Apolipoprotein A-IV mRNA overexpression in early preneoplastic hepatic foci induced by low-number pancreatic islet transplants in streptozotocin-diabetic rats

Very Low Density Lipoprotein Assembly Is Required for cAMP-responsive Element-binding Protein H Processing and Hepatic Apolipoprotein A-IV Expression

Hepatic apolipoprotein A-IV (apoA-IV) expression is correlated with hepatic triglyceride (TG) content in mouse models of chronic hepatosteatosis, and steatosis-induced hepatic apoA-IV gene expression is regulated by nuclear transcription factor cAMP-responsive element-binding protein H (CREBH) processing. To define what aspects of TG homeostasis regulate hepatic CREBH processing and apoA-IV gene expression, several mouse models of attenuated VLDL particle assembly were subjected to acute hepatosteatosis induced by an overnight fast or short term ketogenic diet feeding. Compared with chow-fed C57BL/6 mice, fasted or ketogenic diet-fed mice displayed increased hepatic TG content, which was highly correlated (r² = 0.95) with apoA-IV gene expression, and secretion of larger, TG-enriched VLDL, despite a lower rate of TG secretion and a similar or reduced rate of apoB100 secretion. When VLDL particle assembly and secretion was inhibited by hepatic shRNA-induced apoB silencing or genetic or pharmacologic reduction in microsomal triglyceride transfer protein (MTP) activity, hepatic TG content increased dramatically; however, CREBH processing and apoA-IV gene expression were attenuated compared with controls. Adenovirus-mediated reconstitution of MTP expression proportionately restored CREBH processing and apoA-IV expression in liver-specific MTP knock-out mice. These results reveal that hepatic TG content, per se, does not regulate CREBH processing. Instead, TG mobilization into the endoplasmic reticulum for nascent VLDL particle assembly activates CREBH processing and enhances apoA-IV gene expression in the setting of acute steatosis. We conclude that VLDL assembly and CREBH activation play key roles in the response to hepatic steatosis by up-regulating apoA-IV and promoting assembly and secretion of larger, more TG-enriched VLDL particles.

Hepatocarcinogenesis following pancreatic islet transplantation in streptozotocin- and autoimmune-diabetic rats

Epidemiological studies assign a role to insulin signalling deregulation and diabetes mellitus in human hepatocarcinogenesis. The underlying mechanisms, however, remain largely unknown. To unravel the molecular pathogenesis of insulin-induced hepatocarcinogenesis, we generated an experimental animal model: after transplantation of only a low number of isologous pancreatic islets into the livers of diabetic rats, mild diabetes persists and the beta cells are maximally stimulated to permanently secrete insulin. As a consequence, liver acini, draining the hyperinsulinemic blood from islet grafts, show insulin-induced adaptive alterations simultaneously resembling preneoplastic foci of chemically-induced hepatocarcinogenesis models. These lesions progress to hepatocellular tumours within 6 and 24 months. Hepatocarcinogenesis is accompanied by alterations in hepatocytes metabolisms and changes in signal transduction pathways that, in the beginning, can be attributed solely to insulin action. In this review, we summarize our findings that may help understanding the oncogenic potential of diabetes mellitus in the human liver.

Revelation of simple and complex liver acini after portal transplantation of pancreatic islets or thyroid follicles in rats

The microarchitecture of the liver is still not completely understood although various concepts of structural liver organization have been proposed. Among them, Rappaport's liver acinus stands out as one of the most accepted models. The correctness of this model, however, has also been doubted, and its applicability is hampered by the fact that the outlines of the liver acinus are disguised and nobody was ever able to give visual evidence by "unmasking" a simple liver acinus from the surrounding liver tissue. After intraportal transplantation of pancreatic islets or thyroid follicles into diabetic or thyroidectomized rats, respectively, the transplants engraft in small portal tracts and morphologically alter the downstream liver tissue due to excessive hormone secretion. Using a combined approach of perfusion fixation, stereomicroscopy, and light microscopy, we demonstrate in this study that these foci of altered liver tissue represent simple and complex liver acini, exactly as described by Rappaport. We present stereomicroscopical and histological examples of all important cut levels of altered simple and complex liver acini, including their topographical relation to the supplying and draining vessels and to the "central vein" liver lobule. Moreover, by computer-aided reconstruction of serial semi-thin sections, we were able to present the first 3-dimensional images of simple and complex liver acini.

CONCLUSION

Our results prove the correctness of Rappaport's acinus model and confirm the simple liver acinus as the principal microcirculatory unit of the liver.

