Phenylephrine Enhances the Mitogenic Effect of S-Allyl-L-cysteine on Primary Cultured Hepatocytes through Protein Kinase C-Induced B-Raf Phosphorylation

The co-mitogenic effects of the α1-adrenoceptor agonist phenylephrine on S-allyl-L-cysteine (SAC)-induced hepatocyte proliferation were examined in primary cultures of adult rat hepatocytes. The combination of phenylephrine (10-10-10-6 M) and SAC (10-6 M) exhibited a significant dose-dependent increase in the number of hepatocyte nuclei and viable cells compared to SAC alone. This combination also increased the progression of hepatocyte nuclei into the S-phase. The potentiating effect of phenylephrine on SAC-induced cell proliferation was counteracted by prazosin (an α1-adrenergic receptor antagonist) and GF109203X (selective protein kinase C (PKC) inhibitor). In addition, PMA (direct PKC activator) potentiated the proliferative effects of SAC similarly to phenylephrine. In essence, these findings suggest that PKC activity plays a crucial role in enhancing SAC-induced cell proliferation. Moreover, the effects of phenylephrine on SAC-induced Ras activity, Raf phosphorylation, and extracellular signal-regulated kinase 2 (ERK2) phosphorylation were investigated. Phenylephrine (or PMA) in combination with SAC did not augment Ras activity, but further increased ERK2 phosphorylation and its upstream B-Raf phosphorylation. These results indicate that PKC activation, triggered by stimulating adrenergic α1 receptors, further amplifies SAC-induced cell proliferation through enhanced ERK2 phosphorylation via increased B-Raf-specific phosphorylation in primary cultured hepatocytes.

Cell proliferation effects of S-allyl-L-cysteine are associated with phosphorylation of janus kinase 2, insulin-like growth factor type-I receptor tyrosine kinase, and extracellular signal-regulated kinase 2 in primary cultures of adult rat hepatocytes

The cell proliferation effect of S-allyl-L-cysteine (SAC) and its mechanisms were examined in primary cultures of adult rat hepatocytes. In serum-free cultivation, SAC (10^-6 M)-stimulated hepatocytes showed significant proliferation compared to control at 5-h culture; the effect was dependent on the culture time and the dose of SAC (EC₅₀ value 8.58 × 10^-8 M). In addition, SAC-stimulated hepatocytes significantly increased mRNA expression levels of c-Myc and c-Fos at 1 h and cyclin B1 at 3.5 and 4 h, respectively. In contrast, alliin and allicin, structural analogs of SAC, did not show these effects observed with SAC. The SAC-induced hepatocyte proliferation effects were completely suppressed by monoclonal antibodies against growth hormone receptor and insulin-like growth factor type-I (IGF-I) receptor, respectively. Furthermore, the Janus kinase 2 (JAK2) inhibitor TG101209, phospholipase C (PLC) inhibitor U-73122, IGF-I receptor tyrosine kinase (RTK) inhibitor AG538, PI3 kinase inhibitor LY294002, MEK inhibitor PD98059, and mTOR inhibitor rapamycin completely suppressed the SAC-induced hepatocyte proliferation. JAK2 (p125 kDa) phosphorylation in cultured hepatocytes peaked 5 min after SAC stimulation. SAC-induced IGF-I RTK (p95 kDa) and ERK2 (p42 kDa) phosphorylation had slower rises than JAK2, peaking at 20 and 30 min, respectively. These results indicate that SAC promoted cell proliferation by growth hormone receptor/JAK2/PLC pathway activation followed by activation of the IGF-I RTK/PI3K/ERK2/mTOR pathway in primary cultures of adult rat hepatocytes.