Signal Transduction Mechanism for Serotonin 5-HT_2B Receptor-Mediated DNA Synthesis and Proliferation in Primary Cultures of Adult Rat Hepatocytes

Abstract

Key words serotonin (5-hydroxytryptamine); signal transduction; DNA synthesis; proliferation (cultured hepatocyte); transforming growth factor-α Although adult rat hepatocytes rarely undergo cell division under normal physiological conditions, due to the ensuing regenerative response to recruit mature hepatocytes into the cell cycle in vivo after 70% partial hepatectomy, a phenomenon known as liver regeneration, 1,2) they do remain capable of proliferation. Among the numerous growth-promoting factors that pertain to liver regeneration, platelet-derived serotonin (5-hydroxytryptamine; 5-HT) is reportedly directly involved. 3-7)5-HT is not only a neurotransmitter, but also a kind of hormone with various extraneural functions. 5-HT mediates a diverse array of responses-including platelet aggregation, vascular constriction, and cell proliferation-by interacting with 5-HT receptor subtypes in mammals.8) Seven receptor classes including 14 subtypes of 5-HT receptors have been identified to date, reflecting the diversity of 5-HT actions. With the exception of the 5-HT 3 receptors, which are ligand-gated ion channels, all known 5-HT receptors are G-protein-coupled receptors that are positively linked to phosphatidylinositol turnover or cyclic AMP production and are further linked to a variety of downstream pathways. [9][10][11] In the presence of epidermal growth factor (EGF) and insulin, 5-HT has been shown to cause a dose-dependent increase in DNA synthesis in primary cultures of rat hepatocytes, suggesting that it is also a co-mitogen.12) In addition, interaction has also been seen between 5-HT and various hematological factors such as hepatocyte growth factor (HGF) and platelet-derived growth factor (PDGF). Although 5-HT has been shown to play significant roles in the stimulation and acceleration of hepatocyte proliferation, 6) few studies have been conducted on the possible involvement of 5-HT 2 receptor subtypes and their intracellular signal transduction pathways in 5-HT-induced hepatocyte DNA synthesis and proliferation in primary cultures of adult rat hepatocytes.Some recently identified selective 5-HT receptor agonists and antagonists have shown promise as useful tools for differentiating between closely related 5-HT 2 receptor subtypes. 8,[13][14][15][16] Therefore, in this study, we used these agents to investigate the involvement of 5-HT 2 receptor subtypes and their intracellular signal transduction pathways in 5-HT-induced hepatocyte DNA synthesis and proliferation in primary cultures of adult rat hepatocytes. MATERIALS AND METHODSAnimals Male Wistar rats (weight range, 200-220 g) were purchased from Tokyo Experimental Animal Co. (Tokyo, Japan) and adapted to a light-, humidity-and temperature-controlled room over a minimum 3-d period prior to the start of the experiments. All rats were fed with a standard laboratory diet and given tap water ad libitum. All rats used in this study were given human care in compliance with Guiding Principles for the Care and Use of Laboratory Animals approved by The Japanes...

Serotonin 5-HT_2B Receptor-Stimulated DNA Synthesis and Proliferation Are Mediated by Autocrine Secretion of Transforming Growth Factor-α in Primary Cultures of Adult Rat Hepatocytes

“…33) We previously showed that DNA synthesis and proliferation are induced by 5-HT in hepatocytes via 5-HT 2B receptors. 9) Moreover, 5-HT 2B receptor-mediated hepatocyte mitogenesis involves activation of both the Gq/PLC pathway and the EGF/ TGF-α RTK/PI3K/ERK2/mTOR pathway. However, how these signaling pathways are associated with each other remains unknown.…”

“…These results were in agreement with our previous report. 9,15) In addition, exogenously added TGF-α (1.0 ng/mL) also significantly increased time-dependent DNA synthesis and proliferation in primary cultures of adult rat hepatocytes (Figs. 1A, B).…”

“…7,8) In a previous report, we demonstrated that 5-HT-induced hepatocyte DNA synthesis and proliferation are mediated through 5-HT 2B receptors. 9) Moreover, 5-HT 2B receptor-mediated hepatocyte mitogenesis involves activation of the Gq/ phospholipase C (PLC) pathway and the epidermal growth factor (EGF)/transforming growth factor (TGF)-α receptor tyrosine kinase (RTK)/phosphatidylinositol 3-kinase (PI3K)/ extracellular signal-regulated kinase (ERK)2/mammalian target of rapamycin (mTOR) pathway. However, how these two signaling pathways are associated with each other remains unknown.…”

“…We hypothesized that 5-HT 2B receptor stimulation induces secretion of a putative primary mitogen via the Gq/ PLC pathway in hepatocytes, followed by induction of DNA synthesis and proliferation that is mediated by the EGF/TGF-α RTK/PI3K/ERK2/mTOR pathway. 9) To test this model, demonstrating that treatment of primary cultured hepatocytes with 5-HT or the selective 5-HT 2B receptor agonist BW723C86 10) leads to an increase in secretion of a putative primary mitogen is important. If so, determining how autocrine secretion of this mitogen via 5-HT 2B receptor activation is regulated is also important.…”

Effect of Selective Serotonin (5-HT)_2B Receptor Agonist BW723C86 on Epidermal Growth Factor/Transforming Growth Factor-α Receptor Tyrosine Kinase and Ribosomal p70 S6 Kinase Activities in Primary Cultures of Adult Rat Hepatocytes

“…[7][8][9] As for the liver, there are some reports that treatment with 5-HT and epidermal growth factor (EGF) enhances hepatocyte proliferation in primary culture and promotes liver regeneration in vivo after 70% partial hepatectomy. 10,11) Previously, we demonstrated that 5-HT and a selective 5-HT 2B receptor agonist BW723C86, 12) induce DNA synthesis and proliferation and increase EGF/transforming growth factor (TGF)-α receptor tyrosine kinase (RTK) phosphorylation and extracellular signal-regulated kinase 2 (ERK2) phosphorylation in primary cultures of rat adult hepatocytes. 13) In these hepatocytes, the proliferative effect induced by 5-HT or BW723C86 is mediated through the activation of a signaling pathway involving 5-HT 2B receptor/Gq/phospholipase C (PLC)/Ca 2+ via autocrine secretion of TGF-α.…”