Impact of aryl hydrocarbon receptor activation on the cortisol response to stress in rainbow trout

Persistent organic pollutants like halogenated aromatic hydrocarbons (HAHs) are known to disrupt endocrine function in a wide range of wild life species, including fish. HAHs act as endocrine disruptors by either mimicking hormone action or interfering with hormone synthesis, transport, binding and metabolism, all of which can lead to homeostatic dysfunction. Cortisol, the principal glucocorticoid in teleost fishes, is involved in the physiological and metabolic adjustments that are important for animals to cope with stress. The stress-induced elevation of plasma cortisol levels is under the control of the hypothalamus-pituitary-interrenal (HP)) axis and this function is disrupted by HAHs in fish. HAHs are known to elicit the toxic responses by acting via aryl hydrocarbon receptor (AhR). However, the mechanism of action is not known. The objective of this research was to elucidate the mode of action of HAHs in impairing corticosteroidogenesis in rainbow trout in vivo and in vitro using interrenal tissue preparations. A known AhR agonist (beta-naphthoflavone; BNF) and antagonist (alpha-naphthoflavone; ANF) either alone or in combination were used to elucidate the role played by AhR in the steroidogenic process. BNF induced CYP1A1 protein expression and mRNA abundance in liver and head kidney, respectively, suggesting the activation of AhR signaling, whereas ANF failed to block BNF-induced CYP1A1 expression. BNF attenuated in vitro ACTH- and 8-br-cAMP-stimulated cortisol production. This decrease in steroid production corresponded with BNF-mediated attenuation of ACTH-induced steroidogenic acute regulatory protein (StAR) and P450side chain cleavage enzyme (P450scc) mRNA abundance, the rate-limiting steps in steroidogenesis. In vitro incubation of head kidney tissue slices with radiolabelled steroid substrate (7-3[H] pregnenolone) confirmed the lack of effect on the enzyme activities downstream of P450scc by BNF. This was also reflected in the 11beta-hydroxylase mRNA abundance, which showed no change with BNF. However, these effects were not abolished by ANF and, indeed, further characterization proved that ANF was a weak AhR agonist and not an antagonist in rainbow trout hepatocytes. Consequently, resveratrol, an AhR antagonist used in mammalian systems, was tested for its utility as an AhR antagonist in trout hepatocytes. Resveratrol at high doses (10-5 M) completely abolished BNF-induced CYP1A1 and AhR protein expression confirming the reliability of this drug as an AhR antagonist in fish. (Abstract shortened by UMI.)...