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Endogenous activation of the aryl hydrocarbon receptor (AHR) is required for normal vascular development. This biology led us to investigate the interplay between the AHR and vascular physiology by using an in vitro model of fluid shear stress. Using this system, we show that fluid flow induces a robust AHR-mediated increase in CYP1 expression. Furthermore, we demonstrate that incubation with sheared bovine or human sera is sufficient for AHR activation, indicating that direct cellular exposure to shear stress is not required for this response. Fractionation of sera by size and density revealed the AHR-activating factor to be low-density Iipoprotein (LDL). Purified LDL (0.1 mg/mI) from sheared sera induces a 6-fold increase in AHR-mediated signaling as compared with LDL purified from static sera. Similar results were obtained by exposing a purified fraction of LDL to fluid flow, suggesting that shear stress is capable of directly modifying LDL structure and/or function. In addition, we show that LDL can be converted to an AHR-activating species by conventional methods of lipoprotein modification, such as NaOCI oxidation. Finally, we demonstrate that an increased level of AHR-activating LDL is present in the sera of AHR null mice as compared with heterozygous littermates, suggesting a role for the Ahr locus in the physiological response to modified WI in vivo. Overall, these data demonstrate a previously undescribed relationship between LDL modification and AHR biology and provide a potential explanation for the vascular abnormalities observed in AHR null mice. [ABSTRACT FROM AUTHOR]