Metabolism and binding of benzo[a]pyrene in randomly-proliferating, confluent and S-phase human skin fibroblasts

Comparative evaluation of different cell lysis and extraction methods for studying benzo(a)pyrene metabolism in HT-29 colon cancer cell cultures

Lysis and extraction of cells are essential sample processing steps for investigations pertaining to metabolism of xenobiotics in cell culture studies. Of particular importance to these procedures are maintaining high lysis efficiency and analyte integrity as they influence the qualitative and quantitative distribution of drug and toxicant metabolites in the intra- and extracellular milieus. In this study we have compared the efficiency of different procedures viz. homogenization, sonication, bead beating, and molecular grinding resin treatment for disruption of HT-29 colon cells exposed to benzo(a)pyrene (BaP), a polycyclic aromatic hydrocarbon (PAH) compound and a suspected colon carcinogen. Also, we have evaluated the efficiency of various procedures for extracting BaP parent compound/metabolites from colon cells and culture media prior to High Performance Liquid Chromatography (HPLC) analyses. The extraction procedures include solid phase extraction, solid-supported liquid- liquid extraction, liquid-liquid extraction, and homogeneous liquid- liquid extraction. Our findings showed that bead-beating in combination with detergent treatment of cell pellet coupled with liquid-liquid extraction yielded greater concentrations of BaP metabolites compared to the other methods employed. Our method optimization strategy revealed that disruption of HT-29 colon cells by a combination of mechanical and chemical lysis followed by liquid-liquid extraction is efficient and robust enough for analyzing BaP metabolites from cell culture studies.

Elevation of cellular BPDE uptake by human cells: a possible factor contributing to co-carcinogenicity by arsenite

BACKGROUND

Arsenite (iAsIII) can promote mutagenicity and carcinogenicity of other carcinogens. Considerable attention has focused on interference with DNA repair by inorganic arsenic, especially the nucleotide excision repair (NER) pathway, whereas less is known about the effect of arsenic on the induction of DNA damage by other agents.

OBJECTIVES

We examined how arsenic modulates DNA damage by other chemicals.

METHODS

We used an NER-deficient cell line to dissect DNA damage induction from DNA repair and to examine the effects of iAsIII on the formation of benzo[a]pyrene diol epoxide (BPDE)-DNA adducts.

RESULTS

We found that pretreatment with iAsIII at subtoxic concentrations (10 microM) led to enhanced formation of BPDE-DNA adducts. Reduced glutathione levels, glutathione S-transferase activity and chromatin accessibility were also measured after iAsIII treatment, but none of these factors appeared to account for the enhanced formation of DNA adducts. However, we found that pretreatment with iAsIII increased the cellular uptake of BPDE in a dose-dependent manner.

CONCLUSIONS

Our results suggest that iAsIII enhanced the formation of BPDE-DNA adducts by increasing the cellular uptake of BPDE. Therefore, the ability of arsenic to increase the bioavailability of other carcinogens may contribute to arsenic co-carcinogenicity.

A conundrum in molecular toxicology: molecular and biological changes during neoplastic transformation of human cells

The process of multistage carcinogenesis lends itself to the concept that the effects of carcinogens are mediated through dose-related, multi-hit, linear changes. Multiple in vitro model systems have been developed that are designed to examine the cellular changes associated with the progression of cells through the different stages in the process; however, these systems may have inherent limitations due to the cell lines used for these studies, the manner of assessing the effects of the carcinogens, and the subsequent growth and differentiation of the exposed cells. Each of these variables results in increasing levels of uncertainty relative to the correlation of the events with the actual process of human tumor development. Therefore, the prediction of the ultimate effect of any carcinogen is difficult. Moreover, relationships between individual biological endpoints resulting from carcinogen treatment appear at best to be approximations. The presence of an activated carcinogen inside the cell can give rise to multiple outcomes, only some of which may be critical events. For example, site-specific modification of the 12th and 13th codons of H-ras is different than that in the adjacent 14th and 15th codons. It is interesting to speculate what effect these differences might have on a biological outcome, e.g., transformation to anchorage-independent growth. The use of different model systems to examine the effects of activated carcinogens also creates additional problems. Comparisons of in vitro transformed cells with similar cells isolated from human tumors indicate that the culture environment appears to influence the expression of a particular phenotype, in that human tumor cells in culture express many of the same parameters as those found in cells transformed with carcinogens in vitro. If the process of transformation is linear, then less aggressive phenotypes should progress to a more aggressive transformed stage. However, in carcinogen-transformed human cells, the populations exhibit phenotypic diversity in that many of the transformed cells differentiate and fail to continue to divide in culture. Historically, we have assumed only a limited role for epigenetic modulation of molecular changes that occur during progression; however, our data suggest quite strongly that nonmalignant tumor populations can be converted to a more malignant phenotype without additional mutations taking place and, conversely, malignant populations can be downregulated to a nontumorigenic phenotype. Tumor cell plasticity is not only a fundamental characteristic of diverse types of human tumors, but also appears as an integral characteristic of carcinogen-transformed cells in vitro.

Proliferative responses of quail aortic smooth muscle cells to benzo[a]pyrene: implications in PAH-induced atherogenesis

Repeated exposure of avian and rodent species to benzo(a)pyrene (BaP) has been associated with the development of aortic lesions of atherosclerotic etiology. Because the occurrence of these lesions may involve alterations in the regulation of smooth muscle cell (SMC) growth, the present studies were conducted to evaluate the proliferative responses of quail aortic SMCs to BaP treatment in vivo and in vitro. Measurements of [3H]thymidine incorporation and cell growth were conducted in cultured aortic SMCs isolated from male Japanese quail treated with 10 mg/kg BaP or vehicle weekly for 10 weeks or in naive aortic SMCs exposed in vitro to BaP (0.003-30 microM). Inhibition of DNA synthesis was observed in primary and early passage cultures of aortic SMCs isolated from BaP-treated quail relative to controls. Continued propagation of these cultures yielded a population of BaP cells which proliferated at faster rates than controls. The proliferative phenotype induced by BaP was first observed after the tenth passage and preserved in all subsequent passages tested. In vitro growth of SMCs from BaP-treated animals was serum- and anchorage-dependent. A 24-h exposure of cycling SMC cultures to BaP (0.003-30 microM) was associated with a dose-dependent decrease in DNA synthesis and significant delay in the progression of SMCs through the cell cycle. A time-course study revealed that maximal inhibition of DNA synthesis occurred 10 h after addition of 3 microM BaP to cycling cultures of SMCs. As seen in SMCs isolated from BaP-treated quail, serial subculture of SMCs exposed to 0.3 microM BaP in vitro for 24 h yielded a fast-growing population of cells. In these cultures, expression of the proliferative phenotype was observed after the fifth passage. These data suggest that BaP induces the expression of a proliferative phenotype in aortic SMCs characterized by enhanced serum responsiveness. This phenotypic modulation may contribute to the initiation and/or progression of vascular lesions of atherosclerotic etiology induced by BaP.