Overexpression of latent transforming growth factor-β binding protein 1 (LTBP-1) in dioxin receptor-null mouse embryo fibroblasts

The aryl hydrocarbon receptor (AhR) is a transcriptional regulator of genes involved in xenobiotic metabolism. Increasingly clear is also the role of the AhR in the control of cell growth and proliferation. By analyzing differential patterns of gene expression between wild-type (AhR+/+) and null (AhR-/-) mouse embryo fibroblasts (MEF), we have identified latent transforming growth factor-P binding protein 1 (LTBP-1) as a negatively AhR-regulated gene in the absence of xenobiotics. Ltbp-1 mRNA and protein expression were markedly increased in AhR-/- MEF Furthermore, secreted LTBP-1 was elevated in the culture medium and the extracellular matrix of AhR-null MEF Actinomycin D inhibited Ltbp-1 mRNA overexpression, suggesting regulation at the transcriptional level. AhR activation by dioxin (TCDD) downregulated Ltbp-1, again suggesting an AhR-regulated mechanism. Treatment of AhR+/+ MEF with transforming growth factor-beta (TGF-beta) downregulated AhR and, simultaneously, increased Ltbp-1, further supporting the role of this receptor in LTBP-1 expression. AhR-/- conditioned medium had higher levels of active and total TGF-beta activity, suggesting a role for LTBP-1 in maintaining extracellular TGF-beta concentrations. TGF-beta did not appear to directly regulate Ltbp-1 given that addition of TGFbeta neutralizing antibody or TGFbeta protein to AhR-/- MEF had no effect on Ltbp-1 expression. AhR-/- MEF had lower levels of matrix metalloproteinase 2 (MMP-2) activity, which could not be attributable to MMP-2 mRNA downregulation or MMP-inhibitors Timp-1 and Timp-2 overexpression. These data identify LTBP-I as one of the few AhR-regulated genes not involved in xenobiotic metabolism and also support the implication of the AhR in controlling TGFbeta activity and cell proliferation.