Phenotypic properties of liver tumors induced by dehydroepiandrosterone in F-344 rats

Animal Models: A Useful Tool to Unveil Metabolic Changes in Hepatocellular Carcinoma

Simple Summary

Hepatocellular carcinoma (HCC) represents an important health problem. At the moment, systemic therapies offered only modest clinical benefits. Thus, HCC represents a cancer extremely difficult to treat, and therapeutic breakthroughs are urgently needed. Metabolic reprogramming of neoplastic cells has been recognized as one of the core hallmarks of cancer. Experimental animal models represent an important tool that allows to investigate metabolic changes underlying HCC development and progression. In the present review, we characterize available rodent models of hepatocarcinogenesis. Moreover, we discuss the possibility that pharmacological targeting of Warburg metabolism may represent an additional tool to improve already available therapeutic approaches for HCC.

Abstract

Hepatocellular carcinoma (HCC) is one the most frequent and lethal human cancers. At present, no effective treatment for advanced HCC exist; therefore, the overall prognosis for HCC patients remains dismal. In recent years, a better knowledge of the signaling pathways involved in the regulation of HCC development and progression, has led to the identification of novel potential targets for therapeutic strategies. However, the obtained benefits from current therapeutic options are disappointing. Altered cancer metabolism has become a topic of renewed interest in the last decades, and it has been included among the core hallmarks of cancer. In the light of growing evidence for metabolic reprogramming in cancer, a wide number of experimental animal models have been exploited to study metabolic changes characterizing HCC development and progression and to further expand our knowledge of this tumor. In the present review, we discuss several rodent models of hepatocarcinogenesis, that contributed to elucidate the metabolic profile of HCC and the implications of these changes in modulating the aggressiveness of neoplastic cells. We also highlight the apparently contrasting results stemming from different animal models. Finally, we analyze whether these observations could be exploited to improve current therapeutic strategies for HCC.

The Use of Dehydroepiandrosterone Therapy in Clinical Practice

Dehydroepiandrosterone (DHEA) therapy is controversial due to sensationalized reports of epidemiologic studies and the over-the-counter availability of DHEA. Human clinical trials have investigated the potential efficacy of DHEA therapy in multiple conditions with resultant inconsistencies in findings. DHEA is unique compared with other adrenal steroids because of the fluctuation in serum levels found from birth into advancing age. The lower endogenous levels of DHEA and DHEA sulfate found in advancing age have been correlated with a myriad of health conditions. Also, some studies suggest gender-specific actions of endogenous and exogenous DHEA. We reviewed only pharmacokinetic studies and human clinical trials investigating the efficacy of DHEA therapy that were placebo-controlled as these provided the most reliable scientific basis for the evaluation of DHEA therapy. Pharmacodynamic studies suggest that doses of 30-50mg of oral DHEA may produce physiologic androgen levels, especially in women. These studies report a dose-dependent effect and lack of accumulation of serum androgen levels. Pharmacologic studies also reveal a gender-specific response to DHEA therapy such that testosterone levels are increased in women but not in men. Clinical trials suggest that 50mg of oral DHEA, but not <30mg, can increase serum androgen levels to within the physiologic range for young adults with primary and secondary adrenal insufficiency, possibly improve sexual function, improve mood and self-esteem, and decrease fatigue/exhaustion. Whereas DHEA replacement therapy may be effective in treating patients with adrenal insufficiency, human clinical trials investigating its efficacy in conditions such as systemic lupus erythematosus, HIV, Alzheimer disease, advancing age, male sexual dysfunction, perimenopausal symptoms, depression, and cardiovascular disease have not provided consistent findings.

