Glutathione S-transferases and P-glycoprotein in normal rat hepatocytes and hepatoma cells: analysis using flow cytometry

Building a tiered approach to in vitro predictive toxicity screening: a focus on assays with in vivo relevance

One of the greatest challenges facing the pharmaceutical industry today is the failure of promising new drug candidates due to unanticipated adverse effects discovered during preclinical animal safety studies and clinical trials. Late stage attrition increases the time required to bring a new drug to market, inflates development costs, and represents a major source of inefficiency in the drug discovery/development process. It is generally recognized that early evaluation of new drug candidates is necessary to improve the process. Building in vitro data sets that can accurately predict adverse effects in vivo would allow compounds with high risk profiles to be deprioritized, while those that possess the requisite drug attributes and a lower risk profile are brought forward. In vitro cytotoxicity assays have been used for decades as a tool to understand hypotheses driven questions regarding mechanisms of toxicity. However, when used in a prospective manner, they have not been highly predictive of in vivo toxicity. Therefore, the issue may not be how to collect in vitro toxicity data, but rather how to translate in vitro toxicity data into meaningful in vivo effects. This review will focus on the development of an in vitro toxicity screening strategy that is based on a tiered approach to data collection combined with data interpretation.

The antiepileptic and anticancer agent, valproic acid, induces P-glycoprotein in human tumour cell lines and in rat liver

BACKGROUND AND PURPOSE

The antiepileptic drug valproic acid, a histone deacetylase (HDAC) inhibitor, is currently being tested as an anticancer agent. However, HDAC inhibitors may interact with anticancer drugs through induction of P-glycoprotein (P-gp, MDR1) expression. In this study we assessed whether valproic acid induces P-gp function in tumour cells. We also investigated effects of valproic acid on the mRNA for P-gp and the cytochrome P450, CYP3A, in rat livers.

EXPERIMENTAL APPROACH

Effects of valproic acid on P-gp were assessed in three tumour cell lines, SW620, KG1a and H4IIE. Accumulation of acetylated histone H3 in rats' livers treated for two or seven days with valproic acid was evaluated using a specific antibody. Hepatic expression of the P-gp genes, mdr1a, mdr1b and mdr2, was determined by real-time polymerase chain reaction. The effects of valproic acid on CYP3A were assessed by Northern blot analysis and CYP3A activity assays.

KEY RESULTS

Valproic acid (0.5-2.0 mM) induced P-gp expression and function up to 4-fold in vitro. The effect of a series of valproic acid derivatives on P-gp expression in SW620 and KG1a cells correlated with their HDAC inhibition potencies. Treatment of rats with 1 mmol kg(-1) valproic acid for two and seven days increased hepatic histone acetylation (1.3- and 3.5-fold, respectively) and the expression of mdr1a and mdr2 (2.2-4.1-fold). Valpromide (0.5-2.0 mM) did not increase histone acetylation or P-gp expression in rat livers, but induced CYP3A expression.

CONCLUSIONS

Valproic acid increased P-gp expression and function in human tumour cell lines and in rat liver. The clinical significance of this increase merits further investigation.

The relationship between the cancer cell and the oocyte

What causes cancer? To date, this question is left with no answer. The scientific community keeps defining cancer as a proliferation of cells in an uncontrolled and uncontrollable manner. The hypothesis developed here through an understanding of the cancer cell's behaviour led to its re-definition, providing answers to the questions arising from cancer. Through striking similarities in cell behaviour, I have concluded that the cancer cell is a reprogrammed cell with the launch of the egg cell's genetic program. The unique cell to express this program is the oocyte, therefore this oocyte cell should become the subject of significant study to understand the genesis of cancer. Above all, it will enable us to eradicate in a specific manner the cancer cell.

Glutathione S-transferase alpha expressed in porcine Sertoli cells is under the control of follicle-stimulating hormone and testosterone

Glutathione S-transferases (GSTs) are a family of detoxification isoenzymes present in different tissues including the testis and that conjugate many toxic substrates to glutathione. Among these substrates are carcinogens, mutagens and products of oxidative processes. In the present report we show that GSTalpha is expressed in somatic testicular Leydig cells and Sertoli cells. GSTalpha expression in Sertoli cells is under the hormonal control of FSH, testosterone, and estradiol. In Leydig cells, immunoreactive GSTalpha was present at the neonatal, pubertal, and adult periods. In Sertoli cells, GSTalpha was predominant in pubertal and adult testes (but not in neonatal testes), suggesting that its expression is controlled by gonadotropins. The regulatory action and the mechanisms of action of FSH and testosterone on GSTalpha mRNA and protein levels were studied by using a model of primary cultures of porcine testicular Sertoli cells. FSH increased GSTalpha mRNA levels in a dose-dependent manner (ED50 = 18.5 nm/ml) with a maximal effect observed after 48 h of exposure (a 3-fold increase; P < 0.001). In addition, FSH increased GSTalpha protein, which was detected as a doublet of 28 kDa. Treatment with testosterone enhanced GSTalpha mRNA levels in a dose-dependent (ED50 = 1.4 ng/ml) and time-dependent manner with a maximal effect delayed at 8 h of exposure (a 2-fold increase; P < 0.001). Similarly, Sertoli cell treatment with testosterone metabolites, dihydrotestosterone (DHT) and estradiol, led to an increase in GSTalpha mRNA levels. Because stimulatory effects of FSH and androgens were also observed on GSTalpha protein, we therefore had to determine whether the different hormones were affecting GSTalpha gene transcriptional activity, or GSTalpha mRNA stability, or both. FSH and 8-Br-cAMP (but not testosterone) increased the stability of GSTalpha mRNA. The effects of FSH and testosterone on GSTalpha protein were additive, confirming that both hormones act through distinct mechanisms on the expression of the enzyme. Taken together, the present observations indicate that Sertoli cell GSTalpha is targeted by FSH, testosterone, and its metabolites, and they reinforce the concept that Sertoli cells exert a protective role and are under endocrine control to ward against toxic agents in the context of Sertoli-germ cell interactions during spermatogenesis.

A distinct sequence (ATAAA)n separates methylated and unmethylated domains at the 5'-end of the GSTP1 CpG island

What defines the boundaries between methylated and unmethylated domains in the genome is unclear. In this study we used bisulfite genomic sequencing to map the boundaries of methylation that flank the 5'- and 3'-ends of the CpG island spanning the promoter region of the glutathione S-transferase (GSTP1) gene. We show that GSTP1 is expressed in a wide range of tissues including brain, lung, skeletal muscle, spleen, pancreas, bone marrow, prostate, heart, and blood and that this expression is associated with the CpG island being unmethylated. In these normal tissues a marked boundary was found to separate the methylated and unmethylated regions of the gene at the 5'-flank of the CpG island, and this boundary correlated with an (ATAAA)(19-24) repeated sequence. In contrast, the 3'-end of the CpG island was not marked by a sharp transition in methylation but by a gradual change in methylation density over about 500 base pairs. In normal tissue the sequences on either side of the 5'-boundary appear to lie in separate domains in which CpG methylation is independently controlled. These separate methylation domains are lost in all prostate cancer where GSTP1 expression is silenced and methylation extends throughout the island and spans across both the 5'- and 3'-boundary regions.