Vesicle-associated membrane protein-8/endobrevin negatively regulates phagocytosis of bacteria in dendritic cells

Phagocytosis is a specialized mechanism used by mammalian cells, particularly the cells of the immune system, such as dendritic cells (DC) and macrophages, to protect the host against infection. The process involves a complex cascade of pathways, from the ligation of surface receptors of phagocytes with components of the microorganism's surface, formation of phagosomes and subsequently phagolysosomes, to the eventual presentation of foreign Ags. Vesicle-associated membrane protein (VAMP)-8/endobrevin has been shown previously to function in the endocytic pathways. Our results showed that VAMP-8 colocalized with lysosome-associated membrane protein-2, and a significant amount of VAMP-8 was recruited to the phagosomes during bacterial ingestion. However, overexpression of VAMP-8 significantly inhibited phagocytosis in DC. We also found that the phagocytic activity of VAMP-8-/- DC was significantly higher than wild-type VAMP-8+/+ DC, thus further confirming that VAMP-8 negatively regulates phagocytosis in immature DC.     

Coupling assembly of the E-cadherin/beta-catenin complex to efficient endoplasmic reticulum exit and basal-lateral membrane targeting of E-cadherin in polarized MDCK cells

The E-cadherin/catenin complex regulates Ca++-dependent cell-cell adhesion and is localized to the basal-lateral membrane of polarized epithelial cells. Little is known about mechanisms of complex assembly or intracellular trafficking, or how these processes might ultimately regulate adhesion functions of the complex at the cell surface. The cytoplasmic domain of E-cadherin contains two putative basal-lateral sorting motifs, which are homologous to sorting signals in the low density lipoprotein receptor, but an alanine scan across tyrosine residues in these motifs did not affect the fidelity of newly synthesized E-cadherin delivery to the basal-lateral membrane of MDCK cells. Nevertheless, sorting signals are located in the cytoplasmic domain since a chimeric protein (GP2CAD1), comprising the extracellular domain of GP2 (an apical membrane protein) and the transmembrane and cytoplasmic domains of E-cadherin, was efficiently and specifically delivered to the basal-lateral membrane. Systematic deletion and recombination of specific regions of the cytoplasmic domain of GP2CAD1 resulted in delivery of <10% of these newly synthesized proteins to both apical and basal-lateral membrane domains. Significantly, >90% of each mutant protein was retained in the ER. None of these mutants formed a strong interaction with beta-catenin, which normally occurs shortly after E-cadherin synthesis. In addition, a simple deletion mutation of E-cadherin that lacks beta-catenin binding is also localized intracellularly. Thus, beta-catenin binding to the whole cytoplasmic domain of E-cadherin correlates with efficient and targeted delivery of E-cadherin to the lateral plasma membrane. In this capacity, we suggest that beta-catenin acts as a chauffeur, to facilitate transport of E-cadherin out of the ER and the plasma membrane.     

Overexpression of wild type and mutated human ferritin H-chain in HeLa cells: in vivo role of ferritin ferroxidase activity

Transfectant HeLa cells were generated that expressed human ferritin H-chain wild type and an H-chain mutant with inactivated ferroxidase activity under the control of the tetracycline-responsive promoter (Tet-off). The clones accumulated exogenous ferritins up to levels 14-16-fold over background, half of which were as H-chain homopolymers. This had no evident effect in the mutant ferritin clone, whereas it induced an iron-deficient phenotype in the H-ferritin wild type clone, manifested by approximately 5-fold increase of IRPs activity, approximately 2.5-fold increase of transferrin receptor, approximately 1.8-fold increase in iron-transferrin iron uptake, and approximately 50% reduction of labile iron pool. Overexpression of the H-ferritin, but not of the mutant ferritin, strongly reduced cell growth and increased resistance to H(2)O(2) toxicity, effects that were reverted by prolonged incubation in iron-supplemented medium. The results show that in HeLa cells H-ferritin regulates the metabolic iron pool with a mechanism dependent on the functionality of the ferroxidase centers, and this affects, in opposite directions, cellular growth and resistance to oxidative damage. This, and the finding that also in vivo H-chain homopolymers are much less efficient than the H/L heteropolymers in taking up iron, indicate that functional activity of H-ferritin in HeLa cells is that predicted from the in vitro data.     

Arf6 regulates EGF-induced internalization of E-cadherin in breast cancer cells

E-cadherin internalization facilitates dissolution of adherens junctions and promotes tumor cell epithelial-mesenchymal transition (EMT) and migration. Our previous results have shown that Arf6 exerts pro-migratory action in breast cancer cells after EGF stimulation. Despite the fact that EGF signaling stimulates EMT of breast cancer cells, the effect of Arf6 on internalization of E-cadherin of breast cancer cells under EGF treatment remains to be determined. Here, we showed that EGF dose-dependently stimulated E-cadherin internalization by MCF-7 cells with the maximal effect at 50 ng/ml. Meanwhile, EGF treatment markedly increased Arf6 activation. Arf6 was involved in complexes of E-cadherin, and more E-cadherin was pulled down with Arf6 when the activity of the latter was increased. Immunoblotting and immunofluorescence assays showed that transfection breast cancer cells with Arf6-T27N or Arf6 siRNA suppressed EGF-induced E-cadherin internalization. Taken together, our study demonstrated that Arf6 activation plays a potential role in EGF-induced E-cadherin internalization, providing new mechanism underlying the effect of Arf6 on promoting breast cancer cell metastasis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12935-015-0159-3) contains supplementary material, which is available to authorized users.     

