Studies on the mitoinhibitory effect of orotic acid on hepatocytes in primary culture.

Orotic acid (OA), a promoter of liver carcinogenesis, inhibited proliferation of primary hepatocytes in culture as monitored by labelling index, mitotic index and total DNA content. The mitoinhibitory effect of OA was seen even in the presence of a strong mitogen such as epidermal growth factor (EGF). The growth inhibitory effect of OA was not due to cell killing. Upon exposure to OA the hepatocytes exhibited an increase in the ratio of uridine nucleotides to adenosine nucleotides, and as this ratio increased the response of hepatocytes to proliferate in the presence or absence of EGF decreased. Washing the hepatocytes free of added OA resulted in a gradual decrease in the ratio of uridine nucleotides to adenosine nucleotides, paralleled by an increase in hepatocytic proliferation. Adenine, an agent that inhibits the metabolism of OA to uridine nucleotides, not only inhibited the increase in the ratio of uridine nucleotides to adenosine nucleotides but also counteracted the OA-induced mitoinhibitory effect. These results, together with our earlier observations, suggest that an imbalance in nucleotide pools composed of an increase in uridine nucleotides and a decrease in adenosine nucleotides appears to be important for OA-induced mitoinhibition.