Nina Kokkonen

Elevated Golgi pH impairs terminal N-glycosylation by inducing mislocalization of Golgi glycosyltransferases.

Acidic pH of the Golgi lumen is known to be crucial for correct glycosylation, transport and sorting of proteins and lipids during their transit through the organelle. To better understand why Golgi acidity is important for these processes, we have examined here the most pH sensitive events in N‐glycosylation by sequentially raising Golgi luminal pH with chloroquine (CQ), a weak base. We show that only a 0.2 pH unit increase (20 µM CQ) is sufficient to markedly impair terminal α(2,3)‐sialylation of an N‐glycosylated reporter protein (CEA), and to induce selective mislocalization of the corresponding α(2,3)‐sialyltransferase (ST3) into the endosomal compartments. Much higher pH increase was required to impair α(2,6)‐sialylation, or the proximal glycosylation steps such as β(1,4)‐galactosylation or acquisition of Endo H resistance, and the steady‐state localization of the key enzymes responsible for these modifications (ST6, GalT I, MANII). The overall Golgi morphology also remained unaltered, except when Golgi pH was raised close to neutral. By using transmembrane domain chimeras between the ST6 and ST3, we also show that the luminal domain of the ST6 is mainly responsible for its less pH sensitive localization in the Golgi. Collectively, these results emphasize that moderate Golgi pH alterations such as those detected in cancer cells can impair N‐glycosylation by inducing selective mislocalization of only certain Golgi glycosyltransferases. J. Cell. Physiol. 220: 144–154, 2009.

Functional Organization of Golgi N- and O-Glycosylation Pathways Involves pH-dependent Complex Formation That Is Impaired in Cancer Cells

Background: Glycans are synthesized in the Golgi by sequentially acting glycosyltransferases, but it is not known how their functions are coordinated in live cells. Results: N- and O-glycosyltransferases form enzymatically active homo- and/or heteromeric complexes. Conclusion: Glycosyltransferases function as physically distinct enzyme complexes rather than single enzymes. Significance: The results help understand the overall functioning of the Golgi glycosylation pathways both in health and disease. Glycosylation is one of the most common modifications of proteins and lipids and also a major source of biological diversity in eukaryotes. It is critical for many basic cellular functions and recognition events that range from protein folding to cell signaling, immunological defense, and the development of multicellular organisms. Glycosylation takes place mainly in the endoplasmic reticulum and Golgi apparatus and involves dozens of functionally distinct glycosidases and glycosyltransferases. How the functions of these enzymes, which act sequentially and often competitively, are coordinated to faithfully synthesize a vast array of different glycan structures is currently unclear. Here, we investigate the supramolecular organization of the Golgi N- and O-glycosylation pathways in live cells using a FRET flow cytometric quantification approach. We show that the enzymes form enzymatically active homo- and/or heteromeric complexes within each pathway. However, no complexes composed of enzymes that operate in different pathways, were detected, which suggests that the pathways are physically distinct. In addition, we show that complex formation is mediated almost exclusively by the catalytic domains of the interacting enzymes. Our data also suggest that the heteromeric complexes are functionally more important than enzyme homomers. Heteromeric complex formation was found to be dependent on Golgi acidity, markedly impaired in acidification-defective cancer cells, and required for the efficient synthesis of cell surface glycans. Collectively, the results emphasize that the Golgi glycosylation pathways are functionally organized into complexes that are important for glycan synthesis.

Elevated Golgi pH in breast and colorectal cancer cells correlates with the expression of oncofetal carbohydrate T-antigen.

Altered glycosylation has turned out to be a universal feature of cancer cells, and in many cases, to correlate with altered expression or localization of relevant glycosyltransferases. However, no such correlation exists between observed enzymatic changes and the expression of the oncofetal Thomsen‐Friedenreich (T)‐antigen, a core 1 (Gal‐β1 → 3‐GalNAc‐ser/thr) carbohydrate structure. Here we report that T‐antigen expression, instead, correlates with elevated Golgi pH in cancer cells. Firstly, using a Golgi‐targeted green fluorescent protein (GT‐EGFP) as a probe, we show that the medial/trans‐Golgi pH (pHG) in a high proportion of breast (MCF‐7) and colorectal (HT‐29, SW‐48) cancer cells is significantly more alkaline (pHG ≥ 6.75) than that of control cells (pHG 5.9–6.5). The pH gradient between the cytoplasm and the Golgi lumen is also markedly reduced in MCF‐7 cells, suggesting a Golgi acidification defect. Secondly, we show that T‐antigen expression is highly sensitive to changes in Golgi pH, as only a 0.2 pH unit increase was sufficient to increase T‐antigen expression in control cells. Thirdly, we found that T‐antigen expressing MCF‐7 cells have 0.3 pH units more alkaline Golgi pH than non‐expressing MCF‐7 cells. Fourthly, in all cell types examined, we observed significant correlation between the number of T‐antigen expressing cells and cells with a markedly elevated Golgi pH (pHG ≥ 6.75). Consistent with these observations in cultured cells, cells in solid tumors also heterogenously expressed the T‐antigen. Thus, elevated Golgi pH appears to be directly linked to T‐antigen expression in cancer cells, but it may also act as a more general factor for altered glycosylation in cancer by affecting the distribution of Golgi‐localized glycosyltransferases.