Apolipoprotein A-IV is regulated by nutritional and metabolic stress: involvement of glucocorticoids, HNF-4 alpha, and PGC-1 alpha

Apolipoprotein A-IV (apoA-IV) is a 46 kDa glycoprotein that associates with triglyceride-rich and high density lipoproteins. Blood levels of apoA-IV generally correlate with triglyceride levels and are increased in diabetic patients. This study investigated the mechanisms regulating the in vivo expression of apoA-IV in the liver and intestine of mice in response to changes in nutritional status. Fasting markedly increased liver and ileal apoA-IV mRNA and plasma protein concentrations. This induction was associated with increased serum glucocorticoid levels and was abolished by adrenalectomy. Treatment with dexamethasone increased apoA-IV expression in adrenalectomized mice. Marked increases of apoA-IV expression were also observed in two murine models of diabetes. Reporter gene analysis of the murine and human apoA-IV/C-III promoters revealed a conserved cooperative activation by the hepatic nuclear factor-4 alpha (HNF-4 alpha) and the peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1 alpha) but no evidence of a direct regulatory role for the glucocorticoid receptor. Consistent with these in vitro data, induction of apoA-IV in response to fasting was accompanied by increases in HNF-4 alpha and PGC-1 alpha expression and was abolished in liver-specific HNF-4 alpha-deficient mice. Together, these results indicate that the induction of apoA-IV expression in fasting and diabetes likely involves PGC-1 alpha-mediated coactivation of HNF-4 alpha in addition to glucocorticoid-dependent actions.

Differential gene expression in patients genetically predisposed to pancreatic cancer

BACKGROUND

Nearly 10% of all pancreatic cancer (PCA) results from genetic predisposition. Although abnormalities in sporadic PCA have been described, little is known about the genetics of heritable PCA. The purpose of this study was to identify novel genes expressed in patients with a presumed genetic predisposition or "familial" PCA.

PATIENTS AND METHODS

We defined "familial" PCA as patients having one or more first-degree relatives with biopsy-proven adenocarcinoma of the pancreas. Using a PCR-based subtractive and enrichment procedure, representational difference analysis (RDA), pancreatic tumor cDNA was reverse-transcribed from pooled poly(A)+ mRNA from six such patients (tester) and compared to pooled cDNA from five normal pancreata (driver). Tumor-specific gene fragments were identified and confirmed to be overexpressed in familial PCA by comparative RT-PCR. Six PCA cell lines, 11 sporadic tumors, 5 neuroendocrine tumors, and 3 chronic pancreatitis tissues were screened to determine the specificity of these genes.

RESULTS

Sequence analysis revealed several sequences of unknown significance and six genes previously described in neoplasia/carcinogenesis: Apolipoprotein A4, CEA, Keratin 19, Stratifin (14-3-3 sigma), Trefoil Factor, and Calcium Binding Protein S100 A6. Screening of cell lines and pancreatic tissue types showed varying degrees of specificity for familial and sporadic PCA. The APO-A4 gene was up-regulated in familial PCA.

CONCLUSIONS

The pattern of frequency in all screened tissue suggests that these genes are associated with conditions that produce significant desmoplastic responses and are difficult to differentiate from chronic inflammatory processes. Apolipoprotein A4 is preferentially expressed in familial patients, suggesting that the importance of fatty acid synthesis in carcinogenesis be investigated further.

Cystic cholangiomas after transplantation of pancreatic islets into the livers of diabetic rats

Islet transplantation is increasingly used as a therapy for human type 1 diabetes mellitus. In our study, we investigated the effect of the transplantation of a low number (n = 350) of pancreatic islets into the right liver part on the neighboring portal bile ducts. Male streptozotocin- diabetic Lewis or autoimmune-diabetic BB/Pfd rats (n = 1065) were subdivided into 11 experimental groups. A few days after low-number islet transplantation, cholangiocytes adjacent to the grafts showed an increase in proliferative activity. During the next 12-24 months, many peri-insular ductules progressed via tumor-like cystic lesions to large cystic cholangiomas, accompanied by a translocation of the insulin receptor into the cytoplasm and an increase in expression of insulin-related signaling proteins (Insulin-receptor-substrate-1, Raf-1, Mek-1). After 24 months, 53% of rats with low-number transplantation exhibited at least one cholangioma >10 mm, significantly outnumbering tumor development in the transplant-free left liver part and in any control group. No cholangiocarcinomas emerged. A graft cell origin of the tumors was excluded by Y chromosome in situ hybridization in cross-gender transplantations. Conclusively, low-number intrahepatic islet transplantation, most likely acting by permanent local hyperinsulinism, leads to prolonged cholangiocellular proliferation in streptozotocin- and in autoimmune-diabetic rats, resulting in the development of benign cystic cholangiomas.

