Effects of Different T3 Doses on Src, FAK, and PI3K Proteins in Breast Cancer Cells
To determine whether different doses of thyroid hormone could alter the expression patterns of the kinases involved in cell movement, we designed an in vitro model using T-47D breast cancer cells. In this model, the cells were exposed during 48 h to different doses of T3 to recreate a normal, excess, and absence state of T3 (described in “Materials and Methods”), which allowed measuring the expression of Src, FAK, p85α, and Akt1/2/3 (PI3K substrate) proteins and mRNA were then measured by Western blot and quantitative RT-PCR, respectively (Fig. 1a–h). We found no significant differences in the expression of Src, FAK, p85α, and Akt proteins between normal and excess T3 levels. We did, however, observe a marked decrease in the expression of the proteins mentioned above in cells without T3 compared to the other T3 states (Fig. 1a–e). We also found a similar behavior of mRNA and protein expression for total Akt, but the changes were not significant. We did not observe correlation between mRNA and protein expression for Src and FAK, suggesting that post-transcriptional control mechanisms may be operating.
T3 Induces Phosphorylation of FAK and AKT in T-47D Cells
In order to determine if T3 could activate this protein kinases, we performed a rapid treatment of T-47D breast cancer cells with thyroid hormone (T3, 1 nM) resulting in an increased Tyr397-FAK phosphorylation, which corresponds to activation [7–9]. The same effect was observed in Thr308-Akt1/2/3 protein phosphorylation as a result of PI3K activation. This phenomenon was time-dependent and transient; it was maximal after 20 min and returned to baseline after 60 min (Fig. 2a–c).
To establish the T3 dose capable of stimulating FAK and Akt activation, T-47D cells were treated for 20 min with increasing concentrations of T3 from 0.01 to 100 nM (Fig. 2d–f). Maximum Tyr397-FAK and Akt1/2/3 phosphorylation was observed with 1 nM T3 (Fig. 2d–f).
T3 Induces FAK Phosphorylation and Translocation via Src Kinase and Promotes Formation of Focal Adhesion Complexes
Treatment of T-47D breast cancer cells with T3 (1 nM, 20 min) led to a rapid increase of Tyr419- Src and Tyr397-FAK phosphorylation, which corresponds to activation of these proteins compared to control [6] (Fig. 3a–c). Using the specific Src kinase inhibitor PP2 (10 μM), we determined that Src kinase was involved in FAK phosphorylation (Fig. 3d, e) and translocation (Fig. 3f) during T3 exposure.
Treatment with T3 (1 nM) for 20 min led to a rapid change of the spatial organization of actin fibers and subcellular FAK localization. After T3 treatment, we observed a reorganization of the actin cytoskeleton towards the periphery of the membrane, resulting in membrane thickening (Fig. 3f, yellow arrows). Actin translocated from the cytoplasm towards the edge of the cell membrane, where it co-localized with phosphorylated FAK forming typical focal adhesion complexes (Fig. 3f).
T3 Controls a Multi-protein Complex with Src, PI3K, and FAK
Integrins typically initiate signaling via cell adhesion to the extracellular matrix, where they interact with matrix proteins and cluster on the surface of the membrane. This action promotes the assembly of a focal contact containing integrins together with tyrosine kinases, such as Src or FAK, and adaptor proteins that mediate downstream signaling, leading to a wide array of cellular activities [36]. In fact, previous studies suggest that T3 could bind integrin αV/β3 and transmit the signal to Src, FAK, and PI3K [35]. To determine whether T3 initiates its signaling pathway via integrin αV/β3, we treated the cells with T3 in the presence or absence of the integrin αV/β3 receptor antagonist tetraiodothyroacetic acid (Tetrac—10 μM). As mentioned before, treatment with T3 (1 nM) for 20 min induces FAK activation. However, T3 is not able to activate it in the presence of Tetrac, which suggests that αV/β3 integrin receptor signal to FAK after T3 treatment (Fig. 4a, b). Consistent with this hypothesis, we test whether thyroid hormone receptor alpha (TRα) or thyroid hormone receptor beta 1 (TRβ1) affects this post-translational modification in T-47D breast cancer cells. We silenced TRα and TRβ1 with siRNAs. In TRα and TRβ1-silenced cells, T3 induced FAK phosphorylation (Supporting Information S4), suggesting that TRα/β1 is not involved in this rapid signaling cascade.
In order to prove whether T3 binds to integrin αV/β3 at the cell membrane, we studied this effect through the radiolabel displacement assays using T-47D breast cancer cell membranes. To evaluate this possibility, 60 μg of plasma membrane protein/sample was incubated with [125I] T3 (0.01 nM) for 10 min, and varying concentrations of unlabeled T3 (0.1–1000 nM) were then added. Membrane-bound radiolabeled T3 was displaced by unlabeled T3 in a concentration-dependent manner in T-47D plasmatic membranes, as previously described Lin et al. [35] (Fig. 4c).
