Designing a broad-spectrum integrative approach for cancer prevention and treatment

Targeted therapies and the consequent adoption of "personalized" oncology have achieved notable successes in some cancers; however, significant problems remain with this approach. Many targeted therapies are highly toxic, costs are extremely high, and most patients experience relapse after a few disease-free months. Relapses arise from genetic heterogeneity in tumors, which harbor therapy-resistant immortalized cells that have adopted alternate and compensatory pathways (i.e., pathways that are not reliant upon the same mechanisms as those which have been targeted). To address these limitations, an international task force of 180 scientists was assembled to explore the concept of a low-toxicity "broad-spectrum" therapeutic approach that could simultaneously target many key pathways and mechanisms. Using cancer hallmark phenotypes and the tumor microenvironment to account for the various aspects of relevant cancer biology, interdisciplinary teams reviewed each hallmark area and nominated a wide range of high-priority targets (74 in total) that could be modified to improve patient outcomes. For these targets, corresponding low-toxicity therapeutic approaches were then suggested, many of which were phytochemicals. Proposed actions on each target and all of the approaches were further reviewed for known effects on other hallmark areas and the tumor microenvironment. Potential contrary or procarcinogenic effects were found for 3.9% of the relationships between targets and hallmarks, and mixed evidence of complementary and contrary relationships was found for 7.1%. Approximately 67% of the relationships revealed potentially complementary effects, and the remainder had no known relationship. Among the approaches, 1.1% had contrary, 2.8% had mixed and 62.1% had complementary relationships. These results suggest that a broad-spectrum approach should be feasible from a safety standpoint. This novel approach has potential to be relatively inexpensive, it should help us address stages and types of cancer that lack conventional treatment, and it may reduce relapse risks. A proposed agenda for future research is offered.

Broad targeting of resistance to apoptosis in cancer

Apoptosis or programmed cell death is natural way of removing aged cells from the body. Most of the anti-cancer therapies trigger apoptosis induction and related cell death networks to eliminate malignant cells. However, in cancer, de-regulated apoptotic signaling, particularly the activation of an anti-apoptotic systems, allows cancer cells to escape this program leading to uncontrolled proliferation resulting in tumor survival, therapeutic resistance and recurrence of cancer. This resistance is a complicated phenomenon that emanates from the interactions of various molecules and signaling pathways. In this comprehensive review we discuss the various factors contributing to apoptosis resistance in cancers. The key resistance targets that are discussed include (1) Bcl-2 and Mcl-1 proteins; (2) autophagy processes; (3) necrosis and necroptosis; (4) heat shock protein signaling; (5) the proteasome pathway; (6) epigenetic mechanisms; and (7) aberrant nuclear export signaling. The shortcomings of current therapeutic modalities are highlighted and a broad spectrum strategy using approaches including (a) gossypol; (b) epigallocatechin-3-gallate; (c) UMI-77 (d) triptolide and (e) selinexor that can be used to overcome cell death resistance is presented. This review provides a roadmap for the design of successful anti-cancer strategies that overcome resistance to apoptosis for better therapeutic outcome in patients with cancer.

Assessing the carcinogenic potential of low-dose exposures to chemical mixtures in the environment: the challenge ahead

Low-dose exposures to common environmental chemicals that are deemed safe individually may be combining to instigate carcinogenesis, thereby contributing to the incidence of cancer. This risk may be overlooked by current regulatory practices and needs to be vigorously investigated.

