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

Abstract

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-b (TGFb) 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 TGFb 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 TGFb signaling that involves a mechanism requiring EGFR downregulation 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). (Cancer Sci 2009; 100: 813-820) I n a rat two-stage hepatocarcinogenesis model, altered liver cell foci immunoreactive for glutathiuone S-transferase placental form (GST-P) have been shown to increase in number and area because of tumor promotion with hepatocarcinogens, in accordance with the hepatocarcinogenic potential; therefore, these foci have been confirmed as preneoplastic lesions of liver cells.(1,2) However, only a few studies have examined the roles of cellular signaling and molecular linkage in the growth and development of the liver cell foci. (3)(4)(5)(6) Akt is a serine/threonine kinase that is a central regulator of widely divergent cellular processes, including proliferation, differentiation, migration, survival, and metabolism.(7) Akt is activated by a variety of stimuli, through growth factor receptors, in a phosphatidylinositol 3-kinase (PI3K)-dependent manner, and it is also negatively regulated by the tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN).(8) A disruption of normal Akt/PTEN signaling frequently occurs in many human cancers, and thus these molecules play an important role in cancer development, progression, and therapeutic resistance.(9,10) To date, three members of the Akt family have been identified in mammals: Akt1, Akt2, and...

Endoglin regulates PI3-kinase/Akt trafficking and signaling to alter endothelial capillary stability during angiogenesis

“…We also identified PI3K/Akt signaling as a novel endoglin target in regulating endothelial capillary stability and cell survival. Previous studies showed that TGF-β superfamily receptors can signal through non-Smad mechanisms, including the MAPKs (ERK, JNK, and p38) and PI3K/Akt pathways (Derynck and Zhang, 2003; Yi et al., 2005; Taniai et al., 2009; Pardali et al., 2010). Indeed, endoglin also has an important role in non-Smad signaling to the Ras/MAPK and PI3K/Akt in many cell types (Lee and Blobe, 2007; Fujita et al., 2010; Santibanez et al., 2010).…”

Expression Characteristics of Genes Hypermethylated and Downregulated in Rat Liver Specific to Nongenotoxic Hepatocarcinogens

“…The number and area of GST-P + liver cell foci >200 μm in diameter in liver sections from Experiments 1 and 2 ( n = 10/group) were measured according to the method and equipment as described previously (Taniai et al , 2009). In DEN and CCl 4 groups on day 84 of Experiment 1, and AFB 1 , NPYR, TAA, and MP groups on day 90 of Experiment 2 ( n = 10/group), the immunoreactivity of LDLRAD4, PROC, and CDH17 was classified as increased (+) or decreased (−) in the GST-P + foci compared with the surrounding hepatocytes, and the incidence of LDLRAD4 − , PROC − , and CDH17 − expression in total GST-P + foci appeared in liver sections per animal was estimated.…”

Promoter-region hypermethylation and expression downregulation of Yy1 (Yin yang 1) in preneoplastic liver lesions in a thioacetamide rat hepatocarcinogenesis model

“…The number and area of GST-P + foci larger than 200 μm in diameter in liver sections were measured as described previously Mizukami et al, 2010;Taniai et al, 2009). The number of immunoreactive cells in ten 400 × fields for PCNA and ten 200 × fields for cleaved caspase 3, and the ratio of immunoreactive cells to total liver cells was calculated for each of the Yy1 − and Yy1 + foci in the vicinity of GST-P + foci.…”