Novel and diverse roles of STRAP in maintenance of mesenchymal morphology and GSK3Beta signaling.

Abstract

STRAP inhibits transforming growth factor-Beta (TGF-Beta) signaling and enhances tumorigenicity. The aim of our current research project was to identify novel TGF-Beta independent functions of STRAP. STRAP acts as a scaffold for the assembly of multi-protein complexes and our study has uncovered two novel but independent functions of STRAP.

In the first part, we report, for the first time, that deletion of STRAP from Mouse Embryonic Fibroblasts (MEFs) results in a loss of mesenchymal morphology. These cells lose their spindle shape and exhibit cobloid epithelial morphology. Loss of STRAP leads to upregulation of WT1 that subsequently upregulates E-cadherin leading to the formation of adherens junctions, and sequesters Beta-catenin to the cell membrane and downregulation of the mesenchymal markers like LEF1. Finally, stable expression of STRAP in these cells results in a loss of WT1 and E-cadherin expressions, and a reversal from epithelial to the mesenchymal morphology.

In the second part, we validated that STRAP binds with GSK3-Beta, an enzyme that plays multiple roles in a cell, including insulin and Wnt signaling. In a completely novel finding, we observed that STRAP, GSK3-Beta and Axin form a ternary complex. We also, for the first time show that intracellular fragment of Notch3 (ICN3) binds with GSK3-Beta, suggesting that Notch3 may be a novel substrate of GSK3-Beta.

We show that STRAP binds ICN3 in a proteasomal inhibition-dependent manner. Further studies revealed that STRAP binds ICN3 through the same ankyrin repeat region. In-vivo ubiquitination studies indicate that STRAP is able to reduce ubiquitination of ICN3, raising a possibility that STRAP may stabilize ICN3 leading to a longer half life in the cells. STRAP and Notch3 are both known to be upregulated in lung cancers and we observed that STRAP and ICN3 are co-overexpressed in 59 % of lung cancers in a tissue microarray study. STRAP shows oncogenic activity and our results from the two independent studies provide additional insights into how STRAP may behave as an oncogene using diverse mechanisms.