Dietary sphingolipids suppress a subset of preneoplastic rat liver lesions exhibiting high PTEN, low phospho-Akt and high levels of ceramide species

PTEN and PHLPP crosstalk in cancer cells and in TGFβ-activated stem cells

Akt kinase regulates several cellular processes, among them growth, proliferation and survival, and has been correlated to neoplastic disease. We report here crosstalk between several Akt regulatory phosphatases that controls the level of the activated form (phosphorylated) of Akt and affects tumor cell aggressiveness. In prostate cancer cell lines, we observed that transient transfection of PTEN decreased the endogenous level of PHLPPs and in contrast, the transient transfection of PHLPPs decreased the endogenous level of PTEN. Furthermore, silencing of PTEN by siRNA resulted in increased PHLPP levels. This phenomenon was not seen in non-transformed cells or in prostate stem cells. This crosstalk promoted cancer cell invasion and was controlled by epigenetically regulated processes where activation of miRs (miR-190 and miR214), the polycomb group of proteins and DNA methylation were involved. The purinergic P2X4 receptor, which has been shown to have a role in wound healing, was identified to be the mediator of this crosstalk. We also studied prostate stem cells and found this crosstalk in the TGFβ1-activated epithelial-mesenchymal transition (EMT). The crosstalk seemed to be a natural part of EMT. In summary, we identify a crosstalk between Akt phosphatases which is not present in non-transformed prostate cells but occurs in cancer cells and stem cells transformed by TGFβ-1. This crosstalk is important for cellular invasion.

BACKGROUND

Phosphatases regulate the Akt oncogene.

RESULTS

Crosstalk between Akt phosphatases in prostate cancer cells and in TGF-β1 activated stem cells but not in non-transformed cells.

CONCLUSION

This back-up mechanism facilitates invasive migration of prostate stem and cancer cells.

SIGNIFICANCE

Characterization of Akt regulation may lead to a better understanding of tumor development and to novel strategies for treatment.

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and PH domain and leucine-rich repeat phosphatase cross-talk (PHLPP) in cancer cells and in transforming growth factor β-activated stem cells

Akt kinase controls cell survival, proliferation, and invasive growth and is a critical factor for cancer development. Here we describe a cross-talk between phosphatases that may preserve levels of activated/phosphorylated Akt and confer aggressive growth of cancer cells. In prostatic cancer cells, but not in non-transformed cells or in prostate stem cells, we found that the phosphatase and tensin homolog deleted on chromosome 10 (PTEN) overexpression down-regulated PH domain and leucine-rich repeat phosphatase (PHLPP) and that PHLPP overexpression down-regulated PTEN. We also show that silencing PTEN by siRNA increased the levels of PHLPPs. This cross-talk facilitated invasive migration and was mediated by epigenetic alterations, including activation of miR-190, miR-214, polycomb group of proteins, as well as DNA methylation. A role for the purinergic receptor P2X4, previously associated with wound healing, was indicated. We also show that TGF-β1 induced cross-talk concomitant with epithelial-mesenchymal transition in stem cells. The cross-talk emerged as an integrated part of epithelial-mesenchymal transition. We conclude that cross-talk between PTEN and PHLPPs is silenced in normal prostate cells but activated in TGF-β1 transformed prostate stem and cancer cells and facilitates invasive growth.

Crosstalk between PTEN/Akt2 and TGFbeta signaling involving EGF receptor down-regulation during the tumor promotion process from the early stage in a rat two-stage hepatocarcinogenesis model

The present study investigated the involvement of signaling of phosphatase and tensin homolog deleted on chromosome 10 (PTEN)/protein kinase B (Akt) and transforming growth factor-beta (TGFbeta) as well as receptor tyrosine kinases in the tumor promotion processes in a two-stage hepatocarcinogenesis model using male F344 rats. The cellular localization of related molecules was examined in liver cell foci expressing glutathione S-transferase placental form (GST-P) at the early stage of tumor promotion by fenbendazole (FB), piperonyl butoxide, or thioacetamide. Distribution in the liver cell foci and neoplastic lesions positive for GST-P was also examined at the later stage of FB promotion. In contrast to the initiation-alone cases, subpopulations of GST-P-positive foci induced by promotion for 6 weeks, regardless of the promoting chemicals used, enhanced down-regulation of PTEN and up-regulation of phosphorylated (active) Akt2 and phosphorylated substrate(s) of Akt-kinase activity. Also, up-regulation of TGFbeta receptor I and down-regulation of epidermal growth factor receptor (EGFR) were enhanced in the subpopulation of GST-P-positive foci in all promoted cases. A similar pattern of cellular distribution of these molecules was also observed in the neoplastic lesions at the late stage. These results suggest a crosstalk between Akt2 and TGFbeta signaling that involves a mechanism requiring EGFR down-regulation during the entire tumor promotion process starting from the early stage. In particular, a shift in subcellular localization of phosphorylated substrate(s) of Akt from the cell membrane in liver cell foci to the cytoplasm in carcinomas was observed, suggesting an alteration of the function or activity of the corresponding molecule(s).

Cellular distributions of molecules with altered expression specific to the tumor promotion process from the early stage in a rat two-stage hepatocarcinogenesis model

A global gene expression profiling specific to the early process of tumor promotion by fenbendazole (FB) or phenobarbital (PB) in a rat two-stage hepatocarcinogenesis model revealed 33 genes to show altered expression in common with both chemicals. The immunohistochemical distribution of transferrin receptor (Tfrc), nuclear receptor subfamily 0, group B, member 2 (Nr0b2) and minichromosome maintenance deficient 6 (MCM6), included in the altered expression profile, were therefore examined in FB- and PB-induced proliferative lesions at both early and late stages of tumor promotion. In addition, immunoexpression of transforming growth factor beta receptor (TGFbetaR) I, TGFbetaRII, phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and phosphorylated phosphatase and tensin homolog deleted on chromosome 10 (pPTEN) was also examined. In the early stage, most hepatocellular foci positive for glutathione S-transferase placental form (GST-P) showed co-expression of TGFbetaRI and lack of PTEN and pPTEN, some GST-P-positive foci co-expressing Tfrc and Nr0b2. In the late stage, selective expression of TGFbetaRI, but not TGFbetaRII, was also observed in many adenomas and carcinomas consistently expressing GST-P. Nr0b2 was variably expressed in the proliferative lesions, irrespective of the carcinogenic stage. Like the GST-P-positive foci, adenomas and carcinomas consistently lacked PTEN and pPTEN. Expression of Tfrc and MCM6 was increased in parallel with the carcinogenic stage. In conclusion, loss of PTEN and dysregulation of transforming growth factor beta signaling can be considered to be involved in rat hepatocarcinogenesis from early stages. Selective expression of Tfrc in proliferative lesions suggests an involvement of changes in iron homeostasis during the process of tumor promotion/progression driven by FB or PB.