TGFβ and IGF1R signaling activates protein kinase A through differential regulation of ezrin phosphorylation in colon cancer cells

Abstract

Aberrant cell survival plays a critical role in cancer progression and metastasis. We have previously shown that ezrin, a cAMP-dependent A-kinase anchoring protein (AKAP), is upregulated in colorectal cancer (CRC) liver metastasis. Phosphorylation of ezrin at threonine 567(T567) activates ezrin and plays an important role in CRC cell survival associated with XIAP and survivin upregulation. In this study, we demonstrate that, in FET and GEO colon cancer cells, knockdown of ezrin expression or inhibition of ezrin phosphorylation at T567 increases apoptosis through PKA activation in a cAMPindependent manner. TGFβ signaling inhibits ezrin phosphorylation in a Smad3-dependent and Smad2-independent manner and regulates proapoptotic function through ezrin-mediated PKA activation. On the other hand, ezrin phosphorylation at T567 by IGF1R signaling leads to cAMP-dependent PKA activation and enhances cell survival. Further studies indicate that phosphorylated ezrin forms a complex with PKA RII and dephosphorylated ezrin dissociates from the complex and facilitates the association of PKA RII with AKAP149, both of which activate PKA yet lead to either cell survival or apoptosis. Thus, our studies reveal a novel mechanism of differential PKA activation mediated by TGFβ and IGF1R signaling through regulation of ezrin phosphorylation in CRC resulting in different cell fate. This is of significance because TGFβ and IGF1R signaling pathways are well characterized tumor suppressor and oncogenic pathways respectively with important roles in CRC tumorigenesis and metastasis. Our studies indicate that they crosstalk and antagonize each other's function through regulation of ezrin activation. Therefore, ezrin may be a potential therapeutic target in CRC.

Prognostic and therapeutic implications of NHERF1 expression and regulation in colorectal cancer

“…Insulin‐like growth factor 1 receptor (IGF1R) signaling has been shown to be upregulated in many cancers including CRC, and approximately 30% to 40% of all CRCs are IGF1R‐dependent . IGF1R signaling has been shown to play a critical role in CRC cell survival . Recent in vitro and in vivo studies by our group demonstrated that inhibition of IGF1R signaling induces apoptosis, in association with downregulation of XIAP and survivin, while activation of IGF1R by transferrin and insulin (TI) increases CRC cell survival .…”

“…IGF1R signaling has been shown to play a critical role in CRC cell survival . Recent in vitro and in vivo studies by our group demonstrated that inhibition of IGF1R signaling induces apoptosis, in association with downregulation of XIAP and survivin, while activation of IGF1R by transferrin and insulin (TI) increases CRC cell survival . Since GEO and CBS CRC cells are dependent on IGF1R signaling for survival, we hypothesized that inhibition of the IGF1R pathway would affect NHERF1‐mediated CRC cell survival.…”

“…Furthermore, treatment of GEO and CBS CRC cells with OSI‐906 in vitro led to a reduction in NHERF1 expression, together with downregulation of XIAP and survivin expression (Figure D) and dephosphorylation of IGF1R at tyrosine 1135 (Y1135; Figure S1C). In contrast, treatment of GEO cells with transferrin and insulin (TI), which activates IGF1R signaling, increased NHERF1 expression and upregulated XIAP and survivin together with IGF1R hyperphosphorylation (Figure S1D). Overall, these results indicate that NHERF1 promotes CRC cell survival, modulates XIAP/survivin expression, and is downstream of the IGF1R signaling pathway.…”

Overview of the oncogenic signaling pathways in colorectal cancer: Mechanistic insights

“…Cyclic-AMP-dependent protein kinase A-anchoring proteins (AKAPs) structurally interacted with PKA and targeted these supra-molecular complexes to particular subcellularly localized regions where they phosphorylated different proteins. Ezrin inhibition mediated activation of PKA was found to be dependent on AKAP149 in colorectal cancer cells [70].…”

“…TGFβ inhibited ezrin phosphorylation at 567 th threonine residue which resulted in protein kinase-A (PKA) activation and apoptosis. TGFβ1 treatment time-dependently decreased ezrin phosphorylation [70]. Cyclic-AMP-dependent protein kinase A-anchoring proteins (AKAPs) structurally interacted with PKA and targeted these supra-molecular complexes to particular subcellularly localized regions where they phosphorylated different proteins.…”