Regulation of cell turnover in the livers of tumour-bearing rats: occurrence of apoptosis

A new meroterpenoid functions as an anti-tumor agent in hepatoma cells by downregulating mTOR activation and inhibiting EMT

Liver cancer, also known as primary liver cancer, is cancer that starts in the liver. JNU-144, a new meroterpenoid purified from Lithospermum erythrorhizon, has exhibited promising anticancer activity; however, the molecular mechanisms of action of JNU-144 on malignant cells remain unclear. Our studies revealed that JNU-144 suppressed cell viability and proliferation in hepatoma cells by downregulating mTOR activation. Meanwhile, JNU-144 activated the intrinsic apoptosis pathway and subsequently triggered apoptotic cell death in SMMC-7721 cells. We also found that JNU-144 inhibited the epithelial–mesenchymal transition in both SMMC-7721 and HepG2 cells through reprogramming of epithelial–mesenchymal transition (EMT)-related gene expression or regulating protein instability. These findings indicate that JNU-144 exerts potent anticancer activity in hepatoma cells and may be developed as a potential therapeutic drug.

Bacterial ceramides and sphingophospholipids induce apoptosis of human leukaemic cells

The genus Sphingobacterium, whose members are Gram-negative non-fermentative rods, possesses ceramides and related sphingophospholipids (SPLs) with isoheptadecasphinganine and 2-hydroxy or non-hydroxy isopentadecanoic acid. This paper reports evidence that ceramides isolated from Sphingobacterium spiritivorum ATCC 33861 induce endonucleolytic DNA cleavage in human myeloid leukaemia HL-60 cells in vitro, which is the primary characteristic biochemical marker for apoptosis or programmed cell death. Ceramides and SPLs also induced DNA fragmentation and caspase-3 activation, followed by changes in morphology, such as alterations in the size of nuclei and cells, and cell cycle shortening. Apoptotic activity correlated with the ceramide structure. Ceramide with a 2-hydroxy fatty acid showed stronger apoptotic activity than ceramide with a non-hydroxy fatty acid. Furthermore, the major five SPLs (ceramide phosphorylethanolamine-1 and -2, ceramide phosphorylinositol-1 and -2, and ceramide phosphorylmannose-1) showed apoptosis-inducing activity in HL-60 cells, indicating that the ceramide moiety of the SPLs plays a crucial role as the intracellular second messenger but that their hydrophilicity is less important in this regard. The hydrophilic part of SPLs may play a role in other cellular response systems. The involvement of Fas antigen was implicated in the apoptotic event since Fas antigen expression was observed after 3 or 4 h stimulation of HL-60 cells with bacterial ceramides. However, a time-course study for caspase-3 activation indicated maximal activity at 1 h after stimulation with bacterial ceramides, suggesting that two (or possibly more) mechanisms of signal transduction, Fas-dependent and Fas-independent, may be involved. Fas antigen expression and caspase-3 activation by five kinds of SPLs were observed after 3 or 4 h. These results indicate that there is a difference in the response of HL-60 cells to bacterial ceramides and SPLs.

Kupffer cells promote lead nitrate-induced hepatocyte apoptosis via oxidative stress

BACKGROUND: Apoptosis and its modulation are crucial factors for the maintenance of liver health, allowing hepatocytes to die without provoking a potential harmful inflammatory response through a tightly controlled and regulated process. Since Kupffer cells play a key role in the maintenance of liver function, the aim of this study was to verify whether Kupffer cells are involved in the induction of liver apoptosis after i.v. injection of Pb(NO3)2 likely by secretion mechanisms. RESULTS: The in vivo hepatic apoptosis, induced by Pb(NO3)2 was prevented by a pre-treatment with gadolinium chloride (GdCl3), a Kupffer cells toxicant, that suppresses Kupffer cell activity and reduces to a half the apoptotic rate. In addition, in vivo Pb(NO3)2 administration deprives hepatocytes of reduced glutathione, whereas the loss of this important oxidation-preventing agent is considerably mitigated or abolished by pre-treatment with GdCl3. However, incubation of isolated hepatocytes and Kupffer cells and HepG2 cells with Pb(NO3)2 for 24 hours induced necrotic but not apoptotic cells. Apoptosis of hepatocytes and HepG2 cells was observed only after the addition of conditioned medium obtained from Kupffer cells cultured for 24 hours with Pb(NO3)2, thus indicating the secretion of soluble mediators of apoptosis by Kupffer cells. Apoptosis in the HepG2 cells was observed upon 24-hours incubation of HepG2 cells with 1 mM buthionine sulfoximine, a glutathione depleting agent, thus showing that there is an oxidative apoptogenic pathway in HepG2 cells. CONCLUSION: Pb(NO3)2 has, at most, a direct necrotic (but not apoptogenic) effect on hepatocytes and HepG2 cells, giving a clue about the regulatory role of Kupffer cells in the induction of liver apoptosis after a single Pb(NO3)2 injection without pre-treatment with GdCl3, probably via secreting soluble factors that trigger oxidative stress in target cells.

