Kodavanti P R, Tilson H A
Cellular and Molecular Toxicology Branch, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, USA.
Neurotoxicology. 1997;18(2):425-41.
We have explored the effects of PCBs on Ca(2+)-homeostasis and inositol phosphates in an attempt to understand cellular mechanism(s) for neurotoxicity of PCBs. The selected prototypic congeners have non-dioxin-like (2,2'-dichlorobiphenyl; 2,2'-DCB; IUPAC # 4; ortho-substituted) and dioxin-like (3,3',4,4',5-pentachlorobiphenyl; 3,3',4,4',5-PeCB; IUPAC # 126; non-ortho substituted) properties. The hypothesis is that some PCBs in vitro alter Ca(2+)-homeostasis and interfere with intracellular second messengers. One of the consequences of this perturbation is protein kinase C (PKC) translocation, and these events could lead to cytotoxicity. Our results indicate that the non-dioxin like PCB (ortho-substituted one) is active in vitro and perturbed signal transduction mechanism including Ca(2+)-homeostasis and PKC translocation. The effects were seen at relatively low concentrations (5-50 microM), whereas higher concentrations (> 200 microM) were required to produce cytotoxicity. Results from SAR, in general, indicate that congeners with chlorine substitutions at ortho-position or low lateral substitutions (mostly meta-) are active in vitro where as non-ortho congeners are inactive. In summary, these results indicate that low lateral substitution (especially without para-substitution that favor coplanarity) or high lateral content in the presence of ortho-substitution (to hinder coplanarity) may be the most critical structural requirement underlying the activity of PCB congeners in vitro. Additional experiments with polychlorinated diphenyl ethers (PCDEs) and their analogs, where coplanarity is difficult regardless of degree and pattern of chlorination, provided important information supporting our hypothesis that coplanarity plays a key role in the activity of PCBs in vitro. For example, a PCB congener with 3,3',4,4'-chlorine substitutions is not active whereas a PCDE with the same chlorine substitutions is active. Similarity, 4,4'-DCB is not active whereas PCDE with 4,4'-substitutions is active. One major structural difference in PCDE when compared to the corresponding PCB is non-coplanarity. The PCBs compared here are coplanar and not active, whereas PCDEs are non-coplanar and active in vitro in neuronal preparations. Molecular mechanics calculations and conformational searches confirmed the extent of coplanarity among PCBs and PCDEs. Non-ortho PCBs are more coplanar in nature when compared to ortho-PCBs and PCDEs. These results demonstrate that the extent of coplanarity of certain chlorinated aromatic hydrocarbons can affect their potency in vitro, and ortho-substitutions on the biphenyl, which increase non-coplanarity, are characteristics of the most active PCB congeners.
我们研究了多氯联苯（PCBs）对钙离子稳态和肌醇磷酸的影响，以试图了解PCBs神经毒性的细胞机制。所选的典型同系物具有非二噁英类（2,2'-二氯联苯；2,2'-DCB；IUPAC编号4；邻位取代）和二噁英类（3,3',4,4',5-五氯联苯；3,3',4,4',5-PeCB；IUPAC编号126；非邻位取代）特性。我们的假设是，某些PCBs在体外会改变钙离子稳态并干扰细胞内第二信使。这种干扰的后果之一是蛋白激酶C（PKC）易位，而这些事件可能导致细胞毒性。我们的结果表明，非二噁英类PCB（邻位取代的那种）在体外具有活性，并扰乱了包括钙离子稳态和PKC易位在内的信号转导机制。在相对较低的浓度（5 - 50微摩尔）下就能观察到这些效应，而产生细胞毒性则需要更高的浓度（>200微摩尔）。一般来说，构效关系研究结果表明，邻位有氯取代或低侧链取代（大多为间位）的同系物在体外具有活性，而非邻位同系物则无活性。总之，这些结果表明，低侧链取代（尤其是没有有利于共平面性的对位取代）或在存在邻位取代（以阻碍共平面性）的情况下高侧链含量，可能是PCBs同系物在体外具有活性的最关键结构要求。对多氯二苯醚（PCDEs）及其类似物进行的额外实验提供了重要信息，支持了我们的假设，即共平面性在PCBs体外活性中起关键作用。在PCDEs中，无论氯化程度和模式如何，共平面性都很难实现。例如，具有3,3',4,4'-氯取代的PCB同系物无活性，而具有相同氯取代的PCDE则有活性。同样，4,4'-DCB无活性，而具有4,4'-取代的PCDE有活性。与相应的PCB相比，PCDE的一个主要结构差异是非共平面性。此处比较的PCBs是共平面的且无活性，而PCDEs是非共平面的且在神经元制剂中体外有活性。分子力学计算和构象搜索证实了PCBs和PCDEs之间的共平面程度。与邻位PCBs和PCDEs相比，非邻位PCBs本质上更具共平面性。这些结果表明，某些氯化芳烃的共平面程度会影响其体外效力，并且联苯上的邻位取代增加了非共平面性，这是最具活性的PCB同系物的特征。