Lifestyle factors are responsible for a considerable portion of cancer incidence worldwide, but credible estimates from the World Health Organization and the International Agency for Research on Cancer (IARC) suggest that the fraction of cancers attributable to toxic environmental exposures is between 7% and 19%. To explore the hypothesis that low-dose exposures to mixtures of chemicals in the environment may be combining to contribute to environmental carcinogenesis, we reviewed 11 hallmark phenotypes of cancer, multiple priority target sites for disruption in each area and prototypical chemical disruptors for all targets, this included dose-response characterizations, evidence of low-dose effects and cross-hallmark effects for all targets and chemicals. In total, 85 examples of chemicals were reviewed for actions on key pathways/mechanisms related to carcinogenesis. Only 15% (13/85) were found to have evidence of a dose-response threshold, whereas 59% (50/85) exerted low-dose effects. No dose-response information was found for the remaining 26% (22/85). Our analysis suggests that the cumulative effects of individual (non-carcinogenic) chemicals acting on different pathways, and a variety of related systems, organs, tissues and cells could plausibly conspire to produce carcinogenic synergies. Additional basic research on carcinogenesis and research focused on low-dose effects of chemical mixtures needs to be rigorously pursued before the merits of this hypothesis can be further advanced. However, the structure of the World Health Organization International Programme on Chemical Safety ‘Mode of Action’ framework should be revisited as it has inherent weaknesses that are not fully aligned with our current understanding of cancer biology.

Disruptive environmental chemicals and cellular mechanisms that confer resistance to cell death

Cell death is a process of dying within biological cells that are ceasing to function. This process is essential in regulating organism development, tissue homeostasis, and to eliminate cells in the body that are irreparably damaged. In general, dysfunction in normal cellular death is tightly linked to cancer progression. Specifically, the up-regulation of pro-survival factors, including oncogenic factors and antiapoptotic signaling pathways, and the down-regulation of pro-apoptotic factors, including tumor suppressive factors, confers resistance to cell death in tumor cells, which supports the emergence of a fully immortalized cellular phenotype. This review considers the potential relevance of ubiquitous environmental chemical exposures that have been shown to disrupt key pathways and mechanisms associated with this sort of dysfunction. Specifically, bisphenol A, chlorothalonil, dibutyl phthalate, dichlorvos, lindane, linuron, methoxychlor and oxyfluorfen are discussed as prototypical chemical disruptors; as their effects relate to resistance to cell death, as constituents within environmental mixtures and as potential contributors to environmental carcinogenesis.

β-ESTRADIOL INDUCES CYTOTOXIC EFFECTS TO HUMAN T-LYMPHOMA (JURKAT) CELLS THROUGH OXIDATIVE STRESS

β-estradiol is the most potent estrogen of a group of endogenous estrogen steroids which includes estrone and estriol. This steroid hormone is the most potent natural estrogen, produced mainly by the ovary, placenta, and in smaller amounts by the adrenal cortex, and the male testes. Although β-estradiol protects the renal and cardiovascular systems, the mechanisms involved remain unclear. In this research, we performed the MTT [3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay to evaluate the effect of β-estradiol on human T-lymphoma (Jurkat) cells upon 24 and 48 hours, respectively. Lipid peroxidation assay was also performed to estimate the levels of malondialdehyde (MDA) production in β-estradiol-treated cells. The results of MTT assay demonstrated that low, physiological levels of β-estradiol induce cellular proliferation in Jurkat T-cells. At higher dose of exposure, β-estradiol decreases the viability of Jurkat T-cells compared to the control cells. Data generated from lipid peroxidation assay resulted in a significant increase (p < 0.05) in MDA production in β-estradiol treated sample. Upon 48 h of exposure, MDA concentrations in the sample [µM] (mean ±SE, n = 3) compared to untreated control were 4.9 ± 1.7, 8.1 ± 1,6 11.5 ± 2.2, 21.1 ± 2.3, 19.5 ± 1.4, and 21.5 ± 2.6 in 0, 1, 2, 4, 8, and 16 µM β-estradiol, respectively. In summary, findings from this study demonstrated that high dose of β-estradiol is cytotoxic to Jurkat T-cells. This cytotoxicity is found to be associated with oxidative stress.

