In vitro stimulation of cell growth by hepatic stimulator substance

An extract of normal weanling or adult regenerating rat liver called hepatic stimulator substance (HSS) stimulates incorporation of tritiated thymidine ([3H]dThd) into liver DNA. In vivo, it is organ specific, species nonspecific, displays diurnal rhythm, and has a molecular weight of approximately 10,000-20,000. It has now been further characterized in vitro. HSS retained organ specificity, stimulating normal adult hepatocytes in primary culture, the HTC, PLC, and MH1C1 hepatoma cell lines, and the PRC rat liver cell line. It did not stimulate human or mouse lymphocytes, 3T3 mouse fibroblasts, Y1BS1 mouse adrenal cells, Chinese hamster ovary cells, rat hypernephroma cells, C6 rat glial cells, or L1210 mouse leukemic cells. More detailed studies of the response of HTC cells to HSS revealed a dose-dependent increase in [3H]dThd incorporation (up to 30-fold) with a response to as little as 140-ng purified extract/ml culture medium. Autoradiography confirmed that the thymidine was taken up by nuclear DNA. This represented a true stimulation of growth as shown by an almost twofold increase in cell number after 6 days of exposure to low doses of HSS.