Hypoxia upregulates carcinoembryonic antigen expression in cancer cells

Carcinoembryonic antigen (CEA, ceacam5) is an important tumor‐associated antigen with reported roles, e.g., in immunological defense, cell adhesion, cell survival and metastasis. Its overexpression in cancer cells is known to involve transcriptional activation of the CEA gene, but the underlying molecular details remain unclear. Here, we show that hypoxia and intracellular alkalinization, 2 factors commonly found in solid tumors, increase CEA protein expression in breast (MCF‐7) and colorectal (CaCo‐2 and HT‐29) cancer cells. The increase was comparable (2–3‐fold) to that observed in colorectal carcinomas in vivo. CEA promoter analyses further revealed that this upregulation involves a known binding site for HIF‐1 transcription factor (5′‐ACGTG‐3′) within one of the CEA promoters positive regulatory elements (the FP1 site; the E‐box). Accordingly, deletion or targeted mutagenesis of this motif rendered the CEA promoter unresponsive to hypoxia. Our chromatin immunoprecipitation data confirmed that endogenous HIF‐1α binds to the CEA promoter in hypoxic cells but not in normoxic cells. Moreover, overexpression of the hypoxia‐inducible factor (HIF‐1α) was sufficient to increase CEA protein expression in the cells. In contrast, c‐Myc, which is known to bind to the overlapping E‐box, did not potentiate HIF‐1α‐induced CEA expression. CEA overexpression in vivo was also found to coincide with the expression of carbonic anhydrase IX, a well‐known hypoxia marker. Collectively, these results define CEA as a hypoxia‐inducible protein and suggest an important role for the tumor microenvironmental factors in CEA overexpression during tumorigenesis.

Deletion of the Major Bullous Pemphigoid Epitope Region of Collagen XVII Induces Blistering, Autoimmunization, and Itching in Mice

Bullous pemphigoid (BP) is the most common autoimmune subepidermal blistering skin disease with a characteristic of pruritus and blistering. BP patients carry inflammation-triggering autoantibodies against the collagen XVII (ColXVII, also known as BP180) juxtamembraneous extracellular noncollagenous 16A (NC16A) domain involved in ectodomain shedding. Deletion of the corresponding NC14A region in a genetically modified mouse model (ΔNC14A) decreased the amount of ColXVII in skin, but it did not prevent ectodomain shedding. Newborn ΔNC14A mice had no macroscopic phenotypic changes. However, subepidermal microblisters, rudimentary hemidesmosomes, and loose basement membrane zone were observed by microscopy. ΔNC14A mice grow normally, but at around 3 months of age they start to scratch themselves and develop crusted erosions. Furthermore, perilesional eosinophilic infiltrations in the skin, eosinophilia, and elevated serum IgE levels are detected. Despite the removal of the major BP epitope region, ΔNC14A mice developed IgG and IgA autoantibodies with subepidermal reactivity, indicating autoimmunization against a dermo-epidermal junction component. Moreover, IgG autoantibodies recognized a 180-kDa keratinocyte protein, which was sensitive to collagenase digestion. We show here that ΔNC14A mice provide a highly reproducible BP-related mouse model with spontaneous breakage of self-tolerance and development of autoantibodies.

