Introduction
Despite significant advances in early detection and treatment of breast cancer, mortality due to metastatic disease remains high. A growing body of evidence supports the notion that acquisition of epithelial-to-mesenchymal transition (EMT) by breast cancer cells is an important mechanism in the progression and pathogenesis of cancer [
1,
2]. EMT is a developmentally regulated process in which adherent epithelial cells lose their epithelial characteristics and acquire mesenchymal properties, including fibroid morphology, characteristic changes in gene expression and increased invasion and resistance to chemotherapy [
3]. In addition to eliciting the invasive phenotype, EMT also induces cancer stem cell (CSC)-like traits that are considered to provide cancer cells with the ability to self-renew and colonize at metastatic sites [
4]. Thus aberrant expression of EMT regulators in breast cancer cells may contribute to disease progression, and their identification could yield novel therapeutic targets for improved patient outcomes.
In our quest to determine the significance of elevated tissue transglutaminase 2 (TG2) expression in drug-resistant and metastatic breast cancer cells [
5,
6], we found that stable expression of TG2 in mammary epithelial cells is associated with EMT. TG2-induced EMT was associated with constitutive activation of the NF-κB and increased expression of transcription repressors such as
Snail1,
Twist1,
Zeb1 and
Zeb2 [
7]. The TG2-induced EMT is related to TGF-β signaling in that cells transfected with TG2-shRNA prior to TGF-β treatment failed to undergo EMT compared with control shRNA-transfected cells, which showed morphologic and molecular alterations typical of mesenchymal cells in response to TGF-β treatment. Importantly, TG2-induced EMT was associated with enrichment of the CD44
high/CD24
-/low cell population, increased ability to form mammospheres and self-renewal ability [
8], traits that are considered to endorse the CSC phenotype. These observations revealed a novel function for TG2 and suggested that TG2-regulated pathways play an important role in acquisition of drug resistance and metastasis by conferring the EMT-CSC phenotype in mammary epithelial cells.
TG2 is structurally and functionally a complex protein that has been implicated in diverse processes such as inflammation, wound-healing, celiac disease and cancer [
9,
10]. In addition to catalyzing calcium-dependent transamidation reactions, TG2 can bind and hydrolyze GTP. Under physiological conditions, low calcium and high GTP levels sustain TG2 in a latent form with respect to transamidation activity. Under pathological conditions, however, perturbation in calcium homeostasis and decreased GTP reserves could activate TG2 to its transamidation configuration. Researchers in several recent studies have demonstrated increased expression of TG2 in multiple cancer cell types [
11‐
15]. Importantly, TG2 expression in cancer cells has been associated with increased resistance to chemotherapy, metastasis and poor patient outcomes [
5,
13,
14]. Inhibition of TG2 by siRNA, antisense RNA or small-molecule inhibitors reversed the sensitivity of cancer cells to chemotherapeutic drugs and attenuated their invasion, both
in vitro and in animal models [
6,
12‐
14,
16][
17]. In view of these observations, we initiated studies to determine which of the two well-characterized activities of TG2 (protein cross-linking activity and GTP-binding activity) is responsible for promoting the oncogenic functions. Herein we provide evidence that, similar to wild-type TG2, expression of transamidation-inactive mutants (C277S and W241A) is able to induce EMT-CSC in mammary epithelial cells. In contrast, the expression of the GTP-binding-deficient TG2 mutant (R580A) failed to induce EMT-CSC-related changes. Our current studies suggest that cancer cells utilize the GTP-binding and GTP-signaling function of TG2 to acquire chemoresistance and the metastatic phenotype and that this information can be exploited to develop small-molecule inhibitors to inhibit TG2-regulated pathways and reverse the EMT and CSC phenotypes.
Discussion
Several reports have documented elevated expression of TG2 in multiple cancer cell types [
27,
28]. TG2 expression in cancer cells has been linked to drug resistance and metastasis, the phenotypes that account for nearly 90% of cancer-related deaths [
29]. These observations clearly imply that TG2 could be an attractive therapeutic target for the treatment of drug-resistant and metastatic cancers. As a first step in this direction, we determined the activity of TG2 that is responsible for promoting its oncogenic functions. We employed TG2-C277S and TG2-W241A mutants (data not shown) as the transamidation-inactive forms and TG2-R580A mutant as the GTP-binding-deficient form. The GTP-binding and transamidation activities of constructs in stably transfected MCF10A cell lines were found to be consistent with predicted activities reported in the literature and based on the structure and function information of TG2 [
30‐
32].