Phagocytosis of apoptotic cells by liver: a morphological study

The present review deals with the morphological features of the removal of apoptotic cells by liver. The engulfment of cells undergoing apoptosis can be considered a specialized form of phagocytosis, playing a major role in the general tissue homeostasis in physiological and pathological conditions. In fact, defects of phagocytosis of apoptotic cells might have deleterious consequences for neighboring healthy cells, i.e., pathogenesis of inflammatory disease or dysregulation of the immune system. Phagocytosis of apoptotic cells by liver is a complex phenomenon, involving multiple molecular mechanisms of recognition (i.e., lectin-like receptors and receptors for externalized phosphatydilserine) of both parenchymal (hepatocytes) and nonparenchymal (Kupffer and endothelial cells) liver cells, often operating in cooperation. The data discussed in the present review are drawn from studies of phagocytosis of apoptotic cells in the liver, carried out with in vivo and in situ adhesion experiments as well as in vitro assays. Our results indicate that the three main liver cell types (hepatocytes, Kupffer, and endothelial cells) are able to recognize and internalize apoptotic cells by means of specific receptors (galactose and mannose-specific receptor; receptor for phosphatydilserine) and by cytoskeletal reorganization that favors the engulfment of the apoptotic cells. The "flags" for the identification of apoptotic cells by the liver are modifications of the surface of dead cells, i.e., sugar residues and phosphatydilserine exposition. Vitronectin receptor is not involved in such a recognition. The adhesions between modified cell surfaces of apoptotic cells and phagocytes generate cytoplasmatic signaling pathways that drive apoptotic cells to their final fate within the phagocytes (i.e., lysosomal digestion).

Fasting-induced apoptosis in rat liver is blocked by cycloheximide

The effect of cycloheximide (CH) on the fasting-induced changes of rat liver cell and protein turnover has been investigated. Late starvation phase (3-4-day-fasting period) was characterised by a decrease in liver weight and protein and DNA content. The loss of DNA was not related to liver cell necrosis but due not only to depression of cell proliferation as shown by the drop in the labelling index but also induction of apoptosis. This type of apoptosis was documented by the increase in the apoptotic index (cells labelled by TUNEL) and transglutaminase activity as well as by DNA fragmentation. The liver cells of fasted rats appeared smaller as shown by the higher cell density and DNA/protein ratio than in controls. Females were more resistant to fasting-induced apoptosis than males. A single dose of CH, a drug primary known as inhibitor of protein synthesis, induced or enhanced apoptosis in fed and 2-days fasted male rats, respectively, without any sign of cell necrosis. On the contrary, the administration of repeated doses of CH blocked apoptosis induced by fasting. CH "froze" protein and DNA content as well as apoptotic process at the level of 2 days-fasted rats. While fasting-induced liver protein loss resulted from a marked reduction in protein synthesis with a slight decrease in degradation, repeated treatment with CH virtually blocked protein loss by abolishing protein catabolism. These data suggest a direct relationship between the catabolic side of protein turnover and the apoptotic process.

Compensatory apoptosis in preneoplastic liver of a transgenic mouse model for viral hepatocarcinogenesis

The HBx protein of hepatitis B virus has been shown to induce hepatocellular carcinoma in transgenic mice as direct evidence for its involvement in hepatocarcinogenesis. In these transgenic mice, however, it is not clear why hepatocytes do not acquire a neoplastic phenotype by 13 months old despite the continuous growth stimulation by the HBx protein from 2 months old. In this study, we show that the accelerated proliferation of hepatocytes is counterbalanced by apoptosis, which maintains liver homeostasis. A decrease in the extent of apoptosis seems to precede the emergence of neoplasia in the transgenic mouse liver. The disappearance or block of apoptotic signals, which may be the result of additional genetic or epigenetic aberrations, may result in the preneoplastic hepatocytes becoming neoplastic.