Abstract 2291: Arsenic trioxide induces cell cycle arrest, apoptosis and MAPKinase signaling cascade in acute promyelocytic leukemia cells

Arsenic trioxide (ATO) has recently been successfully used in the treatment of all-trans retinoic acid resistant relapsing acute promyelocytic leukemia (APL) patients both alone or combination with other drugs. Its use as induction and consolidation therapy has resulted in complete remission rate of both de novo and relapsed APL patients. However, the detailed molecular mechanisms of its therapeutic action are poorly known. We have used human leukemia (HL-60) cells as a model to elucidate the anti-cancer properties of arsenic trioxide. We hypothesized that ATO arrests cell cycle progression of HL-60 cells at G1 phase and leading to cell death by intrinsic pathway of apoptotic signaling. To test the hypothesis, we used western blotting, confocal imaging and spectrofluoremetric techniques to identify detailed cellular and molecular mechanisms of ATO action in HL-60 cells. We found that the expression levels of pro-apoptotic molecules (Bax) and cytochrome C were up-regulated, while that of anti-apoptotic Bcl2 protein was down-regulated at 4 and 6mg/mL ATO doses significantly. We also found that ATO stimulates signaling molecules like P38, JNK in a dose dependent manner in HL-60 cells. Oxidative stress and related imbalance of pro- and anti-apoptotic molecules lead to change of mitochondrial membrane potential and opening of the mitochondrial membrane transition pores. Our confocal imaging data shows a clear translocation of Bax from cytosol to mitochondria and of cytochrome c from mitochondria to cytosol in ATO-treated cells. A dose-dependent decrease/drop-down in mitochondrial membrane potential was also observed in ATO-treated cells, as evidenced by JC-1 leveling and spectrofluoremetric potential recording. On the basis of these findings, we conclude that mitochondrial pathway of apoptosis plays a key role in ATO pharmacology and should be further assessed as a drug target in APL chemotherapy.

Citation Format: Sanjay Kumar, Clement Yedjou, Paul B. Tchounwou. Arsenic trioxide induces cell cycle arrest, apoptosis and MAPKinase signaling cascade in acute promyelocytic leukemia cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2291. doi:10.1158/1538-7445.AM2014-2291

Abstract 3846: Vitamin D3 potentiates the antitumor effects of arsenic trioxide in human leukemia (HL-60) cells

Arsenic trioxide (ATO) is an innovative form of therapy that has recently been found to benefit acute promyelocytic leukemia (APL) patients. Our laboratory has demonstrated that ATO stimulates oxidative stress therefore causing ATO-induced cytotoxicity in human leukemia (HL-60) cells. Pro-oxidants have been known to play a role in free radical-mediated oxidative stress. Vitamin D3, an active metabolite of vitamin D, inhibits the growth of a number of cancer types such as prostate, breast, colorectal, leukemia, and skin cancers. The specific aim of this research was to determine whether co-exposure to vitamin D3 increases the efficacy of ATO through cytotoxicity, oxidative stress and apoptosis associated with increased ATO toxicity in human leukemia (HL-60) cells. In our current investigation, we performed the MTT assay and trypan blue exclusion test to measure cell viability. We also performed the lipid peroxidation assay to determine the levels of malondialdehyde (MDA) production in HL-60 cells co-exposed to ATO and vitamin D3 in combination. The results of trypan blue exclusion and MTT assay indicated that vitamin D3 exposure potentiates the toxic activity of ATO in HL-60 cells in a dose dependent manner. Co-administration of vitamin D3 and ATO resulted in a significant (P&lt;0.05) increase in MDA level compared to ATO alone. A statistically significant and dose-dependent increase (p &lt;0.05) was recorded in annexin V positive cells (apoptotic cells) with increasing doses of vitamin D3 in ATO-treated cells. This finding was confirmed by the result of DNA laddering assay showing clear evidence of DNA fragmentation in ATO and vitamin D3 treated HL-60 cells. Overall, the present study indicates that vitamin D3 potentiates the antitumor effects of ATO at least part, via cytotoxicity, oxidative stress, nucleosomal fragmentation and phosphatidylserine externalization.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3846. doi:1538-7445.AM2012-3846

Basic mechanisms of arsenic trioxide (ATO)-induced apoptosis in human leukemia (HL-60) cells

Background

Acute promyelocytic leukemia (APL) is a blood cancer that affects people of all ages and strikes about 1,500 patients in the United States each year. The standard treatment of APL has been based on the combined administration of all-trans retinoic acid and chemotherapy including anthracyclins and cytarabine. However, 10-20% of patients relapse, with their disease becoming resistant to conventional treatment. Recently the Food and Drug Administration has approved the use of arsenic trioxide (ATO) or Trisenox for the treatment of APL, based on clinical studies showing a complete remission, especially in relapsed patients. In a recently published study we demonstrated that ATO pharmacology as an anti-cancer drug is associated with its cytotoxic and genotoxic effects in human leukemia cells.

