Center for Integrative Toxicology, Food Safety and Toxicology Center, Department of Pediatrics and Human Development, College of Human Medicine, Michigan State University, East Lansing, MI 48824, USA.
Toxicology. 2010 Mar 30;270(1):18-34. doi: 10.1016/j.tox.2009.11.019. Epub 2009 Dec 3.
Given the reality of the inadequacies of current concepts of the mechanisms of chemical toxicities, of the various assays to predict toxicities from current molecular, biochemical, in vitro and animal bioassays, and of the failure to generate efficacious and safe chemicals for medicines, food supplements, industrial, consumer and agricultural chemicals, the recent NAS Report, "Toxicity Testing in the 21st Century: A Vision and a Strategy", has drawn attention to a renewed examination of what needs to be done to improve our current approach for better assessment of potential risk to human health. This "Commentary" provides a major paradigm challenge to the current concepts of how chemicals induce toxicities and how these various mechanisms of toxicities can contribute to the pathogenesis of some human diseases, such as birth defects and cancer. In concordance with the NAS Report to take "... advantage of the on-going revolution in biology and biotechnology", this "Commentary" supports the use of human embryonic and adult stem cells, grown in vitro under simulated "in vivo niche conditions". The human being should be viewed "as greater than the sum of its parts". Homeostatic control of the "emergent properties" of the human hierarchy, needed to maintain human health, requires complex integration of endogenous and exogenous signaling molecules that control cell proliferation, differentiation, apoptosis and senescence of stem, progenitor and differentiated cells. Currently, in vitro toxicity assays (mutagenesis, cytotoxicity, epigenetic modulation), done on 2-dimensional primary rodent or human cells (which are always mixtures of cells), on immortalized or tumorigenic rodent or human cell lines do not represent normal human cells in vivo [which do not grow on plastic and which are in micro-environments representing 3 dimensions and constantly interacting factors]. In addition, with the known genetic, gender, and developmental state of cells in vivo, any in vitro toxicity assay will need to mimic these conditions in vitro. More specifically, while tissues contain a few stem cells, many progenitor/transit cells and terminally differentiated cells, it should be obvious that both embryonic and adult stem cells would be critical "target" cells for toxicity testing. The ultimate potential for in vitro testing of human stem cells will to try to mimic a 3-D in vitro micro-environment on multiple "organ-specific and multiple genotypic/gender "adult stem cells. The role of stem cells in many chronic diseases, such as cancer, birth defects, and possibly adult diseases after pre-natal and early post-natal exposures (Barker hypothesis), demands toxicity studies of stem cells. While alteration of gene expression ("toxico-epigenomics") is a legitimate endpoint of these toxicity studies, alteration of the quantity of stem cells during development must be serious considered. If the future utility of human stem cells proves to be valid, the elimination of less relevant, expensive and time-consuming rodent and 2-D human in vitro assays will be eliminated.
鉴于当前对化学毒性机制的认识不足，各种当前分子、生化、体外和动物生物测定方法预测毒性的能力有限，以及未能为药物、食品补充剂、工业、消费和农业化学品生成有效和安全的化学品，最近的 NAS 报告《21 世纪的毒性测试：愿景与策略》提请人们注意需要重新审视哪些工作来改进我们目前的方法，以更好地评估对人类健康的潜在风险。这篇“评论”对当前化学诱导毒性的概念以及这些不同的毒性机制如何导致某些人类疾病（如出生缺陷和癌症）的发病机制提出了重大的范式挑战。为了符合 NAS 报告中“利用生物学和生物技术的持续革命”的要求，这篇“评论”支持使用在模拟“体内生态位条件”下体外培养的人类胚胎和成体干细胞。人类应该被视为“大于其各部分的总和”。维持人类健康所需的人体层次结构“涌现特性”的体内稳态控制需要复杂地整合控制干细胞、祖细胞和分化细胞增殖、分化、凋亡和衰老的内源性和外源性信号分子。目前，在二维原代啮齿动物或人类细胞（通常是细胞混合物）上进行的体外毒性测定（诱变、细胞毒性、表观遗传调节），以及在永生化或致瘤性啮齿动物或人类细胞系上进行的体外毒性测定，均不能代表体内正常的人类细胞[这些细胞不能在塑料上生长，并且处于代表三维的微环境中，并且不断与各种因素相互作用]。此外，由于体内细胞的已知遗传、性别和发育状态，任何体外毒性测定都需要在体外模拟这些条件。更具体地说，虽然组织中含有少量的干细胞，但有许多祖细胞/过渡细胞和终末分化细胞，显然胚胎和成体干细胞都将是毒性测试的关键“靶”细胞。人类干细胞的体外测试的最终潜力将是尝试在多个“器官特异性和多种基因型/性别”的成人干细胞上模拟 3-D 体外微环境。干细胞在许多慢性疾病（如癌症、出生缺陷，以及可能在产前和产后早期暴露后的成人疾病）中的作用，要求对干细胞进行毒性研究。虽然基因表达的改变（“毒代-表观基因组学”）是这些毒性研究的一个合理终点，但在发育过程中干细胞数量的改变必须认真考虑。如果未来人类干细胞的应用被证明是有效的，那么将消除不那么相关、昂贵且耗时的啮齿动物和 2-D 体外人类测定。