Retinoic acid treatment induces apoptosis or expression of a more differentiated phenotype on different fractions of cultured fetal rat hepatocytes

The present study reports the effects of retinoic acid (RA) on cultured fetal rat hepatocytes. We show that RA treatment induces both differentiation and apoptosis. Hepatocytes cultured for 48 hours in the presence of 5 microl/L RA form junctional complexes in the areas of contact between neighboring cells and develop bile canaliculi, typical features of mature and well-differentiated cells. At the same time, about 20% of cells are induced to die by apoptosis, and the percentage of apoptotic cells increases according to the concentration of RA used and the duration of treatment. The induction of apoptosis, studied at the morphological and biochemical levels, revealed that, in our system, the classical compaction of chromatin occurs only during the final stages of the process; instead of the common marker of apoptosis, i.e., the "DNA ladder" pattern of fragmentation, megabase-sized fragments were found. These observations provide further evidence of the existence of fundamental differences in the mechanisms of apoptosis among cell types. To investigate the molecular mechanism of the effects of RA, we evaluated the expression of two proteins, c-myc and p53, which are known to be involved in both cell differentiation and apoptosis. The data obtained show that the amount of p53 remained unchanged after RA treatment. On the contrary, a dose-dependent reduction in c-myc levels was found, suggesting that RA action may be mediated by modulation of this oncogene. Our findings regarding the apoptosis-inducing effect of RA, which was not found in adult hepatocytes, suggest a possible relationship between this phenomenon and the proliferative capacity and/or differentiation state of hepatocytes.

Regression of metastatic liver tumors in rats treated with angiogenesis inhibitor TNP-470: occurrence of apoptosis and necrosis

To clarify the mechanism of the reduction of metastatic liver tumors in rats treated with angiogenesis inhibitor TNP-470, the death of tumor cells was examined pathologically and ultrastructurally. Liver metastases were developed by intravenous injection of AH-130 cells. TNP-470 was given subcutaneously after tumor cell injection. Alterations in the size and number of metastatic tumors were examined at various time points, in association with the analysis of cell death pattern. The metastatic nodules were divided into 4 groups according to the morphological patterns of cell death; no cell death, scattered apoptosis, central necrosis, and diffuse necrosis. The number and size of the metastatic tumors at 2 weeks in untreated rats were larger than those in treated rats. The number of tumors in untreated rats decreased, but the tumor size increased. All rats treated with TNP-470 were alive and free from tumors after 4 weeks, whereas all the untreated rats died of liver metastases. The percentages of the tumors with necrosis in untreated rats (61.2% at 2 weeks and 100% at 4 weeks) were significantly higher than that (31.8% at 2 weeks) in treated rats (P < 0.01). The percentage of the tumors containing apoptotic cells in treated rats was significantly higher than that in untreated rats (54.5% vs. 30.6%; P < 0.05). The growth of metastatic tumors without treatment might be faster than the growth of vessels in untreated tumors, resulting in central necrosis due to ischemia. On the other hand, the reduction of metastatic liver tumors treated with TNP-470 might be caused by inhibition of angiogenesis, providing a weak ischemic stimulus which triggers apoptosis, rather than by a direct cytotoxic effect on tumor cells, because previous in vivo experiments demonstrated that TNP-470 affected endothelial cells but not tumor cells.

The apoptosis protector, bcl-2 protein, is downregulated in bile duct epithelial cells of human liver allografts

BACKGROUND/AIMS

Apoptosis of bile duct cells occurs in hepatic allografts and is correlated with acute rejection. bcl-2 protein counteracts apoptosis and prolongs cell survival. We therefore tested the expression of bcl-2 protein in bile ducts of liver grafts in comparison with those of liver cirrhosis.

METHODS

115 biopsies from 17 liver allografts and 47 biopsies of liver cirrhosis were analyzed and compared with 22 normal controls and with biopsies from patients with primary sclerosing cholangitis or primary biliary cirrhosis. bcl-2 protein and PCNA (proliferating cell nuclear antigen) expression was assessed using immunohistochemistry, and apoptosis was analyzed employing in situ DNA end-labeling.