Induction and modulation of hepatic preneoplasia and neoplasia in the rat by dehydroepiandrosterone

Dehydroepiandrosterone (DHEA), the main adrenal steroid in humans and a precursor in androgen and estrogen biosynthesis, acts as a peroxisome proliferator and as a hepatocarcinogen in rats. Neoplasms emerge from a glycogenotic/amphophilic/basophilic preneoplastic cell lineage. A higher female tumor incidence suggests a relevant influence of sex hormones. DHEA enhances hepatocarcinogenesis induced by N-nitrosomorpholine (NNM), which is characterized by the glycogenotic/basophilic cell lineage. The tumor promoting effect is related to an additional amphophilic/basophilic preneoplastic lesion sequence and to faster proliferation of the basophilic preneoplastic lesions. Nevertheless, hepatocellular carcinomas provided under DHEA treatment seem to have a less malignant phenotype compared to tumors induced by NNM only. Further, DHEA treatment reduces growth and generation of glycogen storage foci (GSF) in initial NNM-treated rats. Thus, DHEA treatment results in both, a growth stimulation of the late basophilic lesion type with an additional amphophilic lesion sequence, and in a growth inhibition of early preneoplastic lesions, addressing especially GSF. DHEA also inhibits the growth of physiologically proliferating liver tissue. This might be explained by a DHEA related cellular metabolism, which results in significant energy consumption. Additionally, a DHEA-induced alteration of cytokine levels might contribute to this growth inhibition as well.

Modulation of dihydroxy-di-n-propylnitrosamine-induced liver lesion development in Opisthorchis-infected Syrian hamsters by praziquantel treatment in association with butylated hydroxyanisole or dehydroepiandrosterone administration

The effects of praziquantel coupled with dehydroepiandrosterone (DHEA) or butylated hydroxyanisole (BHA) administration 16 weeks subsequent to dihydroxy-di-n-propylnitrosamine (DHPN) treatment and infection with Opisthorchis viverrini (OV) on lesion development in the liver of Syrian hamsters were investigated. Animals were given 80 OV metacercariae and then two i.p. injections of DHPN (500 mg/kg body weight) 4 and 5 weeks thereafter. At week 16, groups received praziquantel (250 mg/kg, i.g.) and were placed on normal diet or diet supplemented with BHA (1%) or DHEA (0.6%) until they were killed at week 24. Histopathological assessment revealed that, whereas antihelminthic treatment alone resulted in a clear reduction in hepatocellular lesion development, effects on cholangiocellular lesions were equivocal. BHA and DHEA, in contrast, were both associated with a significant reduction in frequency of cholangiofibrosis and cholangiocellular carcinoma. The former chemical, however, increased the numbers of liver nodules while the hormone brought about a decrease as well as a shift in the phenotype of the lesions. The results thus indicate that although cholangiocellular lesion development may, unlike generation of hepatocellular nodules, be to a certain extent independent of the continued presence of parasite, it can be influenced by exogenous treatments.

Sex differences in dehydroepiandrosterone-induced hepatocarcinogenesis in the rat

Dehydroepiandrosterone (DHEA), a steroid secreted by the adrenal gland, is a peroxisome proliferator and a hepatocarcinogen. Previously, we have shown that 15-week-old male rats given DHEA in AIN-76 diet without vitamin E developed liver tumors. In the present study, we have examined the carcinogenic effect of DHEA in 5-6-week-old male rats and in intact and ovariectomized female rats. Rats were fed Purina chow containing DHEA at a concentration of 0.45% for 100 weeks and livers were evaluated for tumor incidence and multiplicity. In male rats the incidence of total tumors and hepatocellular carcinomas (HCC) was 94 and 81%, respectively, with 1.9 +/- 0.3 (mean +/- SD) tumors per liver. In intact and ovariectomized females the total tumor incidence was 46 and 60%, respectively. However, the incidence of HCC and the mean number of tumors were similar in both intact and ovariectomized groups. Phenotypically the neoplastic nodules and HCC in female rats were negative for gamma-glutamyltranspeptidase. The results of these studies further confirm that DHEA is a hepatocarcinogen in male and female rats. The possible reasons for the decreased incidence of liver tumors in females in relation to the peroxisome proliferative effect of DHEA is discussed.

