Biphasic hCAR Inhibition-Activation by Two Aminoazo Liver Carcinogens

Detailed dose-response data recently archived by the National Center for Biotechnology Information (NCBI) identified 853 human CAR (hCAR) agonists by quantitative high-throughput screening (qHTS) assays applied to >9,000 chemicals tested at ≥14 concentrations using n = 3–48 replicates. By re-examining NCBI data on 746 agonists with replicate data sets each satisfying additional quality criteria, ∼95% had average values of agonist-specific Hill-model slopes estimated by NCBI that exceed 1 (i.e., exhibited an overall sublinear low-dose dose-response), and two unambiguously biphasic hCAR inhibitor-agonists were identified, 4-aminoazobenzene (n = 37) and ortho-aminoazotoluene (n = 3), both of which also cause rodent liver tumors. Although evidently rare among hCAR agonists, such biphasic responses add to evidence that nuclear receptors can exhibit complex patterns of low-dose response, consistent with previous observations and theoretical predictions for endpoints governed by ultrasensitive molecular switches. The pronounced biphasic hCAR response pattern observed for 4-aminoazobenzene is particularly noteworthy insofar as it was identified with statistical power that exceeds that of most if not all other receptor-mediated biphasic cellular responses to any single-chemical exposure reported to date.