RESULTS

A high apoptotic rate was detected in bile duct cells of allograft biopsies. In contrast to controls, bile duct cells of allografts and liver cirrhosis had high proliferative activity (mean PCNA labeling index: 1.0% vs. 40.7% and 18.6%, respectively). In liver grafts, bcl-2 protein positivity of bile duct and ductular cells was found in 3.6% and 4.4% of sections, respectively, and in cirrhosis in 44% and 79%, respectively (allografts vs. cirrhosis p<0.01). In controls, only one biopsy was bcl-2 positive.

CONCLUSIONS

Whereas increased proliferative activity of small bile ducts and ductules in cirrhosis is associated with a high degree of bcl-2 expression, bile duct and ductular cells in liver grafts have a very low bcl-2 protein reactivity, even though their proliferative activity is high. These findings suggest that downregulation of bcl-2 expression in allograft bile duct cells might play a role in the increased apoptosis of these cells in acute rejection.

Liver apoptosis

Apoptosis, also called programmed cell death, is a peculiar form of cell death different from cell necrosis in many morphological and biochemical aspects. Like mitosis or differentiation, apoptosis is a normal cell phenomenon which depends on the expression of genes capable of inducing or inhibiting this type of cell destruction. But apoptosis can also be triggered by many external factors and has been described in many diseases. The very different conditions where programmed cell death occurs suggest that the mechanisms leading to the activation of apoptosis-controlling genes are variable. As in other cells, apoptosis occurs in the liver cells, first in the normal state during liver development and then in the adult liver, respectively for liver organogenesis and the renewal of hepatocytes. But apoptosis is also present in various viral, immunological, malignant or drug-induced human liver diseases. In addition, in the animal, hepatocyte apoptosis can be triggered either in vivo or in vitro by many toxic agents. In contrast to other cells, the mechanisms leading to liver cell apoptosis remain poorly investigated. However, two proteins could play an important role in this field, the fas/apo-1 protein present at the surface of hepatocytes and the bcl-2 protein localized in biliary cells. Analysis of the genes controlling the expression of these two proteins could provide essential information on the mechanisms of liver apoptosis.

Sexual dimorphism in the regulation of cell turnover during liver hyperplasia

A sexual dimorphism occurs in liver cell proliferation following partial hepatectomy, female liver regenerating faster than male, while a continuous excess of choline to females shifts their growth pattern toward that of males (L. Tessitore, P. Pani and M.U. Dianzani, Carcinogenesis, 13 (1992) 1929). In this study we have investigated (a) if the same sexual modulation occurs in a different type of liver growth, hyperplasia induced by a direct mitogen and (b) if the pre-administration of choline to females is able to modulate this dimorphism. Liver hyperplasia induced by lead nitrate, a potent mitogen, has also shown a peculiar sexual dimorphism in all phases of the proliferative process. In contrast with liver regeneration after partial hepatectomy, the mitogenic action of lead nitrate was less effective and was delayed in females as compared with males, by evaluating liver weight, protein accumulation, DNA synthesis and mitotic index. These results were also confirmed by the trend of liver regression by apoptosis. The apoptotic index was higher in males than in females. A prolonged administration of an excess of choline has partially filled these sexual differences, since choline has moved, in females, all the observed parameters (liver weight, protein accumulation, DNA synthesis, mitotic and apoptotic indexes) to values closer to those observed in males.

In situ detection of DNA fragmentation after focal cerebral ischemia in mice

Brains from 5 mice subjected to focal ischemia (2 hours)--reperfusion (22 h), revealed a significant increase (P < 0.01) in cells exhibiting DNA fragmentation (100-200 per section)--ipsilateral hemisphere compared to 0-3 per section found in the contralateral hemisphere and normal (n = 5) and sham operated (n = 5) mice. Neurons were the predominant cells (90-95%) exhibiting DNA fragmentation, and were primarily located in the inner boundary zone to the infarct. Apoptosis may contribute to the development of infarct after transient focal cerebral ischemia.

