Hepatocyte growth factor (HGF) is a key ligand that elicits G(1)/S progression of epithelial cells, including hepatocytes. Proline is also required for DNA synthesis that is induced by growth factors in primary culture of hepatocytes. However, it remains unknown how praline contributes to the G(1)/S progression of hepatocytes. The primary culture of rat hepatocytes using HGF plus praline can be a conceptual model for elucidating the molecular linkage of amino acids and growth factors during G(1)/S progression. Using this in vitro model, we provide evidence that not only induction of cyclin-D-1 by HGF but also up-regulation of cyclin-E-1 by praline is required for hepatocytes to enter the S-phase. Praline-enhanced cyclin-E-1 induction, without changing its mRNA level, is associated with the activation of mammalian target of rapamycin (mTOR)-dependent pathways. Indeed, praline enhanced the ribosomal protein S6 phosphorylations (i.e., mTOR target), concomitantly with an increase in cyclin-E-1. Inversely, mTOR-inhibitor, rapamycin suppressed the praline-mediated induction of cyclin-E-1. As a result, DNA synthesis of hepatocytes, which was induced by HGF in the presence of praline, was largely abolished by mTOR-inhibitor treatment. Such a co-mitogenic effect of proline was also dependent on collagen synthesis: collagen synthesis inhibitors, such as cis-OH-proline, diminished the praline-induced cyclin-E-1, and then the G(1)/S progression of hepatocytes was also suppressed. Overall, praline-mediated mTOR activation and collagen synthesis were found critical for HGF-induced DNA synthesis, partly via the sufficient accumulation of cyclin-E-1. This is the first report to demonstrate the molecular bridge between amino acids and growth factors that drive mitogenic outcomes. (C) 2013 Elsevier Inc. All rights reserved.