Transforming growth factor beta mRNA increases during liver regeneration: a possible paracrine mechanism of growth regulation.

Abstract

Transforming growth factor a (TGF-,) is a growth factor with multiple biological properties including stimulation and inhibition of cell proliferation. To determine whether TGF-P is involved in hepatocyte growth responses in vivo, we measured the levels of TGF-8 mRNA in normal liver and during liver regeneration after partial hepatectomy in rats. TGF-8 mRNA increases in the regenerating liver and reaches a peak (about 8 times higher than basal levels) after the major wave of hepatocyte cell division and mitosis have taken place and after the peak expression of the ras protooncogenes. Although hepatocytes from normal and regenerating liver respond to TGF-fi, they do not synthesize TGF-0 mRNA.Instead, the message is present in liver nonparenchymal cells and is particularly abundant in cell fractions enriched for endothelial cells. TGF-8 inhibits epidermal growth factorinduced DNA synthesis in vitro in hepatocytes from normal or regenerating liver, although the dose-response curves vary according to the culture medium used. We conclude that TGF-P may function as the effector of an inhibitory paracrine loop that is activated during liver regeneration, perhaps to prevent uncontrolled hepatocyte proliferation.The regenerating rat liver is an ideal model system in which to study the mechanisms that control cell proliferation. Under normal physiological conditions, adult rat hepatocytes rarely divide. However, in response to tissue injury or surgical removal of portions of the liver (partial hepatectomy), mature hepatocytes undergo a partially synchronous wave of DNA replication followed by cell division. After partial hepatectomy in rats, the major wave of DNA synthesis starts -14 hr after the operation and reaches a peak at 24 hr. The mass of the liver remnant doubles in the first 36 hr of the growth process, and within a week the normal liver mass is fully restored and the quiescent state reestablished (1, 2). The factors that control this precisely regulated growth process are not known but are often assumed to involve the turning on and off of a positive stimulus for cell proliferation. More likely, however, is the possibility that this tight regulation requires an interplay between growth-stimulatory factors, operative in the early prereplicative stage of the regenerative process, and inhibitory factors, which may be important in later stages as cell division ceases. We have suggested that when quiescent hepatocytes enter the cell cycle, progression to DNA synthesis is controlled by events specifically occurring in the liver-that is, by autocrine or paracrine secretion of growth-stimulatory and -inhibitory factors by hepatic cells (3, 4).It has been shown by several laboratories that transforming growth factor /3 (TGF-f3) is a potent inhibitor of hepatocyte proliferation in vitro (5-7). Under serum-free conditions, as little as 0.1 ng of TGF-,3 per ml (4 pM) causes 85-90% inhibition of epidermal growth factor (EGF)-induced DNA synthesis of hepatocytes in primary culture. TGF-/3 is a 25-kDa, dimeric peptide...

Cloning and characterization of a mouse liver-specific gene mfrep-1, upregulated in liver regeneration

“…This is consistent with the fact that c/ebp α is related with the differentiated state, and in some cells c/ebp α can arrest the proliferative phase [23], [24]. Moreover, the expression of some growth-inhibitory genes, or example, tgf-β and activin a, are also up-regulated and kept high during this period in liver regeneration [25][26][27][28][29]. Here we demonstrated that mfrep-1 maintained at more than 2.5-fold the control level after the wave of DNA synthesis.…”

Screening for candidate hepatic growth factors by selective portal infusion after canine Eck's fistula

“…Against the background of heightened cell renewal, TGF-~, a powerful inhibitor of hepatocyte proliferation in vitro (51,52), was shown in our current experiments to inhibit ongoing proliferation in addition to reducing further the size of the already atrophic hepatocytes. The other test substances that were without activity in our Eck's fistula experiments but which have been reported to be inhibitory in tissue culture systems were tamoxifen (28), IL-l and IL-2 (53).…”

Norepinephrine reverses the effects of activin A on DNA synthesis and apoptosis in cultured rat hepatocytes

“…were recovered by centrifugation at 13,0001 g for 20 minutes, MATERIALS AND METHODS resuspended in TE buffer (10 mmol/L Tris/HCl [pH 8.0] and 1 mmol/L EDTA) for 1 hour at 65ЊC and then incubated with Culture of Hepatocytes. Six-week-old male Wistar rats (about 170 to 200 g) were used for all the experiments.…”