Hepatocellular neoplasms induced by low-number pancreatic islet transplants in autoimmune diabetic BB/Pfd rats

It has been shown that combined high local hyperinsulinism and hyperglycemia after low-number islet transplantation into the livers of streptozotocin-diabetic rats lead to the development of hepatocellular neoplasms but a substantial cocarcinogenic effect of genotoxic streptozotocin could not be ruled out completely. Thus, we herein investigated this model in BB/Pfd rats (n = 805; nine experimental groups), which develop spontaneous autoimmune diabetes similar to human type 1 diabetes. After low-number islet transplantation (n = 450), the liver acini downstream of the islets show insulin-induced alterations: massive glycogen and/or fat accumulation, translocation of the insulin receptor, decrease in glucose-6-phosphatase activity, increase in expression of insulin-like growth factor (IGF)-I, IGF-II/mannose-6-phosphate receptor, insulin receptor substrate-1, Raf-1, and Mek-1, corresponding to clear cell preneoplastic foci of altered hepatocytes known from chemical hepatocarcinogenesis and identical to that in streptozotocin-diabetic Lewis rats. After 6 months, many altered liver acini progressed to other types of preneoplasias often accompanied by an overexpression of the glutathione-S transferase (placental form), IGF-I receptor, and transforming growth factor (TGF)-alpha. After 12 to 15 and 15 to 18 months, 52% and 100% of the animals showed one or multiple hepatocellular adenomas or hepatocellular carcinomas (HCCs), respectively. Conclusively, this study identifies combined hyperinsulinism and hyperglycemia as a carcinogenic mechanism for the development of HCCs in diabetic rats. Hepatocarcinogenesis is independent from additional genotoxic compounds (i.e., streptozotocin), but is primarily triggered by increased intracellular insulin signaling via pathways associated with cell growth and proliferation, such as the Ras-Raf-mitogen-activated protein kinase pathway and the IGF system, and secondarily involves other growth factors, such as TGF-alpha.

Cocarcinogenic Effects of Islet Hormones and N-Nitrosomorpholine in Hepatocarcinogenesis after Intrahepatic Transplantation of Pancreatic Islets in Streptozotocin-Diabetic Rats

In contrast to high local insulin levels obtained after low-number transplantation (n = 350) of islets of Langerhans into the livers of diabetic rats, low insulin levels after high-number transplantation (n = 1,000) do not suffice to induce hepatocarcinogenesis. Herein, we investigated the possible cocarcinogenic potential of high and, in particular, low insulin levels, combining this in vivo model with a chemical model of hepatocarcinogenesis after administration of N-nitrosomorpholine (NNM). In three main experiments, different schemes of single or continuous NNM administration were combined with different transplantation procedures in diabetic or nondiabetic animals, i.e., low-number and high-number islet transplantation, transplantation of polystyrene particles, and sham transplantation. Animals were sacrificed between 3 and 53 weeks after transplantation procedures. Evidence for the cocarcinogenic effects of NNM and insulin was provided in each main experiment. NNM treatment after low-number islet transplantation resulted in an increase in the number of preneoplastic hepatocellular foci, and a significant increase in the number and an earlier appearance of hepatocellular adenomas and carcinomas compared with controls. Most intriguing was the increase in preneoplastic foci after combined NNM treatment and high-number islet transplantation, proving that insulin, even in lower doses, has at least cocarcinogenic effects on the downstream hepatocytes and thus promotes an otherwise initiated hepatocarcinogenic process. Conclusively, intrahepatic transplantation of pancreatic islets acts as a strong cocarcinogenic factor together with NNM in streptozotocin-diabetic rats.

Insulin-mediated down-regulation of apolipoprotein A5 gene expression through the phosphatidylinositol 3-kinase pathway: role of upstream stimulatory factor

The apolipoprotein A5 gene (APOA5) has been repeatedly implicated in lowering plasma triglyceride levels. Since several studies have demonstrated that hyperinsulinemia is associated with hypertriglyceridemia, we sought to determine whether APOA5 is regulated by insulin. Here, we show that cell lines and mice treated with insulin down-regulate APOA5 expression in a dose-dependent manner. Furthermore, we found that insulin decreases human APOA5 promoter activity, and subsequent deletion and mutation analyses uncovered a functional E box in the promoter. Electrophoretic mobility shift and chromatin immunoprecipitation assays demonstrated that this APOA5 E box binds upstream stimulatory factors (USFs). Moreover, in transfection studies, USF1 stimulates APOA5 promoter activity, and the treatment with insulin reduced the binding of USF1/USF2 to the APOA5 promoter. The inhibition of the phosphatidylinositol 3-kinase (PI3K) pathway abolished insulin's effect on APOA5 gene expression, while the inhibition of the P70 S6 kinase pathway with rapamycin reversed its effect and increased APOA5 gene expression. Using an oligonucleotide precipitation assay for USF from nuclear extracts, we demonstrate that phosphorylated USF1 fails to bind to the APOA5 promoter. Taken together, these data indicate that insulin-mediated APOA5 gene transrepression could involve a phosphorylation of USFs through the PI3K and P70 S6 kinase pathways that modulate their binding to the APOA5 E box and results in APOA5 down-regulation. The effect of exogenous hyperinsulinemia in men showed a decrease in the plasma ApoAV level. These results suggest a potential contribution of the APOA5 gene in hypertriglyceridemia associated with hyperinsulinemia.

