Impact of the mitogen-activated protein kinase pathway on the subproteome of detergent-resistant microdomains of colon carcinoma cells.

Lipid rafts play a key role in the regulation of fundamentally important cellular processes, including cell proliferation, differentiation, and survival. The composition of such detergent-resistant microdomains (DRMs) is altered under pathologic conditions, including cancer. Although DRMs have been analyzed in colorectal carcinoma little information exists about their composition upon treatment with targeted drugs. Hence, a quantitative proteomic profiling approach was performed to define alterations within the DRM fraction of colorectal carcinoma cells upon treatment with the drug U0126, an inhibitor of the mitogen-activated protein kinase pathway. Comparative expression profilings resulted in the identification of 300 proteins, which could partially be linked to key oncogenic signaling pathways and tumor-related cellular features, such as cell proliferation, adhesion, motility, invasion, and apoptosis resistance. Most of these proteins were downregulated upon inhibitor treatment. In addition, quantitative proteomic profilings of cholesterol-depleted versus intact lipid rafts were performed to define, which U0126-regulated target structures represent bona fide raft proteins. Selected differentially abundant raft proteins were validated at the mRNA and/or protein level using U0126- or Trametinib-treated cells. The presented data provide insights into the molecular mechanisms associated with the response to the treatment with MEK inhibitors and might also lead to novel candidates for therapeutic interventions.

The cellular localization of autotaxin impacts on its biological functions in human thyroid carcinoma cells.

Autotaxin (ATX/NPP2) shows a nucleotide pyrophosphatase/phosphodiesterase and lysophospholipase D (lysoPLD) activity and is a member of a family of structurally-related mammalian ecto-nucleotide pyrophosphate/phosphodiesterases (E-NPP1-3). ATX is unique among E-NPP as it is secreted and not membrane-bound as are NPP1 and -3. The ATX gene activity is significantly higher in undifferentiated anaplastic (UTC) as compared to follicular (FTC) and papillary thyroid carcinomas (PTC) or goiter tissues. ATX also enhances the motility of thyroid tumor cells. We bio-engineered stable transfectants of the human thyroid carcinoma cell line FTC-238 expressing either bioactively-secreted (sATX) or membrane-anchored ATX (mATX) to identify the biological functions of ATX which critically depend on the E-NPP member being secreted and provide insight into the effects of high local ATX concentrations and cellular responses. An increased cell motility was exclusively observed with FTC-238 sATX transfectants, whereas membrane-anchored ATX appeared to impair motility. We identified IL-1beta as an upstream suppressor of ATX expression in FTC-238, ATX-mediated motility in FTC-238 and stable transfectants, with IL-1beta having the strongest motility-suppressive effect on FTC-238 sATX clones. sATX and mATX strongly increased the anchorage-independent colony formation of FTC-238 but the size and number of colonies formed in the soft agar were significantly smaller in FTC-238 mATX versus the FTC-238 sATX clones. The cancer-testis antigen BAGE was identified as a novel target gene of ATX in FTC-238. Transcript levels for BAGE were 6-fold higher in FTC-238 mATX versus sATX clones. Increased BAGE transcript levels were also detected in tissues of patients with UTC versus FTC, PTC or goiter tissues. In summary, enhanced tumor cell motility and tumorigenic capacity critically depended on sATX in thyroid carcinoma cells. Irrespective of its compartmentalization, the cancer-testis antigen BAGE was identified as a novel target gene of ATX in FTC-238 and a potential new tissue marker in UTC tissues, which we had previously shown to express high levels of ATX.

Aminopeptidase N (APN)/CD13-dependent CXCR4 downregulation is associated with diminished cell migration, proliferation and invasion.

Aminopeptidase N (APN/CD13) is a 150 kDa membrane-bound ubiquitously expressed protease with a broad functional repertoire. It hydrolyzes small peptide mediators, modulates cell motility and adhesion to extracellular matrix and also acts as a viral receptor. In order to dissect the function of enzymatically active and inactive APN/CD13, substitutions of different enzymatic active amino acid residues were generated by site-directed mutagenesis and stably transfected into human embryonic kidney cells. All APN variants analyzed exhibited a complete loss of enzymatic activity, whereas wild type APN transfectants exerted a strong aminopeptidase-specific activity. Furthermore, wild type APN expression was associated with a significant decrease in proliferation, migration and also reduced anchorage-independent growth when compared to enzymatically inactive APN variants and controls. This appeared to be due to a downregulated mRNA and protein expression of the chemokine receptor CXCR4 and an inhibition of the stromal cell-derived factor (SDF)-1alpha/CXCL12-mediated migration. Thus, high APN enzyme activity may antagonize the cellular properties regulated by the CXCR4/SDF-1alpha system in embryonic kidney cells.

Functional co-localization of monocytic aminopeptidase N/CD13 with the Fc gamma receptors CD32 and CD64.

Information about the function of aminopeptidase N/CD13 on monocytes is limited. In order to gain more insight into its interaction with other proteins, we have identified molecules that co-localize with the membrane ectoenzyme at the cell surface of monocytes. Using laser scanning and electron microscopy as well as fluorescence resonance energy transfer (FRET) measured by flow cytometry we show that monocytic CD13 co-localized with the Fc gamma receptor II/CD32 after Fc receptor ligation by a CD32-specific antibody. FRET was also observed between CD13 and the Fc gamma receptor I/CD64, but not with the myeloid marker CD33 representing a member of the sialoadhesin family. Our results imply a novel functional role of CD13 and Fc gamma receptors as members of a multimeric receptor complex. Further studies have to be done to elucidate common signaling pathways of these molecules.