Amino Acids Regulate Hepatocyte Proliferation through Modulation of Cyclin D1 Expression

Abstract

The mechanisms by which amino acids regulate the cell cycle are not well characterized. In this study, we examined the control of hepatocyte proliferation by amino acids and protein intake. In short-term culture, hepatocytes demonstrated normal entry into S phase and cell cycle protein expression in the absence of essential amino acids. However, deprivation of a set of nonessential amino acids (NEAA) potently inhibited cell cycle progression and selectively down-regulated the expression of proliferation-control proteins. Notably, NEAA withdrawal after the mitogen restriction point still inhibited entry into S phase, suggesting that these amino acids regulate a distinct checkpoint. Cyclin D1, an important mediator of hepatocyte proliferation, was markedly inhibited at the transcriptional level by NEAA deprivation, and transfection with cyclin D1 (but not cyclin E) overcame the cell cycle arrest. As previously shown, protein-deprived mice demonstrated impaired hepatocyte proliferation in vivo after 70% partial hepatectomy. The expression of cyclin D1 and downstream cell cycle proteins after partial hepatectomy was inhibited in these mice. Transfection with cyclin D1 in vivo triggered hepatocyte DNA synthesis and the expression of S phase proteins in the absence of dietary protein. Cyclin D1 also induced global protein synthesis in NEAA-deprived hepatocytes and promoted liver growth in vivo in the setting of protein deprivation. These results indicate that cyclin D1 is a key target of amino acid signaling in hepatocytes.In addition to providing the substrates for protein synthesis, amino acids serve as regulatory molecules that modulate numerous cellular functions (1). It has long been recognized that the availability of amino acids controls growth and proliferation in cell culture systems, but the mechanisms have not been well defined (2, 3). A substantial body of literature indicates that amino acids regulate overall protein synthesis, an essential component of growth and proliferation, through several pathways that control the translational apparatus (4, 5). However, the downstream elements that link amino acids to the cell cycle remain to be determined. In yeast systems, nutrient insufficiency inhibits the expression of G 1 cyclins, which are required for normal entry into S phase (6 -9). However, a similar mechanism has not been clearly established in mammalian cells.In the normal adult liver, hepatocytes rarely replicate, but these cells rapidly enter the cell cycle following injuries that reduce functional liver mass (10). Hepatocyte proliferation is an important component of the adaptive response to liver diseases. In the best-studied model of hepatocyte proliferation in vivo, that of 70% partial hepatectomy (PH) 1 in rodents, most of the remaining hepatocytes enter the cell cycle in a relatively synchronous manner, and liver mass is restored within 1-2 weeks. In addition, primary hepatocytes in short-term culture proliferate readily in response to appropriate mitogens. Thus the hepatocyte culture and PH mo...

Interleukin 22 (IL-22) plays a protective role in T cell-mediated murine hepatitis: IL-22 is a survival factor for hepatocytes via STAT3 activation

“…It has been shown that overexpression of cyclin D1 promotes hepatocyte replication and growth in vitro 35 and in vivo, 36 and overcomes rapamycin-or protein deprivation-mediated suppression of hepatocyte proliferation. 37,38 The mitogenic effect of c-myc has been well documented in a wide variety of cell types, including hepatocytes. 39,40 Collectively, induction of both cyclin D1 and c-myc may play an important role in IL-22 stimulation of hepatocyte proliferation.…”

Rapamycin-sensitive induction of eukaryotic initiation factor 4F in regenerating mouse liver

“…11 We have recently found that down-regulation of cyclin D1 plays an apparently important role in the cell cycle arrest produced by amino acid deficiency or rapamycin treatment in models of hepatocyte proliferation. 17,38 These studies suggest that perturbations of the ribosome or translation initiation apparatus significantly regulate cell cycle progression in the liver and that further investigation of the protein synthetic machinery may provide insight into mechanisms of altered liver regeneration.…”

Cell cycle regulation and hepatocarcinogenesis

“…Numerous studies have demonstrated that cell cycle progression through the G 1 /S transition is dependent on induction of Cyclin D. Overexpression of cyclin D1 in the liver is sufficient to induce hepatocyte replication 62 and in the regenerating liver, the importance of cyclin D1 as an integrator of growth, cytokine and metabolic signals that signal the hepatocyte to progress through the G 1 /S restriction point has been highlighted as a result of studies using genetically engineered mice and pharmacologic inhibitors. [62][63][64][65][66] Multiple signaling pathways activate Cyclin D1 after partial hepatectomy, including JNK activation which has been linked to HGF and TNF-α signaling pathways, target of rapamycin (TOR), IL-6 and CREM. [67][68][69][70]64,71,72 Cyclin D1 is regulated at the transcriptional level by JNK 70 and posttranscriptional level by TOR.…”