Effect of aging on EGF-induced proliferative response in primary cultured periportal and perivenous hepatocytes.

BACKGROUND/AIMS: Aging relates to declined proliferative capacity of the liver, but the molecular mechanism is not well understood. We examined whether functional changes of epidermal growth factor (EGF) receptor (EGFR) are involved in age-related decline in EGF-induced DNA synthesis using hepatocytes isolated in periportal and perivenous regions of the liver, which differ in the proliferative capacity. METHODS: Periportal hepatocytes (PPH) and perivenous hepatocytes (PVH) in 7-, 30-, and 90-week-old rats were isolated using the digitonin/collagenase perfusion technique. DNA synthesis was assessed by [methyl-(3)H]thymidine incorporation. EGFR binding affinity to EGF was analyzed by Scatchard analysis using [(125)I]EGF. EGFR dimerization and phosphorylation were determined by Western blot analysis. RESULTS: EGF-induced DNA synthesis was greater in PPH than in PVH from rats of 7 weeks, but the zonal difference disappeared with aging. [(125)I]EGF binding studies indicated that high-affinity EGFR in both subpopulations also disappeared with aging. Furthermore, EGF-induced dimerization in both subpopulations was down-regulated with aging, and the pattern of EGFR phosphorylation was parallel to that of dimerization. CONCLUSIONS: These data suggest that age-related decline in EGF-induced DNA synthesis of PPH and PVH is caused by down-regulation of EGFR dimerization through the decrease of high-affinity EGFR.

Effect of retinoic acid on transglutaminase and ornithine decarboxylase activities during liver regeneration.

Liver regeneration is regulated by several factors, including growth factors, cytokines, and post-translational modifications of several proteins. It is suggested that transglutaminase 2 (TG2) and ornithine decarboxylase (ODC) are involved in liver regeneration. To investigate the role of TG2 and ODC activities in regenerating liver, we used retinoic acid (RA), an inducer of TG2 and a suppressor of ODC. Regenerating rat liver was prepared by 70% partial hepatectomy (PH). Rats were sacrificed at 1, 2, 3, 4, and 6 days after surgery. RA was intraperitoneally injected immediately after PH. TG2 and ODC activities and products (epsilon-(gamma-glutamyl) lysine isopeptide (Gln-Lys) and polyamines, respectively) were examined at the indicated times. In RA-treated rat, DNA synthesis and ODC activity declined and the peak shifted to 2 days after PH, whereas TG2 activity increased at 1 day after PH. At that time, protein-polyamine, especially the protein-spermidine (SPD) bond, transiently decreased, whereas the formation of the Gln-Lys bond increased after PH. These results suggested that in regenerating liver, enhanced the formation of Gln-Lys bonds catalyzed by TG2 led to reduced DNA synthesis, whereas when ODC produced newly synthesized SPD, the inhibition of Gln-Lys bond production by the preferential formation of protein-SPD bonds led to an increase in DNA synthesis.

Symbioimine and neosymbioimine, amphoteric iminium metabolites from the symbiotic marine dinoflagellate Symbiodinium sp.

Two amphoteric iminium metabolites, symbioimine (1) and neosymbioimine (2), were isolated from a cultivated symbiotic marine dinoflagellate Symbiodinium sp. Compounds 1 and 2 have a characteristic 6,6,6-tricyclic iminium ring structure and an aryl sulfate moiety. The plausible biogenetic pathway of 1 and 2 can be explained by an intramolecular Diels-Alder reaction followed by imine cyclization. Symbioimine (1) inhibited the differentiation of RAW264 cells into osteoclasts (EC50 = 44 microM), and significantly inhibited cyclooxygenase-2 activity at 10 microM. Thus, symbioimine is a potent anti-resorptive and anti-inflammatory drug.