Tumor Necrosis Factor Primes Hepatocytes for Dna Replication in the Rat

Abstract

Signaling through tumor necrosis factor receptor type 1 (TNFR-1) using a pathway that involves nuclear factor B (NF-B), interleukin-6 (IL-6), and STAT3 is required for the initiation of liver regeneration. We have proposed that TNF primes hepatocytes to respond to the mitogenic effect of growth factors, but so far, there has been no experimental demonstration that TNF enhances growth factor responses of hepatocytes. To test this hypothesis, we infused hepatocyte growth factor (HGF) and transforming growth factor ␣ (TGF-␣) (40 g/24 h) directly into the portal vein of rats for 24 hours using osmotic pumps and determined whether TNF injection (5 g per rat) would significantly increase hepatocyte DNA labeling in these animals. All rats received 5-bromo-2Ј-deoxyuridine (BrdU) by intraperitoneal delivery during a 48-hour period (i.e., BrdU infusion continued for 24 hours after the end of growth factor administration). Although highly differentiated and normally quiescent, hepatocytes retain an impressive potential for proliferation. This is well demonstrated in vivo by the regeneration of the liver following the removal of tissue by partial hepatectomy (PH) or in response to hepatocyte destruction by toxic agents. 1-3 Hepatocyte proliferation during liver regeneration is mediated by several growth factors, of which hepatocyte growth factor (HGF) and transforming growth factor ␣ (TGF-␣) (a member of the epidermal growth factor [EGF] family) appear to be most important. These growth factors are potent mitogens for hepatocytes in culture, each capable of stimulating hepatocyte DNA synthesis by fivefold or more. [1][2][3][4][5][6][7] Direct infusion of these growth factors into the liver of rats does not, however, cause a major wave of DNA synthesis. 8,9 Although it is possible to obtain hepatocyte replication after infusion of HGF into intact rats, quantitatively significant effects are only achieved by prolonged infusion in the presence of dextran or by multiple injections. 10,11 We have shown previously that a 24-hour infusion of up to 80 µg of HGF, TGF-␣, or EGF into rat livers elicited DNA synthesis in no more than 1% to 2% of hepatocytes at the end of the infusion period. 8 We also demonstrated that a much higher mitogenic response could be obtained by infusion of HGF in rats in which a 30% hepatectomy had been performed immediately before the infusion. 8 In contrast to the standard 70% hepatectomy, removal of 30% of the liver 12 by itself did not cause a wave of DNA synthesis under the conditions of the experiment. 8 Based on these data, we proposed that hepatocytes in vivo have low sensitivity to growth factors and that to effectively respond to these factors, hepatocytes must undergo a set of changes referred to as ''priming. '' 13 Other work has shown that a priming state can be obtained in normal rat liver hepatocytes by nutritional modification 14 or by perfusing livers with small amounts of collagenase. 15

Angiogenesis in chronic inflammatory liver disease

“…During the priming phase of liver regeneration in mice, several protooncogenes, as well as a number of transcription factors involved in the elicitation of inflammation and angiogenesis (including hypoxia-inducible factor-1␣), were also specifically upregulated. 30,32,49,[57][58][59][60] …”

A Fresh Look At Augmenter of Liver Regeneration in Rats

“…7 The assay also has been used increasingly to study other hepatic growth factors whose in vivo role in regeneration has been largely extrapolated from results with in vitro models. [8][9][10][11][12] The principal limitation of this assay is the variability of the mitotic response to the priming hepatectomy, and the additional variability of the mitosis augmentation. 7,8 The far more sensitive canine Eck fistula assay that ultimately guided the steps in purification of HSS 8 also is based on the priming principle, because portacaval shunt causes a tripling of hepatic cell renewal.…”

“…29,30 Cloned mouse and human ALR genes 30 that have highly conserved nucleotide and predicted amino acid sequences have been mapped to allele-rich regions of mouse chromosome 17 and human chromosome 16. 30 Meanwhile, the six additional molecules annotated in Table 1 [6][7][8][9]12,14,15, also had been shown to have primingdependent hepatotrophic qualities comparable with insulin and ALR. Unlike insulin and ALR, two of these molecules (hepatocyte growth factor [HGF] and transforming growth factor [TGF-␣]) initiate mitosis of cultured hepatocytes.…”

“…Like all the others, however, they are only feebly mitogenic when injected into unaltered animals. 9,12 The small number of molecules with genuine hepatotrophic qualities (Table 1) as established by the criteria of the Eck fistula, minimum hepatectomy, or both assays, contrasts with the large number of physical and chemical factors that can initiate mitosis of hepatocytes in vitro or in vivo. 10,11,[46][47][48][49] Epidermal growth factor does not fulfill hepatotrophic criteria even though it is mitogenic for cultured hepatocytes, has a 30% homology with the hepatotrophic TGF-␣, and has the same receptor.…”

Tumor necrosis factor α down-regulates expression of the α1(I) collagen gene in rat hepatic stellate cells through a p20C/EBPβ- and C/EBPδ-dependent mechanism

“…Taken together, these data suggest that the ؊378 to ؊340 region of the col1a1 promoter is the site of convergence of different stimuli that ultimately modulate Tumor necrosis factor ␣ (TNF-␣) is a pleiotropic cytokine that induces apoptosis and generates important signals that, together with interleukin-6 (IL-6), prime hepatocytes to respond to growth factors that induce their proliferation following partial hepatectomy. [1][2][3][4] TNF-␣ and IL-6 are also the main cytokines produced during acute, nonspecific inflammatory stimuli, and are responsible for many manifestations of the acute phase response, including the production of acute phase proteins by hepatocytes and the expression of type I collagen by hepatic stellate cells (HSC). [5][6][7][8] However, while IL-6 up-regulates the expression of type I collagen in vivo 9 and in cultured HSC, 10 TNF-␣ has the opposite effect and down-regulates the expression of collagen both in vitro and in vivo.…”