumor versus wound or normal endothelial cells. Using a series of in vitro and in vivo studies with orthotopic and genetically engineered mouse models, we demonstrate the mechanisms by which EGFL6 stimulates tumor angiogenesis. In contrast to its antagonistic effects on tumor angiogenesis, EGFL6 blockage did not affect normal wound healing. These findings have significant implications for development of anti-angiogenesis therapies.