1.2 Å X-ray structure of the renal potassium channel Kv1.3 T1 domain.

Here we present the structure of the T1 domain derived from the voltage-dependent potassium channel K(v)1.3 of Homo sapiens sapiens at 1.2 Å resolution crystallized under near-physiological conditions. The crystals were grown without precipitant in 150 mM KP(i), pH 6.25. The crystals show I4 symmetry typical of the natural occurring tetrameric assembly of the single subunits. The obtained structural model is based on the highest resolution currently achieved for tetramerization domains of voltage-gated potassium channels. We identified an identical fold of the monomer but inside the tetramer the single monomers show a significant rotation which leads to a different orientation of the tetramer compared to other known structures. Such a rotational movement inside the tetrameric assembly might influence the gating properties of the channel. In addition we see two distinct side chain configurations for amino acids located in the top layer proximal to the membrane (Tyr109, Arg116, Ser129, Glu140, Met142, Arg146), and amino acids in the bottom layer of the T1-domain distal from the membrane (Val55, Ile56, Leu77, Arg86). The relative populations of these two states are ranging from 50:50 for Val55, Tyr109, Arg116, Ser129, Glu140, 60:40 for Met142, 65:35 for Arg86, 70:30 for Arg146, and 80:20 for Ile56 and Leu77. The data suggest that in solution these amino acids are involved in an equilibrium of conformational states that may be coupled to the functional states of the whole potassium channel.

Constitutive HIF-1 activity in malignant melanoma.

The hypoxia-inducible factor-1 (HIF-1), which consists of the constitutive HIF-1beta and the oxygen-responsive HIF-1alpha subunit, is the master activator of the cellular transcriptional response to hypoxia coordinating gene expression during reduced oxygen tension. Overexpression of HIF-1 and increased transcriptional activity induced by hypoxia are linked to progression of many tumour types such as head and neck cancer, cervical carcinoma, leukaemia and renal cell carcinoma. In this study, we demonstrate that HIF activity is increased in malignant melanoma cells already under normoxic conditions in contrast to other tumour types. HIF-1alpha and -2alpha knockdown by siRNA transfection revealed that this effect is due to constitutive HIF-1alpha expression. Furthermore, the inhibition or activation of reactive oxygen species (ROS) decreased or activated, respectively, HIF-1 activity and HIF-1alpha protein expression. Interestingly, the inhibition of the NFkappaB pathway also reduced the accumulation of HIF-1alpha assuming a context between ROS and NFkappaB, and suggesting that ROS and NFkappaB activity contribute to HIF-1alpha accumulation. In summary, we identified an increased HIF-1alpha protein expression and activity in melanoma under normoxia mediated by ROS and the NFkappaB pathway.

Crystallization and preliminary X-ray diffraction studies of the tetramerization domain derived from the human potassium channel Kv1.3.

The tetramerization domain (T1 domain) derived from the voltage-dependent potassium channel Kv1.3 of Homo sapiens was recombinantly expressed in Escherichia coli and purified. The crystals were first grown in an NMR tube in 150 mM potassium phosphate pH 6.5 in the absence of additional precipitants. The crystals showed I4 symmetry characteristic of the naturally occurring tetrameric assembly of the single subunits. A complete native data set was collected to 1.2 A resolution at 100 K using synchrotron radiation.

Bone morphogenetic protein 4 is induced in hepatocellular carcinoma by hypoxia and promotes tumour progression.

Striking similarities exist between molecular mechanisms driving embryonic liver development and progression of hepatocellular carcinoma (HCC). Bone morphogenetic proteins (BMPs), particularly BMP4, have been proposed to regulate embryonic hepatic development. BMP expression has been observed in neoplasia but the expression and biological role of BMP4 in human HCC are unknown. We found increased BMP4 mRNA and protein in HCC cell lines and tissue samples compared to primary human hepatocytes and corresponding non-tumourous tissue. Hypoxia further induced BMP4 expression in HCC cells, which was abolished by transfection of a dominant negative form of HIF-1 alpha (dnHIF-1 alpha). However, gel shift assays revealed only minor binding activity in nuclear extracts from (hypoxic) HCC cells to a putative hypoxia-response element in the BMP4 promoter. Sequence analysis of the BMP4 promoter revealed two Ets-1 binding sites, and Ets-1 activity was increased in HCC cells under hypoxic conditions. Transfection of dnHIF-1 alpha completely abrogated hypoxia-induced Ets-1 activity as well as BMP4 expression. Overexpression of Ets-1 markedly enhanced BMP4 promoter activity, while antisense Ets-1 almost completely abolished basal as well as hypoxia-induced BMP4 expression. These data demonstrate that Ets-1 activity contributes to baseline expression of the BMP4 gene and is the predominant mediator of the HIF-dependent BMP4 induction under hypoxic conditions. To determine the functional relevance of BMP4 expression, HCC cell lines were treated with antisense BMP4 constructs or siRNA against BMP4. BMP4 suppression resulted in a strong reduction of the migratory and invasive potential and anchorage-independent growth. Furthermore, tube formation assays indicated that BMP4 expressed by HCC cells promotes vasculogenesis. Our findings demonstrate that BMP4 is increased in HCC and promotes HCC progression. Therefore, BMP4 expression may have clinical relevance, and interfering with BMP4 signalling appears as an attractive therapeutic target for this highly aggressive tumour.

