Tumor suppression effects of bilberry extracts and enzymatically modified isoquercitrin in early preneoplastic liver cell lesions induced by piperonyl butoxide promotion in a two-stage rat hepatocarcinogenesis model

To investigate the protective effect of bilberry extracts (BBE) and enzymatically modified isoquercitrin (EMIQ) on the hepatocarcinogenic process involving oxidative stress responses, we used a two-stage hepatocarcinogenesis model in N-diethylnitrosamine-initiated and piperonyl butoxide (PBO)-promoted rats. We examined the modifying effect of co-administration with BBE or EMIQ on the liver tissue environment including oxidative stress responses, cell proliferation and apoptosis, and phosphatase and tensin homolog (PTEN)/Akt and transforming growth factor (TGF)-β/Smad signalings on the induction mechanism of preneoplastic lesions during early stages of hepatocellular tumor promotion. PBO increased the numbers and area of glutathione S-transferase placental form (GST-P)+ liver cell foci and the numbers of Ki-67+ proliferating cells within GST-P+ foci. Co-administration of BBE or EMIQ suppressed these effects with the reductions of GST-P+ foci (area) to 48.9–49.4% and Ki-67+ cells to 55.5–61.4% of the PBO-promoted cases. Neither BBE nor EMIQ decreased microsomal reactive oxygen species induced by PBO. However, only EMIQ suppressed the level of thiobarbituric acid-reactive substances to 78.4% of the PBO-promoted cases. PBO increased the incidences of phospho-PTEN– foci, phospho-Akt substrate+ foci, phospho-Smad3– foci and Smad4– foci in GST-P+ foci. Both BBE and EMIQ decreased the incidences of phospho-PTEN– foci in GST-P+ foci to 59.8–72.2% and Smad4– foci to 62.4–71.5% of the PBO-promoted cases, and BBE also suppressed the incidence of phospho-Akt substrate+ foci in GST-P+ foci to 75.2–75.7% of the PBO-promoted cases. These results suggest that PBO-induced tumor promotion involves facilitation of PTEN/Akt and disruptive TGF-β/Smad signalings without relation to oxidative stress responses, but this promotion was suppressed by co-treatment with BBE or EMIQ through suppression of cell proliferation activity of preneoplastic liver cells.