Alternative exon usage creates novel transcript variants of tumor suppressor SHREW-1 gene with differential tissue expression profile.

Petra A B Klemmt,Eduard Resch,Anna Starzinski-Powitz,Knut Engels,Isabell Smyrek,Ernst H K Stelzer

doi:10.1242/bio.019463

Abstract

ABSTRACTShrew-1, also called AJAP1, is a transmembrane protein associated with E-cadherin-mediated adherence junctions and a putative tumor suppressor. Apart from its interaction with β-catenin and involvement in E-cadherin internalization, little structure or function information exists. Here we explored shrew-1 expression during postnatal differentiation of mammary gland as a model system. Immunohistological analyses with antibodies against either the extracellular or the cytoplasmic domains of shrew-1 consistently revealed the expression of full-length shrew-1 in myoepithelial cells, but only part of it in luminal cells. While shrew-1 localization remained unaltered in myoepithelial cells, nuclear localization occurred in luminal cells during lactation. Based on these observations, we identified two unknown shrew-1 transcript variants encoding N-terminally truncated proteins. The smallest shrew-1 protein lacks the extracellular domain and is most likely the only variant present in luminal cells. RNA analyses of human tissues confirmed that the novel transcript variants of shrew-1 exist in vivo and exhibit a differential tissue expression profile. We conclude that our findings are essential for the understanding and interpretation of future functional and interactome analyses of shrew-1 variants.

Highlights

Shrew-1, known as adherence junction-associated protein-1 (AJAP1) is a vertebrate-specific transmembrane protein characterized by an unusually long N-terminal signal peptide and one hydrophobic transmembrane segment (Bharti et al, 2004; Resch et al, 2008)
The function of shrew-1 in epithelial tissues remains elusive, it is known from progressive promoter silencing or gene deletion in brain tumors that shrew-1 can act as a tumor suppressor protein (Cogdell et al, 2011; Ernst et al, 2009; McDonald et al, 2006; Milde et al, 2009)
More recent evidence indicates that the SHREW-1/AJAP1 gene is silenced in other tumor types such as gastric (Matsusaka et al, 2011), cervical (Chen et al, 2014) and endometrial cancer (Lai et al, 2014), or hepatocellular carcinoma (Ezaka et al, 2015)

Summary

Introduction

Shrew-1, known as adherence junction-associated protein-1 (AJAP1) is a vertebrate-specific transmembrane protein characterized by an unusually long N-terminal signal peptide and one hydrophobic transmembrane segment (Bharti et al, 2004; Resch et al, 2008). In polarized cells, targeting to the basolateral plasma membrane depends on targeting motifs in the cytoplasmic tail Once delivered to the plasma membrane, shrew-1 localizes basolaterally in polarized epithelial cells and is associated with components of the adherence junction complex and a direct interaction partner of β-catenin (Bharti et al, 2004). It is implicated in modulating the internalization of E-cadherin upon growth factor stimulation in breast cancer cells (Gross et al, 2009). More recent evidence indicates that the SHREW-1/AJAP1 gene is silenced in other tumor types such as gastric (Matsusaka et al, 2011), cervical (Chen et al, 2014) and endometrial cancer (Lai et al, 2014), or hepatocellular carcinoma (Ezaka et al, 2015)

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