Ornithine decarboxylase as an early indicator of in vitro hepatocyte DNA synthesis

The enzyme ornithine decarboxylase, one of the key enzymes involved in polyamine biosynthesis, catalyzes the decarboxylation of ornithine to give putrescine. The activity of this enzyme in an in vitro hepatocyte culture assay system was measured because it is known that ornithine decarboxylase levels increase in instances where active protein synthesis, DNA synthesis, and cell growth is initiated. A good correlation was found between ornithine decarboxylase activity and the rate of tritiated thymidine incorporation into hepatocyte DNA. The increase in enzyme activity precedes the incorporation of tritiated thymidine into DNA (enzyme activity increases 2-3 hr following stimulation of cell growth; whereas the tritiated thymidine uptake increases at about 14-18 hr). Experimental results obtained with this assay system, suggest that hepatocytes from the regenerating liver remnant, grown in vitro, secrete a factor(s) into the culture medium which stimulates DNA synthesis of normal hepatocytes. Use of the increase in ornithine decarboxylase activity in this hepatocyte monolayer culture system confirmed the observation made by several investigators: that the serum of rats which underwent partial hepatectomy contains a factor(s) which stimulates hepatocyte DNA synthesis in vitro. In conclusion, these results suggest that ornithine decarboxylase activity can be used as a sensitive, early indicator of the degree of stimulation of hepatocyte DNA synthesis and thus be of use in determining the effect of various trophic factors on hepatocyte DNA synthesis in vitro.