The untransformed stable cell line Met murine hepatocytes (MMH), generated from liver explants of transgenic mice expressing a constitutively active truncated form of the human hepatocyte growth factor receptor (cyto-Met), represents an innovative tool for in vitro studies of liver function. In the present report, we show that the MMH-D3 line isolated from the liver of a 3-day-old mouse is a useful model to investigate the regulation of the synthesis and secretion of retinol-binding protein (RBP) by retinol (vitamin A alcohol). Experiments with Northern blot hybridization, metabolic labeling of cellular proteins followed by immunoprecipitation, and Western blot analysis demonstrated that, similarly to the in vivo situation, in MMH-D3 cells the presence of retinol does not affect transcriptional and translational rate of the REP gene but is essential for regulating the secretion rate of the protein. Unlike HepG2 human hepatocarcinoma cells used thus far in studies of retinoid metabolism, including the synthesis and secretion of REP, vitamin A deficiency causes, in MMH-D3 cells, the inhibition of REP secretion and the protein accumulation in the cell, whereas retinol repletion promptly results in REP secretion. This model will be very useful in future studies on vitamin A distribution in the organism