Methods

In the present study, we further investigated the apoptotic mechanisms of ATO toxicity using the HL-60 cell line as a test model. Apoptosis was measured by flow cytometry analysis of phosphatidylserine externalization (Annexin V assay) and caspase 3 activity, and by DNA laddering assay.

Results

Flow cytometry data showed a strong dose-response relationship between ATO exposure and Annexin-V positive HL-60 cells. Similarly, a statistically significant and dose-dependent increase (p <0.05) was recorded with regard to caspase 3 activity in HL60 cells undergoing late apoptosis. These results were confirmed by data of DNA laddering assay showing a clear evidence of nucleosomal DNA fragmentation in ATO-treated cells.

Conclusion

Taken together, our research demonstrated that ATO represents an apoptosis-inducing agent and its apoptotic mechanisms involve phosphatidylserine externalization, caspase 3 activation and nucleosomal DNA fragmentation.

Abstract 4380: Cadmium-induced toxicity in human liver carcinoma (HepG2) cells: Involvement of oxidative stress, phosphatidylserine externalization, and nucleosomal DNA fragmentation

Human liver carcinoma (HepG2) cells were used as a test model to evaluate the in vitro cytotoxicity of cadmium and to determine whether oxidative stress plays a key role in cadmium induced apoptosis. To achieve this goal, we performed the MTT assay to measure cell viability and lipid peroxidation to determine the level of oxidative cell/tissue damage. The annexin V assay and DNA fragmentation analysis were performed for early and late apoptosis, respectively. The results of MTT assay indicated that cadmium chloride induces toxicity to HepG2 cells in a dose-dependent manner, showing a 24 h-LD50 of 3.6 µg/mL. We detected a significant increase in lipid peroxidation (malondialdehyde) concentrations in cadmium chloride-treated HepG2 cells compared to the control. Similarly, a strong dose-response relationship (p &lt; 0.05) was obtained in connection with cadmium chloride-induced apoptosis. Cadmium chloride-induced apoptosis was characterized by a significant increase in the percentage of annexin-V positive cells (apoptotic cells), as well by the occurrence of nucleosomal DNA fragmentation. In summary, this study indicated cadmium chloride is highly cytotoxic to HepG2 cells. This cytotoxic effect may be mediated at least in part through oxidative cell/tissue damage, phosphatidylserine externalization and occurrence of nucleosomal DNA fragmentation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4380.

Abstract A39: In vitro mechanisms of cadmium-induced apoptosis in human liver carcinoma (HEPG2) cells

Cadmium is a heavy metal that has been shown to manifest its toxicity in humans and animals. Many documented in vitro studies have shown that cadmium produces various genotoxic effects such as DNA damage and chromosomal aberrations. Diseases like autoimmune thyroidism are thought to be caused, in large part, by overexposure to cadmium. Although there have been numerous studies examining the effects of cadmium in animal models and a few case studies involving communities where cadmium contamination has occurred, the mechanisms of the carcinogenic activity of cadmium is not clearly defined. Therefore, the purpose of this study was to elucidate some of the molecular mechanisms that are involved in cadmium-induced toxicity in human liver carcinoma (HepG2) cells. To achieve this goal, we performed the MTT assay to measure cell viability. Annexin V assay and DNA fragmentation analysis were performed for early and late apoptosis, respectively. The result of MTT assay indicated that cadmium induces toxicity to HepG2 cells in a dose-dependent manner, showing a 24 h-LD50 of 3.6 µg/mL cadmium chloride. Similarly, a strong dose-response relationship (p &lt; 0.05) was obtained in connection with cadmium chloride-induced apoptosis. Cadmium chloride-induced apoptosis was characterized by a significant increase in the percentages of annexin-V positive cells, as well by the occurrence of nucleosomal DNA fragmentation. In summary, this study indicated cadmium chloride is highly cytotoxic to HepG2 cells. This cytotoxic effect may be mediated at least in part through phosphatidylserine externalization and occurrence of nucleosomal DNA fragmentation.