On the other hand, previous studies have linked thyroid hormone signaling with integrin αV/β3 and estrogen receptor alpha (ERα) suggesting that T3 activates crosstalk between both receptors in human lung carcinoma cells [37]. To test whether ERα is involved in this rapid signaling system in T-47D (ER+) breast cancer cells, we silenced ERα with siRNAs. In ERα-silenced cells, T3 induced FAK phosphorylation, suggesting that ERα is not involved in this non-genomic signaling cascade (Supporting Information S6).
We performed co-immunoprecipitation assays (IP) in breast cancer cells to further explore the signaling pathways through which integrin αV/β3 transmits the signal to Src, FAK, and PI3K. With this approach we found that integrin αV/β3, Src, p85α, and FAK interacted in T-47D breast cancer control cells (Fig. 4d, e). This interaction was partially disrupted after treatment with T3 for 20 min (Fig. 4d, e) and membrane translocation of FAK was observed (Figs. 3f and 4f) in the presence of T3. Furthermore, the dissociation of integrin αV/β3 with this complex (Src/FAK/p85α) was prevented by the use of Tetrac (Supporting Information S8). These findings support the concept that T3 signals to FAK through an integrin αV/β3/Src/PI3K cascade.
Integrin αV/β3 Signals to FAK through a Src and PI3K-Dependent Signaling Pathway
We used specific pharmacological inhibitors in cells exposed or not to T3 to identify the signaling intermediates involved in FAK activation by T3 hormone in T-47D breast cancer cells. The Src kinase inhibitor PP2 (10 μM), the focal adhesion kinase inhibitor (FAKi 1 μM), and wortmannin (WM 30 nM)—an inhibitor of phosphatidylinositol 3-OH kinase (PI3K)—all significantly inhibited Tyr397-FAK phosphorylation induced by T3 (Fig. 5a, b and Supporting Information S1), suggesting that in the presence of T3, integrin αV/β3 signals to FAK via Src and PI3K.
To examine the effect of T3 on PI3K kinase activation (via AKT phosphorylation, AKT1S473) in more detail, we used three different breast cancer cell lines with or without mutations in the catalytic (p110α) subunit of PI3K (PI3KCA), the lines SKBR-3 (wild type for PI3KCA), MCF-7 (with helical PI3KCA mutations), and T-47D (with kinase-domain PI3KCA mutation), in order to determine the AKT pathway activation. By means of Western blot analysis, we performed phosphoprotein profiles associated with distinct alterations affecting the PI3K pathway. Additionally, we examined the effects of T3 using another specific PI3K inhibitor (LY294002) that is wild type for PIK3CA (Supporting Information S7). Our results have shown that the PI3K kinase activation was promoted by the effects of T3 in three different breast cancer cells, and this induction was disrupted by the use of the specific PI3K inhibitor LY independent of the PIK3CA mutations.
The thyroid hormone analog (Tetrac 10 μM) and MAPK inhibitor (PD 5 mM) markedly decreased ERK1/2 phosphorylation (Supporting Information S5 and Fig. 5c), but did not affect FAK activation during T3 exposure (Fig. 5a, b), indicating that the MAPK pathway is not involved in Src/FAK/PI3K signaling induced by T3 (Fig. 5a–c).
In order to evaluate the participation of direct associations between Src, p85α (regulatory subunit of PI3K), and FAK after T3 treatment, we performed IP using or not the specific inhibitors PP2, WM, and FAKi. With immunoprecipitation assays, we found that, in the presence of T3, Src increased its interaction with FAK and this interaction was disrupted by the use of PP2, WM and FAKi (Fig. 5d). On the other hand, in the presence of T3, Src kinase increased its interaction with p85α and this interaction was disrupted by the use of PP2, WM, and FAKi (Fig. 5f), suggesting that T3 induces the formation of a multi-protein complex between Src with p85α and FAK (Fig. 5d, e).
T3 Controls T-47D Breast Cancer Cell Adhesion and Migration via Src/FAK/PI3K
We performed horizontal migration assays with T-47D cells to relate the T3-induced actin cytoskeleton remodeling and Src/FAK/PI3K activation with increased cell motility. A selective inhibitor of DNA strand separation that does not block RNA synthesis, cytosine β-darabinofuranoside hydrochloride (10 μM), was used to arrest cell proliferation, to distinguish cell migration from cell proliferation. Treatment with T3 (1 nM) for 48 h significantly increased the number of T-47D breast cancer cells that migrated through the starting line as well as the mean length of migration compared to control. This effect was blocked by PP2, WM and FAKi but not by PD (Fig. 6a, b), suggesting that the Src/FAK/PI3K cascade is implicated in the signaling of T3 to T-47D cell migration.
In addition to promoting T-47D cell migration, treatment with T3 (1 nM/48 h) led to significantly higher cellular adhesion to gelatin (Fig. 6c, d). As control, we tested the effect of the different inhibitors individually, demonstrating the specific action of T3 on cellular migration (Supporting Information S2) and adhesion (Fig. 6c, d).