Expression patterns of ornithine decarboxylase and c-met in growing Yoshida AH-130 hepatoma

The expression of the two proto-oncogenes ornithine decarboxylase and c-met was examined during various phases of growth of Yoshida AH-130 ascites hepatoma. Ornithine decarboxylase (ODC) and c-met mRNA levels declined progressively from day 5 (exponential growth-phase) until day 14 (quasi-stationary growth-phase). Transcription rate for both the genes remained constant between days 5 and 10, while decreasing at day 14. ODC activity was consistent with ODC mRNA level during hepatoma growth. In host liver, ODC mRNA accumulated 5 and 14 days after tumor transplantation, while c-met mRNA level was elevated until day 10 and diminished at day 14. ODC activity triplicated at day 14 in host liver. The progressive decline in the expression of ODC and c-met observed in hepatoma might be one of the mechanisms important for the control of tumor growth.

Expression of spermidine/spermine N1-acetyltransferase in growing Yoshida AH-130 hepatoma cells

Activity and messenger RNA levels of spermidine/spermine N1-acetyltransferase, the rate-limiting enzyme of the polyamine interconversion pathway, were investigated in host liver and in Yoshida AH-130 ascites hepatoma cells as a function of tumor growth phases. Enzyme activity reached maximal values at day 10 in host liver (2.0-fold increase) and at days 10 and 14 in hepatoma cells (4.2- and 5.4-fold increases)--that is, when the cellular growth was nearly arrested. At day 10 the messenger RNA levels of spermidine/spermine N1-acetyltransferase were augmented concomitantly; they were about two and four times higher, respectively, in host liver and tumor cells than in control liver. The in vitro transcription rate seemed to be constant during hepatoma cell growth. Treatment of the animals with N1,N2-bis-(2,3-butadienyl)-1,4-butanediamine (MDL 72527), a specific inhibitor of polyamine oxidase, caused large accumulation of N1-acetylspermidine in hepatoma cells and in the ascitic fluid; the maximal values were reached at day 14. The levels of putrescine in inhibitor-treated rats decreased in hepatoma cells (day 5) and in ascitic fluid (days 5 and 14), whereas values of spermidine and spermine remained unchanged. The proposed role for spermidine/spermine N1-acetyltransferase-enhanced expression is to regulate the cellular polyamine pool by causing their excretion as acetylderivatives from tumor cells into the ascitic fluid, even if putrescine seems also to be excreted. Eventual repeat uptake of putrescine by hepatoma cells could contribute to the control of cellular polyamine levels.

Programmed cell death: concept, mechanism and control

Programmed cell death or apoptosis occurs under physiological conditions as a result of physiological effectors. It is a relatively slower process and requires active participation of the cell in the suicidal mechanism. Apoptosis is controlled by precise intrinsic genetic programme and may be induced by almost all those stimuli causing necrosis. The role played by the intensity in determining the death process and the underlying mechanism is imperfectly understood. Morphologically apoptotic cells appear as small condensed body. The chromatin is dense and fragmented, packed into compact membrane-bound bodies together with randomly distributed cell organelles. The plasma membrane loses its characteristic architecture and shows extensive blebbing. It buds off projections so that the whole cell may split into several membrane-bound apoptotic bodies. Significant chemical changes take place in the plasma membrane. This helps in recognition of the apoptotic bodies by phagocytes. At this moment it is unclear if all cells can undergo apoptosis or it is a characteristic of only some tissues which are predisposed to apoptotic death being directly under the control of hormones or growth factors. Experimental studies aimed at comparison of induction of apoptosis in cells of different origin are warranted to elucidate this point. Biochemically a pre-commitment step for induction of death programmation through macromolecular synthesis is essential for most systems. The double-stranded linker DNA between nucleosomes is cleaved at regular inter-nucleosomal sites through the action of a Ca2+, Mg(2+)-sensitive neutral endonuclease. Zinc is a potent inhibitor of the enzyme. Calcium probably plays a key controlling role in activation of the enzyme since prevention of Ca2+ increase prevents endonuclease activation. It is becoming evident that signal transduction through appropriate receptors control the Ca2+ flux in the cells. Most apoptotic cells require synthesis of RNA and proteins. Delay or abrogation of apoptosis by inhibition of macromolecular synthesis is well known. The dying cells show high mRNA levels for several enzymes. Several degradative enzymes become active. Regulatory proteins maintain control over the apoptotic cascade. At the molecular level, search has been initiated for the mammalian equivalents of the cell death (ced) gene. Activation of several specific genes is indicated. Specific expression of cell death-associated gene products (e.g. TRPM-2/SGP-2) has been reported in several unrelated apoptotic cell systems. Sequential induction of c-fos, c-myc and 70 kDa heat shock protein is reported. Studies demonstrate that certain genes must remain in a transcriptionally active demethylated state during programmed cell death. Recent evidences clearly indicate that apoptosis may be positively or negatively modulated by certain genes.(ABSTRACT TRUNCATED AT 400 WORDS)

