Liver cell proliferation and incidence of hepatocellular carcinomas in rats fed consecutively a choline-devoid and a choline-supplemented diet.

A group-set of male Fischer 344 rats was kept on a choline-devoid (CD) diet for 3, 6, 9, 12 or 16 months. A second set was fed the same diet for 3, 6 and 9 months, followed by a control, choline-supplemented diet for 3 months. A third set was fed the CD diet for 0, 3, 6, 9 and 12 months and the control diet for the duration (16 months) of the experiment. [3H]Thymidine was injected into some of the animals to assess the extent of liver cell proliferation. In the latter animals, liver triacylglycerols were also determined as an index of the hepatonecrogenic action of a CD diet. Foci of enzyme-altered hepatocytes were detected histochemically and the presence of tumors was established histologically. Cell proliferation and triacylglycerols were both high after 3 months and declined steadily as the length of CD diet feeding increased. Upon subsequent feeding with the control diet, triacylglycerols promptly cleared from the liver, while cell proliferation remained at reduced but still relatively high levels for at least 3 months. Increasing but small numbers of foci of enzyme-altered hepatocytes were detected in some of the rats under experimentation for 6 months or longer. The incidence of hepatocellular carcinomas was 13, 27, 33 and 73% respectively in rats fed the CD diet for 3, 6, 9 or 12 months and the control diet for the duration of the experiment. On the other hand, only 26% of the rats fed exclusively the CD diet for 16 months developed carcinomas. The results were taken as evidence that: (i) liver cell proliferation persists beyond discontinuation of a CD diet; (ii) the liver contains initiated cells--capable of full evolution to cancer even in the absence of active promotion--after a relatively short 3-month exposure to a CD diet; and (iii) occurrence of a late event(s) is critical for the genesis of the tumors. Whether a CD diet is a complete carcinogen able to initiate de novo liver cells or acts merely as a promoter of the evolution of endogenous initiated cells is briefly discussed.