The GTP-binding and transamidation activities of TG2 are mechanistically mutually exclusive and are regulated by GTP and calcium. The transamidase activity of TG2 is stimulated by calcium and inhibited by GTP, whereas GTPase activity is inhibited by calcium [
33]. It is not clear whether under physiological conditions intracellular TG2 acts as a transamidase or as a GTPase; however, the prevailing view is that, due to high GTP and low calcium, TG2 exists as an inactive transamidase inside the cell, where it acts mainly as a GTP-binding protein and converts GTP into GDP. This view is supported by our current results, in which TG2-WT-expressing MCF10A cells failed to show any significant
in situ activity under basal culture conditions. The GTP-binding inactive mutant (TG2-R580A), in contrast, showed some transamidase activity even under basal conditions (Figure
1). The transamidating
in situ activity increased in response to calcium ionophore (A23187) treatment in TG2-WT- and TG2-R580A-transfected cells, but not in transamidase-inactive TG2-C277S-expressing cells. Interestingly, the transamidation activity of TG2 had no relevance to its ability to induce EMT-CSC in mammary epithelial cells. Thus the catalytically inactive TG2-C277S and TG2-W241A forms of TG2 were as effective as TG2-WT in inducing the EMT-CSC-related changes, except for some subtle quantitative differences in the levels of some inducible genes (Figure
2). The GTP-binding-inactive R580A mutant, on the other hand, was completely inactive in inducing the EMT-CSC-related changes, despite its high expression and transamidation activity. The expression of each of the three constructs was associated with decreased growth rates in MCF10A cells and was observed to be Vec > C277S > WT > R580A, in that order (Additional file
7).
TG2 has received considerable attention in recent years for its potential role in cancer cells. There is ample evidence supporting metastatic and drug-resistant cancer cells' expression of high basal levels of TG2 [
5,
6,
11,
12,
14,
15,
17]. Our quest to understand the significance of TG2 in cancer cells led us to some rather surprising findings. We found that stable expression of TG2 in normal and transformed mammary epithelial cells is associated with the induction of EMT [
7] and CSC [
8], the phenotypes that are closely linked with the development of drug resistance and metastasis in cancer cells [
34,
35]. These observations clearly imply that aberrant expression of TG2 in cancer cells could promote drug resistance and metastasis by inducing the EMT-CSC phenotype and hence could serve as a promising therapeutic target for reversing chemoresistance and inhibiting metastasis. Indeed, the observations that inhibition of TG2 by siRNA, small-molecule inhibitors or antisense RNA could render cancer cells sensitive to chemotherapeutic drugs and inhibit their invasiveness both
in vitro and in animal models [
13,
14,
17,
24] strongly support such a contention. Therefore, knowledge of the TG2 domain and function that is essential for promoting the EMT-CSC is foundational for the rational design of small-molecule inhibitors that are able to harness TG2-regulated events in cancer cells.
With this intent, we initiated studies to address whether either or both of the two well-characterized activities (transamidation and GTPase) of TG2 are essential for promoting EMT-CSC in mammary epithelial cells. Our findings strongly suggest that the transamidation activity of TG2 is not essential for promoting the EMT-CSC. However, interference with GTP-binding function could completely abrogate its oncogenic functions. Whether the failure of the TG2-R580A mutant to support the EMT-CSC phenotype is related to its inability to bind and hydrolyze GTP or is a consequence of the conformational change induced as a result of single-amino acid substitution in position 580 remains to be determined. Indeed, it has been suggested that the TG2-R580A mutant acquires a more open and extended conformation compared with the TG2-WT form [
31]. Hence the failure of TG2-R580A to promote EMT-CSC may be related to its altered interaction with some signaling proteins rather than to its inability to bind and hydrolyze GTP. In this context, our recent observation that the interaction of TG2 with the inhibitor of NF-κB (IκB) results in constitutive activation of NF-κB (Kumar S and Mehta K, unpublished data) is of interest. It is likely that TG2-R580A is unable to interact effectively with IκB, owing to its extended conformation. Indeed, the TG2-induced activation of NF-κB was significantly compromised in TG2-R580A-transfected cells compared with the TG2-WT, TG2-C277S (Figure
5A) or TG2-W241A cells (data not shown). Constitutive activation of NF-κB is frequently associated with advanced-stage cancers and is known to confer resistance to chemotherapy and to promote metastasis by inducing EMT [
36‐
38]. As a proof of concept, our data support the hypothesis that TG2-induced EMT in MCF10A cells could be reversed (MET) by inhibiting NF-κB activity. Thus downregulation of the p65 subunit of NF-κB reversed the mesenchymal phenotype to the epithelial phenotype without any noticeable change in TG2 expression (Figure
5B). Recently, Shao
et al. [
25] noted similar involvement of NF-κB in TG2-induced EMT, invasiveness and drug resistance in ovarian cancer cells. Overall, these observations suggest that open or closed conformation of TG2, which depends on the intracellular environment, is the major determining factor in TG2-induced promotion of cell survival or cell death signaling. Thus transamidase-inactive closed conformation of TG2 (due to GTP binding in the presence of low calcium) may promote cell survival processes by serving as a scaffold protein and mediating protein-protein interactions. In this context, the band 4.2 protein in red blood cells, the only catalytically inactive member of the transglutaminase family of proteins, primarily serves as a scaffold protein and promotes interaction with various membrane proteins, such as ankyrin, spectrin and CD47 [
39,
40]. Given the structural similarities between TG2 and protein 4.2, it is tempting to speculate that the primary function of TG2 in cancer cells is to serve as a scaffold protein rather than as an enzyme. In this capacity, TG2 can promote protein-protein interactions, resulting in constitutive activation of cellular events needed for increased cell survival and invasive function during advanced-stage cancer.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
AK and KM designed the experiments. AK, JX, BS and SK performed the experiments. BBA and DY contributed new reagents or analytical tools. AK and KM wrote the article. All authors read and approved the final manuscript.