Reversibility of carcinogen-induced rat forestomach basal cell hyperplasia is due to squamous cell differentiation

The mechanisms of reversibility of basal cell hyperplasia in the rat forestomach were investigated. Male F344 rats were given an initial single gastric intubation of N-methyl-N'-nitro-N-nitorosoguanidine and then received 2% butylated hydroxyanisole in the diet from the third week to the 26th week. Rats were killed at weeks 26 and 46 after return to basal diet and their forestomachs were removed. Bromouracil deoxyriboside (BUdR) was administered as a single i.p. injection 1 h before death or by osmotic minipump (120 micrograms/h) continuously for 7 days before death. Additional animals were maintained for 2 or 4 weeks after removal of osmotic minipumps to allow assessment of the fate of proliferating populations. In each case BUdR-labeled cells were demonstrated by immunohistochemistry by immunohistochemistry. At week 26, hyperplastic changes were more pronounced than at week 46. Squamous cells above basal cell hyperplasias were strongly labeled even 4 weeks after cessation of continuous BUdR Three-dimensional reconstruction of persisting basal cell hyperplasias showed almost all basal cells limited to a thin sheet in direct contact with the squamous cell layer, occasional separate islands demonstrating differentiation to squamous cells and formation of epidermal cysts. The results thus showed that the mechanism of reversibility of basal cell hyperplasia involves differentiation of basal cells to squamous cells.

Decreased protein kinase C activity is associated with programmed cell death (apoptosis) in freshly isolated rat hepatocytes

Apoptosis of freshly isolated rat hepatocytes was induced by either the omission of fetal bovine serum in the culture medium or addition of the protein kinase C inhibitors polymyxin B or staurosporine. The time-course of DNA breakdown into oligonucleosome-sized fragments and the activity of protein kinase C was determined. Hepatocytes were found to be sensitive to bleomycin which induced a high degree of DNA breakdown even within 30 min incubation. Both staurosporine and polymyxin B induced DNA degradation in hepatocytes after three hours incubation, an effect that was partially prevented by phorbol myristate acetate (PMA). After eight hours incubation, PMA failed to counteract this action and itself produced the apoptosis of rat hepatocytes. The results suggest the involvement of protein kinase C in hepatocyte survival.

DNA strand breaks and death of thymocytes induced by N-methyl-N-nitrosourea

N-Methyl-N-nitrosourea (MNU) is a potent carcinogen in various sites of experimental animals and induces thymic lymphoma in rats, which has long been hard to induce by any carcinogen. To analyze the action of MNU on thymocytes, DNA strand breaking in thymocytes from the MNU-treated rat and that in MNU-treated cultured thymocytes were assayed. Fluorometric analysis of DNA unwinding (FADU assay), first reported by Birnboim and Jevcak to detect X-ray-induced DNA damage, was modified and applied to detect DNA damage in thymocytes treated with MNU in vitro or in vivo. In the present modified method, cell lysate was admixed with 0.15 M sodium hydroxide, and DNA unwinding was processed at pH 12.0 for up to 2 h at 0 degree C in iced water. Double-stranded DNA remaining after alkaline reaction was detected by binding ethidium bromide and measuring its fluorescence. The severity of DNA damage, both in vivo and in vitro, depended on the MNU concentration. In addition, the sequential survival rate and cell-size distribution of thymocytes treated with MNU in vitro were investigated. A close relationship between the severity of DNA damage and cell death was demonstrated in MNU-treated thymocytes, and DNA damage by a non-cell-killing dose of MNU was detected with this FADU assay. MNU-induced cell death is not programmed as in apoptosis, which is caused in thymocytes physiologically, immunologically and by X-ray irradiation or corticoids.