Enhancement and phenotypic modulation of N-nitrosomorpholine-induced hepatocarcinogenesis by dehydroepiandrosterone

Hepatocarcinogenesis was induced in male and female rats by continuous administration of the adrenal steroid dehydroepiandrosterone (DHEA; 0.6% in the diet) with and without previous treatment with N-nitrosomorpholine (NNM; 120 mg/l drinking water for 7 weeks). DHEA treatment alone resulted in hepatocellular adenomas (HCA) and carcinomas (HCC) after 72-84 weeks, the incidence of both benign and malignant neoplasms being higher in females than in males. After DHEA administration for up to 32 weeks subsequent to NNM, the incidence of HCA and HCC was significantly higher (HCA, 42%; HCC, 42%) than after NNM alone (HCA, 33%; HCC, 28%). While total tumor incidence was similar in male (63%) and female (60%) rats after NNM treatment alone, it was higher in females (87%) than in males (80%) after NNM/DHEA treatment. The difference between the genders was mainly due to the higher incidence of HCC in females. Morphometric analysis of preneoplastic foci of altered hepatocytes (FAH) yielded that DHEA treatment did not increase the average total number of FAH induced by NNM, but caused a modulation of the phenotype of FAH from the glycogenotic/basophilic to the amphophilic cell lineage. The results confirm that DHEA acts as a hepatocarcinogen and show for the first time that it enhances NNM-induced hepatocarcinogenesis in rats.

Fish models for environmental carcinogenesis: the rainbow trout

Progress over the past 30 years has revealed many strengths of the rainbow trout as an alternative model for environmental carcinogenesis research. These include low rearing costs, an early life-stage ultrasensitive bioassay, sensitivity to many classes of carcinogen, a well-described tumor pathology, responsiveness to tumor promoters and inhibitors, and a mechanistically informative nonmammalian comparative status. Low-cost husbandry, for example, has permitted statistically challenging tumor study designs with up to 10,000 trout to investigate the quantitative interrelationships among carcinogen dose, anticarcinogen dose, DNA adduct formation, and final tumor outcome. The basic elements of the trout carcinogen bioassay include multiple exposure routes, carcinogen response, husbandry requirements, and pathology. The principal known neoplasms occur in liver (mixed hepatocellular/cholangiocellular adenoma and carcinoma, hepatocellular carcinoma), kidney (nephroblastoma), swim bladder (adenopapilloma), and stomach (adenopapilloma). Trout possess a complex but incompletely characterized array of cytochromes P450, transferases, and other enzymic systems for phase I and phase II procarcinogen metabolism. In general, trout exhibit only limited capacity for DNA repair, especially for removal of bulky DNA adducts. This factor, together with a high capacity for P450 bioactivation and negligible glutathione transferase-mediated detoxication of the epoxide, accounts for the exceptional sensitivity of trout to aflatoxin B1 carcinogenesis. At the gene level, all trout tumors except nephroblastoma exhibit variable and often high incidences of oncogenic Ki-ras gene mutations. Mutations in the trout p53 tumor suppressor gene have yet to be described. There are many aspects of the trout model, especially the lack of complete organ homology, that limit its application as a surrogate for human cancer research. Within these limitations, however, it is apparent that trout and other fish models can serve as highly useful adjuncts to conventional rodent models in the study of environmental carcinogenesis and its modulation. For some problems, fish models can provide wholly unique approaches.

Medium-term rat liver bioassay for rapid detection of carcinogens and modifiers of hepatocarcinogenesis

For rapid detection of carcinogenic agents, a medium-term liver bioassay has been established in our laboratory using preneoplastic glutathione S-transferase placental form (GST-P) positive foci in the rat liver as endpoint marker lesions. A total of 237 compounds have so far been tested in this system and the results compared with reported Salmonella/microsome and long-term carcinogenicity test findings. The positive rate was found to be extremely high, 97% (28 of 29 compounds) for genotoxic hepatocarcinogens; and satisfactory, 86% (23 of 27 chemicals) for nongenotoxic ones. The positive rate for carcinogens targeting organs other than the liver, however, is relatively low (24%). Malathion and vinclozolin proved positive, although both have been reported to be noncarcinogenic in rats and mice. Those chemicals which exerted positive results in this system might be hepatopromoting agents even if hepatocarcinogenicity has not been established. Five of the six false-negative hepatocarcinogens could be categorized as peroxisome proliferators. In addition, a number of inhibitory agents for GST-P-positive foci development have been detected and many are categorized as antioxidants. The validity of this system as a tool for rapid detection of carcinogenic and chemopreventive agents is discussed.