Cadherin-7 interacts with melanoma inhibitory activity protein and negatively modulates melanoma cell migration.

Melanoma inhibitory activity (MIA) has been identified as a small protein secreted from malignant melanoma cells, which strongly enhances melanoma cell migration and invasion. Detailed analyses performed by our group showed interaction of MIA with extracellular matrix proteins and integrin alpha4beta1 and alpha5beta1 leading to cellular detachment. In this study, we identified cadherin-7 as a new MIA-binding protein using surface-enhanced laser desorption/ionization-mass spectrometry technology and co-immunoprecipitation. Cadherin-7 is a classical cell-cell adhesion molecule which was shown to be upregulated in malignant melanoma. We demonstrated enhanced expression of cadherin-7 in primary tumor cells compared to metastatic cells. Upregulation of cadherin-7 expression in metastatic cell lines but also downregulation of expression in cells derived from primary melanomas resulted in reduced cell migration. In addition, we speculate that MIA/cadherin-7 interaction may regulate cell-cell adhesion of malignant melanoma cells influencing the migration of the cells. Interestingly, overexpression of cadherin-7 resulted in a decreased MIA mRNA expression. In addition, MIA effects on cell migration were abrogated in cell clones overexpressing cadherin-7. In conclusion, these findings suggest that cadherin-7 regulates the expression and activity of MIA and the migration of melanoma cells playing a role in tumor development of malignant melanoma.

Loss of full length CtBP1 expression enhances the invasive potential of human melanoma.

BACKGROUND: The C-terminal binding protein 1 (CtBP1) is a known co-repressor of gene transcription. We recently revealed that CtBP1 expression is lost in melanoma cells and melanoma inhibitory activity (MIA) expression is subsequently increased. The present study was performed to evaluate a more general role of CtBP1 in human melanoma and identify further CtBP1-regulated target genes. METHODS: Sequence analysis and expression profile of CtBP1 in melanoma cell lines were done by PCR. Boyden Chamber assays and co-immunoprecipitation were performed to investigate the functional role of CtBP1. Gene expression analysis and micro array data were used to define target genes. RESULTS: Interestingly, we detected an alternative splice product of CtBP1 with unknown function whose expression is induced at reduction of full length CtBP1. Overexpression of full length CtBP1 in melanoma cells had no effect on cell proliferation but did influence cell migration and invasiveness. To understand the effect of CtBP1 we identified putative LEF/TCF target genes found to be strongly expressed in melanoma using DNA microarray analysis. We focused on fourteen genes not previously associated with melanoma. Detailed analysis revealed that most of these were known to be involved in tumor metastasis. Eleven genes had expression profiles associated with melanoma cell invasiveness. CONCLUSION: In summary, this study revealed that reduction of CtBP1 expression is correlated with migratory, invasive potential of melanoma cells.

Regulation of integrin activity by MIA.

MIA (melanoma inhibitory activity) has been identified as a small protein secreted from malignant melanoma cells, which interacts with extracellular matrix proteins including fibronectin. Here, we show that MIA negatively regulates the activity of the mitogen-activated protein kinase pathway in malignant melanoma. Using far Western blotting and co-immunoprecipitation we searched for MIA-binding cell surface proteins. We found that MIA interacts with integrin alpha4beta1 and alpha5beta1, leading to down-regulation of integrin activity and reduction of mitogen-activated protein kinase signaling. These findings also suggest that MIA may play a role in tumor progression and the spread of malignant melanomas via mediating detachment of cells from extracellular matrix molecules by modulating integrin activity. Inhibiting MIA functions in vivo may therefore provide a novel therapeutic strategy for metastatic melanoma disease.