Differential regulation of signal transduction pathways in wild type and mutated p53 breast cancer epithelial cells by copper and zinc

Previous studies have suggested that cells may differ in their response to metal stress. This study was undertaken to investigate the role of PI3K/Akt signaling pathway in metal resistance in human breast cancer epithelial cells with different p53 and estrogen receptor status. Exposure to copper and zinc increased Akt phosphorylation with its nuclear localization only in MDA-MB-231 cells with no estrogen receptor and mutated p53. Cyclin D1 expression and cell-cycle progression followed the metal-induced Akt phosphorylation. Treatment with LY294002 abrogated these effects, suggesting the essential role of PI3-kinase. In contrast, in MCF-7 cells with wild type p53 and estrogen receptor, there was no change in Akt activation, while suppression of p53 activity by pifithrin-alpha increased phosphorylation of Akt after the treatment with copper. In MCF-7 cells, the metal treatment increased the phosphorylation of p53 at serine 15, up-regulated p21 expression, and resulted in cell-cycle arrest in G1 phase with apoptosis. These results demonstrate that copper-induced apoptosis in MCF-7 cells is p53 dependent, whereas the metal resistance in MDA-MB-231 cells may be due to activation of Akt in the absence of a functional p53.     

Differential effect of NF-kappaB activity on beta-catenin/Tcf pathway in various cancer cells

beta-Catenin/Tcf and NF-kappaB pathways play an important role in biological functions. We determined the underlying mechanisms of differential interaction between two pathways in various human cancer cell lines. NF-kappaB positively regulated beta-catenin/Tcf pathways in human glioblastoma, whereas it has an opposite effect on beta-catenin/Tcf pathways in colon, liver, and breast cancer cells. Expression of lucine zipper tumor suppressor 2 (lzts2) was positively regulated by NF-kappaB activity in colon, liver, and breast cancer cells, whereas negatively regulated in glioma cells. Downregulation of lzts2 increased the beta-catenin/Tcf promoter activity and inhibited NF-kappaB-induced modulation of the nuclear translocation of beta-catenin. These data indicate that the differential crosstalk between beta-catenin/Tcf and NF-kappaB pathway in various cancer cells is resulted from the differences in the regulation of NF-kappaB-induced lzts2 expression.     

5'-AMP-activated protein kinase (AMPK) regulates progesterone receptor transcriptional activity in breast cancer cells

The steroid hormone progesterone is an essential regulator of the cellular processes that are required for the development and maintenance of reproductive function. The diverse effects of progesterone are mediated by the progesterone receptor (PR). The functions of the PR are regulated not only by ligands but also by modulators of various cell signaling pathways. However, it is not clear which energy state regulates PR activity. AMP-activated protein kinase (AMPK), a serine/threonine protein kinase, is a key modulator of energy homeostasis. Once activated by an increasing cellular AMP:ATP ratio, AMPK switches off ATP-consuming processes and switches on ATP-producing processes. We found that both 5-aminoimidazole-4-carboxamide 1-β-d-ribofuranoside (AICAR) and metformin, traditional pharmacological activators of AMPK, inhibited the PR pathway, as evidenced by progesterone response element (PRE)-driven luciferase activity and PR target gene expression. Compound C, an inhibitor of AMPK, partly but significantly reversed the anti-PR effects of AICAR and metformin. The downregulation of endogenous AMPK by small interfering RNAs (siRNAs) stimulated PR activity. AMPK activation by AICAR decreased the progesterone-induced phosphorylation of PR at serine 294 and inhibited the recruitment of PR to an endogenous PRE. Taken together, our data suggest that AMPK, an energy sensor, is involved in the regulation of PR signaling.     

HSPA5 negatively regulates lysosomal activity through ubiquitination of MUL1 in head and neck cancer

HSPA5/GRP78/BiP plays an important role in cell survival or tumor progression. For these reasons, HSPA5 is an emerging therapeutic target in cancer development. Here we report that HSPA5 contributes to head and neck cancer (HNC) survival via maintenance of lysosomal activity; however, a nonthermal plasma (NTP, considered as a next-generation cancer therapy)-treated solution (NTS) inhibits HNC progression through HSPA5-dependent alteration of lysosomal activity. HSPA5 prevents NTS-induced lysosome inhibition through lysosomal-related proteins or regulation of gene expression. However, NTS-induced MUL1/MULAN/GIDE/MAPL (mitochondrial ubiquitin ligase activator of NFKB 1) leads to downregulation of HSPA5 via K48-linked ubiquitination at the lysine 446 (K446) residue. MUL1 knockdown hinders NTS-induced lysosome inhibition or cytotoxicity through the reduction of HSPA5 ubiquitination in HNC cells. While MUL1 was suppressed, HSPA5 was overexpressed in tissues of HNC patients. NTS strongly inhibited HNC progression via alterations of expression of MUL1 and HSPA5, in vivo in a xenograft model. However, NTS did not induce inhibition of tumor progression or HSPA5 reduction in MUL1 knockout (KO) HNC cells which were generated by CRISPR/Cas9 system. The data provide compelling evidence to support the idea that the regulation of the MUL1-HSPA5 axis can be a novel strategy for the treatment of HNC.