The mechanism of insulin-like growth factor type-I (IGF-I) secretion stimulated by S-allyl-L-cysteine (SAC) was investigated as part of a study of SAC-induced DNA synthesis and cell proliferation in primary cultures of adult rat hepatocytes. When 10-6 M SAC was added to the culture, the amount of IGF-I in the medium was significantly increased at 10 min. The peak IGF-I level (140 pg/mL) was observed 20 min after SAC stimulation. The SAC-induced IGF-I secretion was completely suppressed by a selective Janus kinase 2 (JAK2) inhibitor (TG101209), a selective phospholipase C (PLC) inhibitor (U-73122), an intracellular Ca2+ chelating agent (BAPTA-AM), and a granule secretion inhibitor (somatostatin). On the other hand, 10-6 M SAC-stimulated hepatocytes showed increased intracellular Ca2+ concentration in a time-dependent manner from 0 to 10 min. Phosphorylation of SAC-induced JAK2 and IGF-I receptor tyrosine kinase (RTK) was completely suppressed by TG101209. In addition, U-73122, BAPTA-AM, and somatostatin did not suppress SAC-induced JAK2 phosphorylation, but significantly suppressed SAC-induced IGF-I RTK phosphorylation. Furthermore, binding of the monoclonal antibody against growth hormone (GH) to GH receptor was dose-dependently suppressed by SAC on immunofluorescence. These results showed that SAC promotes cell proliferation by stimulating GH receptor/JAK2/phospholipase C pathways and promoting autocrine secretion of IGF-I in primary cultures of adult rat hepatocytes.
Keywords: Janus kinase 2 (JAK2); S-allyl-L-cysteine (SAC); hepatocyte; insulin-like growth factor type-I (IGF-I); secretion.