A Novel Mechanism by Which Tissue Transglutaminase Activates Signaling Events That Promote Cell Survival

Marc A. Antonyak,Lindsey K. Boroughs,Richard A. Cerione

doi:10.1074/jbc.m113.464693

Abstract

Tissue transglutaminase (tTG) functions as a GTPase and an acyl transferase that catalyzes the formation of protein cross-links. tTG expression is frequently up-regulated in human cancer, where it has been implicated in various aspects of cancer progression, including cell survival and chemo-resistance. However, the extent to which tTG cooperates with other proteins within the context of a cancer cell, versus its intrinsic ability to confer transformed characteristics to cells, is poorly understood. To address this question, we asked what effect the ectopic expression of tTG in a non-transformed cellular background would have on the behavior of the cells. Using NIH3T3 fibroblasts stably expressing a Myc-tagged form of tTG, we found that tTG strongly protected these cells from serum starvation-induced apoptosis and triggered the activation of the PI3-kinase/mTOR Complex 1 (mTORC1)/p70 S6-kinase pathway. We determined that tTG forms a complex with the non-receptor tyrosine kinase c-Src and PI3-kinase, and that treating cells with inhibitors to block tTG function (monodansylcadaverine; MDC) or c-Src kinase activity (PP2) disrupted the formation of this complex, and prevented tTG from activating the PI3-kinase pathway. Moreover, treatment of fibroblasts over-expressing tTG with PP2, or with inhibitors that inactivate components of the PI3-kinase pathway, including PI3-kinase (LY294002) and mTORC1 (rapamycin), ablated the tTG-promoted survival of the cells. These findings demonstrate that tTG has an intrinsic capability to stimulate cell survival through a novel mechanism that activates PI3-kinase signaling events, thus highlighting tTG as a potential target for the treatment of human cancer.

Highlights

Tissue transglutaminase promotes various aspects of oncogenesis, including cell survival
Several of these studies have shown that knocking-down Tissue transglutaminase (tTG) expression in cancer cells with siRNAs, or treating cells with compounds that bind at the transamidation active site of tTG, often sensitizes these cancer cells to chemotherapeutic agents and reduces many of their transformed features [11,12,13,14,15,16]
While these findings point to tTG as being an important player in cancer progression, exactly how tTG contributes to the malignant state and whether it represents a valid target for cancer therapy, remains to be determined

Summary

Introduction

Tissue transglutaminase (tTG) promotes various aspects of oncogenesis, including cell survival. The extent to which tTG cooperates with other proteins within the context of a cancer cell, versus its intrinsic ability to confer transformed characteristics to cells, is poorly understood. To address this question, we asked what effect the ectopic expression of tTG in a non-transformed cellular background would have on the behavior of the cells. Treatment of fibroblasts over-expressing tTG with PP2, or with inhibitors that inactivate components of the PI3-kinase pathway, including PI3-kinase (LY294002) and mTORC1 (rapamycin), ablated the tTG-promoted survival of the cells These findings demonstrate that tTG has an intrinsic capability to stimulate cell survival through a novel mechanism that activates PI3-kinase signaling events, highlighting tTG as a potential target for the treatment of human cancer

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Conclusion