Regulation of intestinal epithelial homeostasis by the desmosomal cadherins Open Access

Kolegraff, Keli (2011)

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Regulation of intestinal epithelial homeostasis by the desmosomal cadherins
By Keli Nicole Kolegraff
Desmocollin-2 (Dsc2) and desmoglein-2 (Dsg2) are the transmembrane cell adhesion proteins of desmosomes. Down-regulation of Dsc2 has been reported in colorectal carcinomas, however no mechanistic studies have examined the contribution of Dsc2-deficiency to tumorigenesis. Using RNAi to down-regulate the expression of Dsc2, I have examined the effect of Dsc2 loss on the behavior of colonic epithelial cells. Down-regulation of Dsc2 in a model colonic epithelial cell line SK-CO15 increases cell proliferation and enhances β-catenin signaling, a well-established pro-proliferative pathway in intestinal epithelial cells. Furthermore, Dsc2-deficient cells exhibit activation of Akt, a serine/threonine kinase which has been reported to regulate the transcriptional activity of β-catenin. Inhibition of Akt with triciribine or siRNA-mediated Akt depletion prevents the activation of β-catenin-dependent transcription and cell proliferation following Dsc2 loss. These data suggest that decreased expression of Dsc2 promotes cell proliferation via activation of Akt/β-catenin signaling. Given that aberrant activation of β-catenin signaling plays an important role in the progression of colorectal carcinomas, I also examined the ability of Dsc2-deficient cells to grow as tumors in immunodeficient mice. Notably, the parental cell line SK-CO15 is non-tumorigenic and does not form tumors in mice, however, down-regulation of Dsc2 enables tumor formation in vivo. Furthermore, inhibition of Akt strongly attenuates the growth of Dsc2-deficient cells. Taken together, this work defines a novel link between the desmosomal cadherins, Akt, and β-catenin and provides mechanistic evidence that loss of Dsc2 may contribute to the progression of epithelial cancers.

Table of Contents

Table of Contents

Chapter 1: Introduction. 1
The Origin and Molecular Basis of Intercellular Adhesion. 2

1.1.1 How do cells adhere to one another? 2
1.1.2 To each (tissue), his own (cadherin). 3
1.1.3 Differential expression of cadherin family members mediates cell-type specific sorting. 4
1.1.4 E-cadherin is required for epithelial cell polarity. 4
1.1.5 Cadherin engagement promotes cell differentiation and altered expression of cadherins perturbs tissue homeostasis. 4
1.2 Intercellular Adhesive Junctions Confer Functional Properties to Tissues. 5
1.2.1 Tight junctions regulate tissue permeability and barrier function. 5
1.2.2 Desmosomes reinforce intercellular adhesive contacts and enable tissues to resist mechanical stress. 7
1.3 Intercellular Adhesive Junctions in the Context of Disease: Chronic Inflammation of the Intestine. 12
1.4 Hypothesis: Altered Expression of the Desmosomal Cadherins Affects Intestinal Epithelia Homeostasis. 14
1.5 References. 16
1.6 Figures and Legends. 21
Chapter 2: Loss of desmocollin-2 confers a tumorigenic phenotype to colonic epithelial cells through activation of Akt/β-catenin signaling. 24
2.1 Abstract. 26
2.2 Introduction. 27
2.3 Results. 30
2.4 Discussion. 38
2.5 Materials and Methods. 46
2.6 References. 52
2.7 Figure Legends. 58
2.8 Figures. 64
2.9 Supplementary Data. 70
Chapter 3: Characterization of full-length and proteolytic cleavage fragments of desmoglein-2 in native human colon and colonic epithelial cell lines. 77
3.1 Abstract. 78
3.2 Introduction. 79
3.3 Results. 81
3.4 Discussion. 85
3.5 Materials and Methods. 89
3.6 References. 92
3.7 Figure Legends. 94
3.8 Figures and Supplementary Data. 98
Chapter 4: Conclusions and Future Directions. 105
4.1 The Desmosomal Cadherins. 105
4.2 Loss of Desmocollin-2 Promotes Tumorigenesis. 106
4.3 Desmosomal Cadherins and the Regulation of Lipid Raft Signaling Pathways. 106
4.4 Evidence of Distinct Signaling Functions for the "Partner" Desmosomal Cadherins Desmocollin-2 and Desmoglein-2. 107
4.5 Controlling Desmoglein-2 Function by Proteolysis. 108
4.6 What is the Significance of Altered Desmoglein-2 Expression in Chronic Intestinal Inflammation? 109
4.7 In Summary. 110
4.8 References. 112
Appendix: Cellular Domains Chapter 19 - The Tight Junction and Desmosomes. 114

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