Studies with Isolated Liver Cells in Culture

While much has been learned from whole-animal studies about the regulation of RBP metabolism, the interpretation of these results is sometimes clouded owing to the interplay between organs. To avoid this problem, studies have been conducted in vitro with isolated liver cells in culture. 

Two lines of differentiated rat hepatoma cells were found that synthesized RBP during culture in vitro (Smith etal., 1978). When the cells were incubated in a vitamin A-free serumless medium, a relatively large proportion of the synthesized RBP was retained within the cells. Addition of retinol to the medium (at levels of 0.1 or 1 p.g/ml) stimulated the release of RBP. In contrast, retinol had no effect on either the synthesis or secretion of rat semm albumin or TTR by these cells. 

Because these cell lines respond to retinol depletion and repletion in a manner similar to the intact rat liver cell in vivo, they should provide a good model to study other hormonal and metabolic factors that may be involved in the regulation of RBP synthesis and secretion. In addition, preliminary experiments have been carried out with primary rat hepatocytes, which have been found to synthesize RBP for several days in culture in vitro (unpublished studies from the author s laboratory). The primary hepatocyte may be useful for studying regulatory factors that cannot be studied with the hepatoma cells. An important general question which these systems may help to address is What are the molecular signals from peripheral tissues that normally stimulate or depress the synthesis and secretion of RBP  

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The synthesis and secretion of RBP has been observed and studied with isolated liver cells in culture in vitro. Detailed studies have been carried out with two differentiated rat hepatoma cell lines (Smith et al., 1978). The production of RBP by primary rat hepatocytes in culture in vitro has also been observed (unpublished studies from the author s laboratory). Finally, the specific in vitro synthesis of rat RBP using isolated messenger RNA from rat liver and the rabbit reticulocyte in vitro protein-synthesizing system has been demonstrated (Soprano et al., 1981). Taken together, these various studies clearly establish the liver, and specifically the hepatic parenchymal cell, as the locus of RBP synthesis. The molecular and cellular mechanisms that are involved in the processes of RBP synthesis and secretion by the liver and the factors that regulate these processes are discussed in detail below. While it is clear that the liver is the major site of RBP synthesis, there is no evidence as to whether or not the liver is the only site of RBP synthesis in the body. 

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