Role of adipokinetic hormones in antioxidative stress response

Adipokinetic hormones (AKHs) are insect neuropeptides that control energy metabolism via the mobilization of stored nutrients, however, their activities also include stimulation of the heart, locomotor and immune systems, and even regulation of some reproductive processes. Thus, AKHs behave as typical stress hormones: they stimulate anti-stress reactions to eliminate or at least to reduce the impact of stress to insect organisms. In a series of publications, we demonstrated that AKHs are also involved in the activation of defensive mechanisms that protect insects against oxidative stress.

Using our insect model species (the firebug Pyrrhocoris apterus and the cotton leafworm Spodoptera littoralis) we showed that the application of oxidative stressors (paraquat, tannic acid) increases the AKH level in both the central nervous system, where those hormones are synthesised, and also in the haemolymph. Further, AKHs mobilize anti-oxidative mechanisms that ameliorate damage incurred by oxidative stress such as increased protein carbonylation, decrease of reduced glutathione (GSH) level and impaired total antioxidant activity in haemolymph (Večeřa et al. 2012, Comp. Biochem. Physiol. C 155, 389-395). AKHs also efficiently modulate the activity of superoxide dismutase, catalase and glutathione-S-trasnferase, the principal enzymes involved in anti-oxidative stress protection of insect body (Bednářová et al. 2013, Physiol. Entomol. 38, 54-62; unpublished data).

Our results suggest that the activity of these enzymes is controlled at the post-translational level rather than via regulation of expression of their genes; at least the catalase and superoxide dismutase mRNA expression was not affected after the AKH injection into the insect body.

We have proposed several biochemical steps of possible mode of AKH in anti-oxidative stress response. Using in vitro assay we proved the importance of extra and intra-cellular Ca²⁺ stores as well as the involvement of protein kinase C (PKC) and cyclic adenosine 3',5'-monophosphate (cAMP) pathways in this process. Lipid peroxidation product (4-HNE) was significantly enhanced and membrane fluidity reduced in microsomal fractions of isolated brains (CNS) of P. apterus when treated with hydrogen peroxide (H₂O₂) as a stressor, whereas these biomarkers of oxidative stress were reduced to control levels when H₂O₂ was co-treated with AKH. The effects of mitigation of oxidative stress in isolated CNS by AKH were negated when these treatments were conducted in the presence of Ca²⁺ channel inhibitors (CdCl₂ and thapsigargin). Presence of either bisindolylmaliemide or chelyrythrine chloride (inhibitors of PKC) in the incubating medium also compromised the anti-oxidative function of AKH. However, supplementing the medium with either phorbol myristate acetate (PMA, an activator of PKC) or forskolin (an activator of cAMP) restored the protective effects of exogenous AKH treatment by reducing 4-HNE levels and increasing membrane fluidity to control levels. Taken together, our results strongly implicate the importance of both PKC and cAMP pathways in AKHs’ anti-oxidative action by mobilizing both extra and intra-cellular stores of Ca²⁺. This point seems to be unique, because parallel activation of both mentioned pathways has never been recorded in other AKH activities (Bednářová et. al. 2013, Comp. Biochem. Physiol. C 158, 142-149).

All these studies came fully from our projects and were done partly in our laboratory and partly in cooperation with our former post-doc Dr. Krishnan during stays of our students in his present lab in the USA (University of Mississippi). One set of experiments was done by our student in the Max Planck Institute in Jena, Germany.

Authors: prof. Ing. Vladimír Košťál CSc., prof. RNDr. Michal Žurovec CSc., prof. RNDr. Dalibor Kodrík CSc.

We acknowledge the use of research infrastructure that has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 316304.

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