Citation Information: Cancer Prev Res 2010;3(1 Suppl):A39.

Ascorbic Acid Potentiation of Arsenic Trioxide Anticancer Activity Against Acute Promyelocytic Leukemia

INTRODUCTION: Acute promyelocytic leukemia (APL) is a malignant disorder of the white blood cells. Arsenic trioxide (As(2)O(3)) has been used as a therapeutic agent to treat APL and other tumors. Studies suggest that ascorbic acid (AA) supplementation may improve the clinical outcome of As(2)O(3) for APL patients. Our aim was to use human leukemia (HL-60) APL-cells as an in vitro test model to evaluate the effect of physiologic doses of AA on As(2)O(3)-induced toxicity and apoptosis of HL-60 cells. METHODS: HL-60 cells were treated either with a pharmacologic dose of As(2)O(3) alone and with several physiologic doses of AA. Cell survival was determined by trypan blue exclusion test. The extent of oxidative cell/tissue damage was determined by measuring lipid hydroperoxide concentration by spectrophotometry. Cell apoptosis was measured by flow cytometry using Annexin-V and propidium iodide (PI) staining. RESULTS: AA treatment potentiates the cytotoxicity of As(2)O(3) in HL-60 cells. Viability decreased from (58 +/- 3)% in cells with As(2)O(3) alone to (47 +/- 2)% in cells treated with 100 microM AA and 6 microg/mL As(2)O(3) with P < 0.05. There was a significant (P < 0.05) increase in lipid hydroperoxide concentrations in HL-60 cells co-treated with AA compared to As(2)O(3) alone. Flow cytometry assessment (Annexin V FITC/PI) suggested that AA co-treatment induces more apoptosis of HL-60 cells than did As(2)O(3) alone, but this was not statistically significant. Taken together, our experiment indicates that As(2)O(3) induced in vitro cell death and apoptosis of HL-60 cells. Administration of physiologic doses of AA enhanced As(2)O(3)-induced cytotoxicity, oxidative cell/tissue damage, and apoptosis of HL-60 cells through externalization of phosphatidylserine. CONCLUSIONS: These suggest that AA may enhance the cytotoxicity of As(2)O(3), suggesting a possible future role of AA/As(2)O(3) combination therapy in patients with APL.

Preclinical assessment of vernonia amygdalina leaf extracts as DNA damaging anti-cancer agent in the management of breast cancer

Breast cancer is the leading cause of death among women between 40 and 55 years of age and is the second overall cause of death among women. Fortunately, the mortality rate from breast cancer has decreased in recent years due to an increased emphasis on early detection and more effective treatments. Despite early detection, conventional and chemotherapeutic methods of treatment, about 7% of women still died every year. Hence, the aim of the present study was to assess the therapeutic efficacy of vernonia amygdalina (VA) leaf extracts as anti-cancer agent against human breast cancer in vitro using the MTT [3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] and alkaline single cell gel electrophoresis (Comet) assays, respectively. In this experiment, human breast adenocarcinoma (MCF-7) cells were treated with different doses of VA leaf extracts for 48 hours. Data obtained from the MTT assay showed that VA significantly ((P < 0.05) reduced the viability of MCF-7 cells in a dose-dependent manner upon 48 hours of exposure. Data generated from the comet assay also indicated a slight dose-dependent increase in DNA damage in MCF-7 cells associated with VA treatment. We observed a slight increase in comet tail-length, tail arm and tail moment, as well as in percentages of DNA cleavage at all doses tested, showing an evidence that VA-induced minimal genotoxic damage in MCF-7 cells. Taken together, our findings suggest that VA treatment moderately (P < 0.05) reduces cellular viability and induces minimal DNA damage in MCF-7 cells. These findings provide evidence that VA extracts represent a DNA-damaging anti-cancer agent against breast cancer and its mechanisms of action functions, at least in part, through minimal DNA damage and moderate toxicity in tumors cells.

