Expression and immunity: new findings on Ahr interactions.

The transcription factor aryl hydrocarbon receptor (Ahr) can activate a multitude of genes that regulate the growth and division of ceils, including processes leading to cancer. To date, little is known about the biochemical processes that mediate the activation of Ahr receptors in the body, or about the endogenous ligands that bind to and activate these receptors. This month, researchers from the University of Louisville, Texas A&M University, and the University of New Mexico, under the leadership of Charles D. Johnson, report on microarray analyses that illuminate the complexity of Ahr interactions and thus provide a focus for future experiments [EHP 112:403-412]. These results include a previously unknown relationship between Ahr and genes involved in the activation of the immune system.

The researchers modeled the identification of relevant components of the biological response to Ahr ligands using transcriptional profiles of cells from murine embryonic heart, kidney, and vascular smooth muscle cells. The data were analyzed using methods developed at the Texas A&M University Genomic Signal Processing Laboratory to decipher multivariate, nonlinear relationships among genes.

The analysis focused on 200 of the 12,000 clones on the microarrays that showed the greatest changes in response to hydrocarbon exposures. The changes in each of these genes were quantified in relationship to five target genes, selected because they are known to respond to ligands of Ahr. The first target gene was Ahr itself. The second was Cyp1b1, which is involved in the synthesis of steroids. The third was Igfbp-5, which regulates the functions of the bones, kidneys, and mammary glands. The fourth was Lox, which transcribes a copper-containing enzyme involved in the creation of connective tissue. And the fifth was Opn, which transcribes osteopontin, a protein involved in the mineralization of bone and a component of human atherosclerotic plaques.

Activity in all five target genes was related to some degree; when one was expressed, so were the other four. The analysis was not designed to determine whether activated genes were upstream or downstream of each other--in other words, which of a given set of genes initiated a reaction.

The expression of Ahr was most closely linked to that of lymphocyte antigen 6, locus e, which is involved in the activation of T cells. This relationship had not been predicted by previous studies and helps fill a gap in the map of relationships among Ahr and retinoids (forms of vitamin A). Lymphocyte antigen 6e responds to the presence of retinoic acid, and recent research has shown that Ahr controls the expression of genes that metabolize this compound. Thus, computational strategies allowed the delineation of connections between Ahr and retinoic acid that otherwise could not have been predicted in the absence of biological information.

In other relationships found in this study, Cyp1b1 was best predicted by the gene that codes for spleen tyrosine kinase, which participates in signaling leading to activation of the transcription factor NF-[kappa]B. Igfbp-5 was most frequently predicted by Opn; Lox was best predicted by lymphocyte antigen 6 complex, locus H; and Opn was most often predicted by brain-derived neurotrophic factor, interleukin 6, and proliferin.

The authors write that the computational approach they used allowed them to begin constructing gene networks that define broad-ranging interactive biological relationships. "Although the biological bases for these theoretical relationships must be investigated further," they conclude, "the number of possible combinations is now reduced to a manageable size that can be systematically scrutinized using established molecular methodologies."