Effect of Selective Serotonin (5-HT)(2B) Receptor Agonist BW723C86 on Epidermal Growth Factor/Transforming Growth Factor-alpha Receptor Tyrosine Kinase and Ribosomal p70 S6 Kinase Activities in Primary Cultures of Adult Rat Hepatocytes

摘要

Serotonin (5-hydroxytryptamine; 5-HT) can induce hepatocyte DNA synthesis and proliferation by autocrine secretion of transforming growth factor (TGF)-alpha through 5-HT2B receptor/phospholipase C (PLC)/Ca2+ and a signaling pathway involving epidermal growth factor (EGF)/TGF-alpha receptor tyrosine kinase (RTK)/extracellular signal-regulated kinase 2 (ERK2)/mammalian target of rapamycin (mTOR). In the present study, we investigated whether 5-HT or a selective 5-HT2B receptor agonist BW723C86, would stimulate phosphorylation of TGF-alpha RTK and ribosomal p70 S6 kinase (p70S6K) in primary cultures of adult rat hepatocytes. Western blotting analysis was used to detect 5-HT- or BW723C86 (10(-6)M)-induced phosphorylation of EGF/TGF-alpha RTK and p70S6K. Our results showed that 5-HT- or BW723C86 (10(-6)M)-induced phosphorylation of EGF/TGF-alpha RTK peaked at between 5 and 10min. On the other hand, 5-HT- or BW723C86 (10(-6)M) induced phosphorylation of p70S6K peaked at about 30 min. Furthermore, a selective 5-HT2B receptor antagonist LY272015, a specific PLC inhibitor U-73122, a membrane-permeable Ca2+ chelator BAPTA/AM, an L-type Ca2+ channel blocker verapamil, somatostatin, and a specific p70S6K inhibitor LY2584702 completely abolished the phosphorylation of p70S6K induced by both 5-HT and BW723C86. These results indicate that phosphorylation of p70S6K is dependent on the 5-HT2B-receptor-mediated autocrine secretion of TGF-alpha. In addition, these results demonstrate that the hepatocyte proliferating action of 5-HT and BW723C86 are mediated by phosphorylation of p70S6K, a downstream element of the EGF/TGF-alpha RTK signaling pathway.