Mitogenic activity in platelet-poor plasma from rats with persistent liver nodules or liver cancer

Different effects of regenerative and direct mitogenic stimuli on the growth of initiated cells in the resistant hepatocyte model

The possible mechanism(s) responsible for the different effects exerted by proliferative stimuli of different nature on the appearance of enzyme-altered hepatic foci, were investigated in male Wistar rats. Rats given an initiating dose of diethylnitrosamine (150 mg/kg body weight) were fed a diet containing 0.03% acetylaminofluorene for 2 weeks. Between the first and the second week, cell proliferation was induced by a proliferative stimulus of compensatory type (partial hepatectomy) or by a direct mitogenic stimulus (lead nitrate, 100 mumol/kg). The effect of the two different proliferative stimuli on the appearance of gamma-glutamyl transferase-positive foci was monitored by killing the rats for examination at 1, 2, 3, 5, and 6 days after the induction of cell proliferation. The results indicate that while enzyme-altered hepatocytes can be observed as early as 3 days after partial hepatectomy and are characterized by a rapid growth, direct hyperplasia did not exert any effect on the growth capacity of initiated cells. No effect of lead nitrate-induced hyperplasia was observed following three administrations of the mitogen. When platelet-poor plasma taken from animals exposed to the different proliferative stimuli was tested in primary cultures of hepatocytes, it was found that it induced a significant increase in the labeling index of normal hepatocytes. However, while serum taken 6 days after partial hepatectomy was still able to induce a significant increase in the labeling index, platelet-poor plasma from lead-treated rats had lost part of its effect at 5 days after treatment. The inability of direct hyperplasia to stimulate the development of enzyme-altered hepatic foci was not unique to lead nitrate since the same phenomenon was observed when three other hepatomitogens, nafenopin, cyproterone acetate, and ethylene dibromide, were used.

Hepatocyte growth factor stimulates phosphoinositide hydrolysis and mitogenesis in cultured renal epithelial cells

Hepatocyte growth factor (HGF), a novel heparin-binding peptide growth factor of MW 97-kDa, is a potent mitogen for parenchymal hepatocytes. HGF is present in normal serum and increases following liver injury or partial hepatectomy. In addition to liver, HGF mRNA has been detected in kidney. In cultured rabbit proximal tubule cells, recombinant human HGF (10(-10) M) increased DNA synthesis, measured as [3H] thymidine incorporation, from 1345 +/- 213 to 2931 +/- 636 cpm/10(6) cells; n = 9; p < 0.005). HGF was found to exert mitogenic effects at lower concentrations than epidermal growth factor (EGF), with half maximal effects seen at 6 x 10(-11) M compared to 7 x 10(-10) M for EGF. HGF was additive with EGF in stimulating [3H] thymidine incorporation. In addition to rabbit proximal tubule cells, HGF increased proliferation in a cultured mouse proximal tubule cell line, MCT, and in rat glomerular epithelial cells. In contrast, HGF did not stimulate proliferation of either rat mesangial cells or a rat aortic smooth muscle cell line, A7r5. The HGF receptor is the product of the c-met proto-oncogene. C-met mRNA was detected in total kidney and in cultured proximal tubule cells but was not detected in cultured mesangial cells. In contrast, HGF mRNA was detected in mesangial cells but not in cultured proximal tubule cells. Preincubation of rabbit proximal tubule cells with the tyrosine kinase inhibitor, genistein (50 microM), prevented HGF-stimulation of [3H] thymidine incorporation. In LiCl pretreated rabbit proximal tubule cells loaded with [3H] myoinositol, HGF increased total inositol phosphate release, measured by anion exchange chromatography (control: 2181 +/- 414 vs HGF: 2609 +/- 478 cpm/10(6) cells; n = 6; p < 0.05). Although genistein did not affect baseline phosphoinositide hydrolysis, it inhibited the HGF stimulation. Thus, HGF is mitogenic for cultured proximal tubule cells as well as glomerular epithelial cells. Inhibition of proliferation and PI turnover by genistein suggests that HGF's actions are mediated in part by tyrosine kinase activity. In mammalian kidney, HGF released from mesangial cells may serve as a paracrine activator of the adjacent epithelial cells.

