Preventing germ cell death by inactivating aryl hydrocarbon receptor (AHR)

The effect of hydroalcoholic extract of Cichorium intybus leaf on aryl hydrocarbon receptor expression in the testis of Wistar rats exposed to cigarette smoke

Objective

Cigarette smoke (CS) contains compounds such as reactive oxygen species (ROS). Oxidative stress caused by excessive ROS eventually leads to germ cell apoptosis and male infertility. The leaves of Cichorium intybus (chicory) are rich in natural antioxidants, but their protective effects on the adverse effects of CS on testicular tissue have not been studied.

Materials and Methods

24 Wistar rats were classified into four groups: control, extract: treatment with chicory extract (200 mg/kg body weight/day) for 13 weeks, smoke: exposed to CS for 13 weeks, and smoke + extract: exposed to CS and treated with the C. intybus extract. Histological and biochemical analyses and apoptosis assay were done, and Ahr, and Cyp1a1 expression was determined.

Results

Treatment with C. intybus compensated for the reduction of Sertoli cells, spermatogonia, spermatocytes, and spermatids caused by CS. Chicory extract reduced free radicals and improved antioxidant status. The lowest and highest percentage of apoptotic cells was observed in the extract and smoke groups, respectively, while simultaneous treatment with C. intybus extract led to a significant reduction of apoptotic cells. The mean Ahr levels in the control, extract, smoke and smoke + extract groups were 1.00±0.57, 1.93±0.25, 5.98±0.42, and 0.62±0.22, respectively (p˂0.05). The mean levels of Cyp1a1 expression in the control, extract, smoke and smoke + extract groups were 1.00±0.31, 2.28±0.65, 5.55±0.40, and 0.21±0.23 (p˂0.05).

Conclusion

The C. intybus extract probably affected Cyp1a1 expression by downregulation of Ahr. These led to a decrease in free radicals and apoptosis, and an improvement in antioxidant status.

Divergent Ah Receptor Ligand Selectivity during Hominin Evolution

We have identified a fixed nonsynonymous sequence difference between humans (Val381; derived variant) and Neandertals (Ala381; ancestral variant) in the ligand-binding domain of the aryl hydrocarbon receptor (AHR) gene. In an exome sequence analysis of four Neandertal and Denisovan individuals compared with nine modern humans, there are only 90 total nucleotide sites genome-wide for which archaic hominins are fixed for the ancestral nonsynonymous variant and the modern humans are fixed for the derived variant. Of those sites, only 27, including Val381 in the AHR, also have no reported variability in the human dbSNP database, further suggesting that this highly conserved functional variant is a rare event. Functional analysis of the amino acid variant Ala381 within the AHR carried by Neandertals and nonhuman primates indicate enhanced polycyclic aromatic hydrocarbon (PAH) binding, DNA binding capacity, and AHR mediated transcriptional activity compared with the human AHR. Also relative to human AHR, the Neandertal AHR exhibited 150-1000 times greater sensitivity to induction of Cyp1a1 and Cyp1b1 expression by PAHs (e.g., benzo(a)pyrene). The resulting CYP1A1/CYP1B1 enzymes are responsible for PAH first pass metabolism, which can result in the generation of toxic intermediates and perhaps AHR-associated toxicities. In contrast, the human AHR retains the ancestral sensitivity observed in primates to nontoxic endogenous AHR ligands (e.g., indole, indoxyl sulfate). Our findings reveal that a functionally significant change in the AHR occurred uniquely in humans, relative to other primates, that would attenuate the response to many environmental pollutants, including chemicals present in smoke from fire use during cooking.