Deciphering posttranslational codes of the dioxin receptor

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The dioxin receptor (DR) is a protein which protects human cells by binding xenobiotics, ie foreign or anti-nutritional chemicals found in food sources and the general environment. When these chemicals bind the DR, it becomes an active gene regulatory protein, turning on genes that are involved in breakdown and excretion of the xenobiotics. Recently it has been found that the DR performs other important functions which are unrelated to xenobiotic breakdown. These include blood vessel development in the embryonic liver and hormone production during the estrous cycle. These observations imply that natural physiological mechanisms also exist for activating the DR, providing it with a separate code to perform these innate functions. A number of man-made chemicals, such as dioxins and PCBs, are especially good at activating the DR. However, they have chlorinated chemical structures, which are not broken down by the protective system. This creates a wide range of severe toxic responses. It has been established that toxicities result from persistent hyperactivation of the DR, but how this hyperactivation induces the toxic outcomes is not known. As the DR has roles in early development and estrogen production, this project will investigate how the DR becomes activated to perform these functions. Our initial experiments have shown that the DR can be activated by normal cell signalling systems, which induce distinct modifications (a distinct code) to the protein. We are comparing this code of modifications to those induced by xenobiotics which are able to be broken down, and dioxins which are resistant to breakdown. We hypothesise that dioxins will give an excessive code of activating modifications, resulting in uncontrolled regulation of genes used in both the developmental and xenobiotic breakdown pathways. We will explore the hypothesis that this gross loss of gene regulatory control underpins the multifarious toxicities of dioxin poisoning.