Oxidative stress, DNA damage, and antioxidant enzyme activity induced by hexavalent chromium in Sprague-Dawley rats

Chromium is a widespread industrial compound. The soluble hexavalent chromium Cr (VI) is an environmental contaminant widely recognized as carcinogen, mutagen, and teratogen toward humans and animals. The fate of chromium in the environment is dependent on its oxidation state. The reduction of Cr (VI) to Cr (III) results in the formation of reactive intermediates leading to oxidative tissue damage and cellular injury. In the present investigation, Potassium dichromate was given intraperitoneally to Sprague-Dawley rats for 5 days with the doses of 2.5, 5.0, 7.5, and 10 mg/kg body weight per day. Oxidative stress including the level of reactive oxygen species (ROS), the extent of lipid peroxidation and the activity of antioxidant enzymes in both liver and kidney was determined. DNA damage in peripheral blood lymphocytes was determined by single-cell gel electrophoresis (comet assay). The results indicated that administration of Cr (VI) had caused a significant increase of ROS level in both liver and kidney after 5 days of exposure, accompanied with a dose-dependent increase in superoxide dismutase and catalase activities. The malondialdehyde content in liver and kidney was elevated as compared with the control animals. Dose- and time-dependent effects were observed on DNA damage after 24, 48, 72, and 96 h posttreatment. The results obtained from the present study showed that Cr (VI) could induce dose- and time-dependent effects on DNA damage, both liver and kidney show defense against chromium-induced oxidative stress by enhancing their antioxidant enzyme activity. However, liver was found to exhibit more antioxidant defense than the kidney.

GENOTOXIC MECHANISMS OF ARSENIC TRIOXIDE IN HUMAN JURKAT T-LYMPHOMA CELLS

Arsenic trioxide (As(2)O(3)) has cytotoxic effects on several cancer cell lines. However, the molecular mechanisms of action remain to be elucidated. Hence, the aim of the present study was to evaluate the cytotoxicity and genotoxicity induced by As(2)O(3) in a human Jurkat T-lymphoma cell line using the trypan blue exclusion test and alkaline single cell gel electrophoresis (Comet) assays, respectively. Jurkat T-cells were treated with different doses of As(2)O(3) for 24 and 48 h prior to cytogenetic assessment. Data obtained from the trypan blue exclusion test indicated that As(2)O(3) significantly (p < 0.05) reduced the viability of Jurkat T-cells in a dose and time-dependent manner. Data generated from the comet assay also indicated a significant dose and time-dependent increase in DNA damage in Jurkat T-cells associated with As(2)O(3) exposure. We observed a significant increase (P < 0.05) in comet tail-length, tail arm and tail moment, as well as in percentages of DNA cleavage at all doses tested, showing an evidence As(2)O(3) -induced genotoxic damage in Jurkat T-cells. This study confirms that the comet assay is a sensitive and effective method to detect DNA damage caused by heavy metals such as arsenic. Taken together, our findings suggest that As(2)O(3) exposure significantly (p < 0.05) reduces cellular viability and induces DNA damage in human Jurkat T-lymphoma cells.

Oxidative effects of lead in young and adult Fisher 344 rats

Lead poisoning has been extensively studied over the years. Many adverse physiological and behavioral impacts on the human body have been reported due to the entry of this heavy metal. It especially affects the neural development of children. The current study investigates the effect of lead exposure in young (1.5 months) and adult (10 months) male Fisher 344 rats. Five weeks of lead administration resulted in a profound change in the lead levels in the red blood cells (RBCs) of the young lead-exposed group (37.0 +/- 4.47 microg/dl) compared to the control (<1 microg/dl) and adult (27.4 +/- 8.38 microg/dl) lead-exposed groups. Therefore, this study confirms the fact that gastrointestinal absorption of lead in young is greater than that of adults. Furthermore, glutathione and glutathione disulfide (GSSG) levels in RBCs, liver, and brain tissues were measured to determine thiol status; malondialdehyde (MDA) levels of lipid peroxidation and catalase activity were measured to assess changes in oxidative stress parameters. Liver GSSG and MDA levels were significantly higher in the young lead-exposed group than those in the adult lead-exposed group. In RBCs and brains, however, adult lead-exposed animals have shown more elevated MDA levels than young animals exposed to the same lead treatment.