Overexpression of fatty acid synthase in chemically and hormonally induced hepatocarcinogenesis of the rat

Fatty acid synthase (FAS) is the key enzyme of de novo fatty acid synthesis and has been shown to be involved in carcinogenesis of numerous human malignancies, including breast, colorectal, and prostate carcinomas, often associated with a worse prognosis. Although FAS is mainly expressed in the liver, an implication of FAS in hepatocarcinogenesis, has not yet been investigated. FAS expression is stimulated by insulin and glucose, and insulin is also the primary trigger of hepatocarcinogenesis in an endocrine experimental model, which is induced by low-number transplantation of islets of Langerhans into the livers of diabetic rats. We therefore investigated, whether FAS is implicated in hepatocarcinogenesis in this model and in comparison to chemically induced hepatocarcinogenesis after N-nitrosomorpholine (NNM) treatment in diabetic and normoglycemic rats. Preneoplastic clear-cell foci of altered hepatocytes (FAH), harvested after laser-microdissection of kryostat sections, showed an overexpression of FAS messenger RNA in gene expression profiles, done by array-hybridization, and in quantitative RT-PCR (Light-Cycler). Virtually, all (96-98%) of the subsequently investigated FAH and the glycogenotic hepatocellular adenomas and carcinomas showed an additional strong FAS protein overexpression. In the NNM-model, FAS protein was also overexpressed in the vast majority (87%) of glycogenotic FAH and neoplasms, in particular in the diabetic animals. Also two spontaneous glycogenotic FAH in control animals displayed strong FAS overexpression. Basophilic lesions and neoplasms, which occasionally develop out of the primary clear-cell FAH at later stages of carcinogenesis, however, lost FAS overexpression. In conclusion, FAS overexpression is an early phenonemon in spontaneous, hormonally and chemically induced rat hepatocarcinogenesis, demonstrable in early clear-cell (glycogenotic) FAH and hepatocellular neoplasms. FAS overexpression can be attributed to the local hyperinsulinemia in the transplantation model and belongs to cellular and metabolic alterations in the chemical model, which are induced by an insulinomimetic but yet unknown mechanism.

Insulin receptor, insulin receptor substrate-1, Raf-1, and Mek-1 during hormonal hepatocarcinogenesis by intrahepatic pancreatic islet transplantation in diabetic rats

Low-number transplantation of pancreatic islets into the livers of diabetic rats leads to transformation of the downstream liver acini into clear-cell foci of altered hepatocytes (FAHs). These FAHs correspond to the glycogen-storing (clear-cell) phenotype of hepatocellular preneoplasias and develop into hepatocellular adenomas (HCAs) and hepatocellular carcinomas (HCCs) within 6 to 24 months. In addition, they show metabolic alterations that resemble well-known insulin effects, most likely constituting the result of the local hyperinsulinemia. Thus, we investigated FAHs, HCAs, and HCCs for altered expression of insulin receptor, insulin receptor substrate-1 (IRS-1), Raf-1 and Mek-1. Light and electron microscopic immunohistochemistry revealed a translocation of insulin receptor from the plasma membrane (normal tissue) into the cytoplasm in clear-cell FAHs and an increase in insulin receptor expression in HCAs and HCCs. FAHs also showed an increase in IRS-1 gene expression, investigated by in situ hybridization and quantitative reverse transcription-PCR. IRS-1, Raf-1, and Mek-1 proteins were strongly overexpressed in FAHs and tumors, as compared with the unaltered liver tissue. These overexpressions were closely linked to the clear-cell phenotype of preneoplastic and neoplastic hepatocytes, because basophilic FAHs (later stages) and basophilic tumors showed no overexpressions. In this endocrine model of hepatocarcinogenesis, severe alterations of insulin signaling were induced by the pathological local action of islet hormones in the livers and may substantially contribute to the carcinogenic process.