Protein Kinase D1 Suppresses Epithelial-to-Mesenchymal Transition through Phosphorylation of Snail

Abstract

Cancer cells undergo epithelial-mesenchymal transition (EMT) as a program of increased invasion and metastasis during cancer progression. Here, we report that a novel regulator of EMT in cancer cells is protein kinase D1 (PKD1), which is downregulated in advanced prostate, breast, and gastric cancers. Ectopic reexpression of PKD1 in metastatic prostate cancer cells reversibly suppressed expression of mesenchyme-specific genes and increased epithelial markers such as E-cadherin, whereas small interfering RNA-mediated knockdown of PKD1 increased expression of mesenchyme markers. Further, PKD1 inhibited tumor growth and metastasis in a tumor xenograft model. PKD1 phosphorylates Ser 11 (S11) on transcription factor Snail, a master EMT regulator and repressor of E-cadherin expression, triggering nuclear export of Snail via 14-3-3σ binding. Snail S11 mutation causes acquisition of mesenchymal traits and expression of stem cell markers. Together, our results suggest that PKD1 functions as a tumor and metastasis suppressor, at least partly by regulating Snail-mediated EMT, and that loss of PKD1 may contribute to acquisition of an aggressive malignant phenotype.

Protein Kinase D1 Mediates Anchorage-dependent and -independent Growth of Tumor Cells via the Zinc Finger Transcription Factor Snail1

“…In addition, phosphorylation of Snail1 was absent when Ser-11 was replaced by Ala (S11A) even in the presence of active PKD1. Thus, in accordance with data published by Du et al (16), Ser-11 is a PKD phosphorylation site in vivo, and it is the only PKD phosphorylation site in Snail1 (Fig. 1B).…”

“…Our data also suggest that phosphorylation at this site is necessary for efficient binding of vital Snail1 co-repressors such as HDAC2 modulating Snail1-dependent HDAC activity. In contrast to Du et al (16), Snail1 phosphorylation at Ser-11 did not affect nucleocytoplasmic shuttling of the protein. This may be explained by 14-3-3 down-regulation in many tumor cells by different mechanisms (39) including promotor methylation and inhibition downstream of p53 mutations, thereby facilitating cancer formation by many routes (40).…”

“…Du et al (16) reported that phosphorylation of Snail1 at Ser-11 by PKDs regulates its nuclear export by interaction with 14-3-3 proteins in epithelial cell lines including C4-2 tumor cells. However, the authors conceded that tumor cells may have different mechanisms for regulating Snail1 transcriptional activity.…”

Post-translational modifications of EMT transcriptional factors in cancer metastasis

“…PKD1-mediated phosphorylation of Snail was required for E-cadherin expression in prostate cancer cells [32]. Furthermore, Ser 11 was involved in the interaction between Snail and Fbxo11, followed by ubiquitylation and degradation of Snail upon Fbxo11 E3 ligase activity.…”

The regulation of snail: on the ubiquitin edge

“…Whether Snail degradation by Fbxo11 is dependent on S11 phosphorylation by the PKD1 is controversial and needs confirmation. The role of PKD1 -induced Snail phosphorylation is complex since PKD1 can either inhibit or enhance Snail transcriptional repression [25,26] . Fbxo45 is an atypical F-box protein induced by estrogen [27] .…”