Stimulation of DNA synthesis by rat plasma following in vivo treatment with three liver mitogens

The present study was undertaken to determine the effect of two different types of liver cell proliferative stimuli, namely compensatory regeneration and direct hyperplasia on DNA synthesis of normal and preneoplastic isolated hepatocytes. Platelet-poor plasma (PPP) isolated from male Wistar rats treated with three different hepato-mitogens, lead nitrate (LN), cyproterone acetate (CPA) and ethylene dibromide (EDB), or subjected to surgical partial hepatectomy (PH), was tested for its ability to stimulate DNA synthesis in normal and preneoplastic hepatocytes in primary cultures. Induction of DNA synthesis was detected as early as 30 min after CPA, EDB and PH administration and persisted up to 5 days after the LN administration. In addition, hepatocytes isolated from preneoplastic liver nodules were also able to respond in culture to the DNA synthesis stimulus induced by these factors.

Hepatocyte growth factor (hepatopoietin A) rapidly increases in plasma before DNA synthesis and liver regeneration stimulated by partial hepatectomy and carbon tetrachloride administration

An enzyme-linked immunosorbent assay was used to measure the level of hepatocyte growth factor in rat plasma at various times after two-thirds partial hepatectomy or CCl4 administration. An initial 17-fold rise and 13-fold rise in the level of hepatocyte growth factor was observed 2 hr after partial hepatectomy and CCl4 treatment, respectively, well before the onset of DNA synthesis in the liver. The peaks of DNA synthesis in remnant livers and livers exposed to CCl4 occurred at 24 hr and 48 hr, respectively, as determined by 5-bromo-2'-deoxyuridine labeling and [3H]thymidine uptake by the liver. A later peak level (17-fold above control) of hepatocyte growth factor at 24 hr after CCl4 treatment coincided with strong immunostaining of damaged or necrotic hepatocytes around central veins with an antibody to hepatocyte growth factor. This suggests a later intrahepatic origin of the signals for liver regeneration after hepatotoxic injury subsequent to the early extrahepatic production of hepatocyte growth factor at 2 hr after CCl4 administration. The absence of staining in the liver remnants in partially hepatectomized rats implies that the increase in hepatocyte growth factor seen in the plasma is caused by production at extrahepatic site(s). Possible sources include the pancreas, brain, thyroid and salivary glands, and Brunner's glands of the duodenum. Norepinephrine also increases in plasma as early as 2 hr after hepatectomy. In vitro, [3H]thymidine incorporation into hepatocyte DNA in the presence of hepatocyte growth factor is greater if 10(-5) mol/L norepinephrine is also present in the media.(ABSTRACT TRUNCATED AT 250 WORDS)

Human carcinogens so far identified

The massive exploitation of natural resources, of which tobacco and asbestos are two conspicuous, though very different examples, and the synthesis of industrial chemicals have generated new hazards and new carcinogens which have been added to older ones. The majority of the over 50 agents that have been firmly identified so far as being human carcinogens belong to the relatively new hazards, that is environmental chemicals or chemical mixtures to which humans have been exposed only during the last century and a half. They are of more importance for cancer occurring in men than in women, and there is no evidence so far that they are related to cancers occurring at some of the most common target sites in either sex. It would be mistaken to believe that complete cancer prevention could be achieved solely by controlling these new, or relatively new, carcinogenic agents, but it would be similarly wrong to deny the importance of trying to control them and of continuing to do so. The experimental approach for the identification of carcinogens has an irreplaceable role to play in preventing the dispersal into our environment of new hazards and in identifying among the chemicals already in use, those that are carcinogenic. That a closer integration between the epidemiological and the experimental approaches may succeed in substantially reducing the size of the unknown region within the spectrum of cancer-causing factors, is today's hope that awaits confirmation. At the same time, advances in the understanding of the mechanisms underlying the different steps of the process leading to the clinical manifestation of cancer may help in the uncovering of agents and risk factors that the approaches used, at least in the way they have been used until now, may not have been apt to identify.