Collagen XVII expression correlates with the invasion and metastasis of colorectal cancer

Collagen XVII has a well-established role as an adhesion molecule and a cell surface receptor located in the type I hemidesmosome of stratified epithelia. Its ectodomain is constitutively shed from the cell surface and suggested to regulate the adhesion, migration, and signaling of cutaneous epithelial cells. Collagen XVII was not previously thought to be expressed by colon epithelial cells. Immunohistochemical analysis of tissue microarray samples of 141 cases of colorectal carcinoma showed that collagen XVII is expressed in normal human colonic mucosa and colorectal carcinoma. In colorectal carcinoma, increased collagen XVII expression was significantly associated with higher TNM stage. It also correlated with infiltrative growth pattern and tumor budding as well as lymph node and distant metastasis. Increased collagen XVII expression was associated with decreased disease-free and cancer-specific survival. Immunofluorescence staining of collagen XVII and its well-known binding partner laminin γ2 chain demonstrated a partial colocalization in normal and tumor tissue. In vitro, the overexpression of murine collagen XVII promoted the invasion of CaCo-2 colon carcinoma cells through Matrigel (BD Biosciences; Bedford, MA). To conclude, this study reports for the first time the expression of collagen XVII in colon epithelium and the association of increased collagen XVII immunoexpression with poor outcome in colorectal carcinoma.

Significant role of collagen XVII and integrin β4 in migration and invasion of the less aggressive squamous cell carcinoma cells

Collagen XVII and integrin α6β4 have well-established roles as epithelial adhesion molecules. Their binding partner laminin 332 as well as integrin α6β4 are largely recognized to promote invasion and metastasis in various cancers, and collagen XVII is essential for the survival of colon and lung cancer stem cells. We have studied the expression of laminin γ2, collagen XVII and integrin β4 in tissue microarray samples of squamous cell carcinoma (SCC) and its precursors, actinic keratosis and Bowen’s disease. The expression of laminin γ2 was highest in SCC samples, whereas the expression of collagen XVII and integrin β4 varied greatly in SCC and its precursors. Collagen XVII and integrin β4 were also expressed in SCC cell lines. Virus-mediated RNAi knockdown of collagen XVII and integrin β4 reduced the migration of less aggressive SCC-25 cells in horizontal scratch wound healing assay. Additionally, in a 3D organotypic myoma invasion assay the loss of collagen XVII or integrin β4 suppressed equally the migration and invasion of SCC-25 cells whereas there was no effect on the most aggressive HSC-3 cells. Variable expression patterns and results in migration and invasion assays suggest that collagen XVII and integrin β4 contribute to SCC tumorigenesis.

Junctional epidermolysis bullosa with LAMB3 splice-site mutations.

© 2015 The Authors. doi: 10.2340/00015555-2073 Journal Compilation

Clinical efficiency of topical calcipotriol/betamethasone treatment in psoriasis relies on suppression of the inflammatory TNFα – IL- 23 – IL-17 axis

The effects of topical calcipotriol/betamethasone combination therapy and betamethasone monotherapy on inflammatory T-cell numbers and molecular markers were compared in patients with psoriasis. Combination therapy down-regulated the expression of tumour necrosis factor (TNF)-α, interleukin (IL)-23A, IL-17A, S100A7, CCL-20 and interferon (IFN)-γ in skin and TNF-α, IL-6, IL-23A, T-bet and IFN-γ in peripheral blood mononuclear cells (PBMCs). Betamethasone monotherapy had less effect. Expression of FoxP3 in both skin and PBMCs was down-regulated by calcipotriol/betamethasone, but not by betamethasone. Immunohistochemical analysis revealed that calcipotriol/betamethasone reduced the numbers of CD4+ and CD8+ T cells and Tregs in psoriatic lesions more than betamethasone. Flow cytometric analyses demonstrated that calcipotriol/betamethasone decreased the numbers of circulating CD8+ T cells, Tregs, skin-homing Th17 memory cells and Th22 memory cells, while betamethasone had little or no effect. Glucocorticoid receptors GRα and GRß were expressed in psoriatic skin. In conclusion, calcipotriol increases the immunosuppressive power of betamethasone by suppressing the inflammatory TNF-α - IL-23 - IL-17 axis.

Expression of Glucocorticoid Receptors GRα and GRβ in Bullous Pemphigoid

First-line treatments of bullous pemphigoid (BP) are topical and systemic glucocorticoids (GC). The actions of GC are mediated by glucocorticoid receptors (GR), which exist in several isoforms, of which GRα and GRβ are the most important. In many inflammatory diseases, up-regulation of GRβ is associated with GC insensitivity. The aims of this study were to determine the expression of GRα and GRβ in patients with BP and to investigate the effect of prednisolone treatment on the expression of GR isoforms in BP. Quantitative real-time PCR (qPCR) analysis demonstrated that GR isoform mRNAs are expressed in peripheral blood mononuclear cells (PBMC) from patients with BP. Expression of GRα and GRβ protein was confirmed by immunohistochemical staining of BP skin biopsies and by Western blot analysis and flow cytometric analysis of PBMCs. During prednisolone treatment, GRα and GRβ expression varied markedly, but changes were not suitable as a clinical marker of GC sensitivity in patients with BP.