Investigation of the inhibition by acetylshikonin of the respiratory burst in rat neutrophils

Abstract

1. The ability of acetylshikonin to inhibit the respiratory burst in rat neutrophils was characterized and the underlying mechanism of action was also assessed in the present study.

2. Acetylshikonin caused an irreversible and a concentration-dependent inhibition of formylmethionyl-leucyl-phenylalanine (fMLP) plus dihydrocytochalasin B (CB)- and phorbol 12-myristate 13-acetate (PMA)-induced superoxide anion (O₂^.−) generation with IC₅₀ values of 0.48±0.03 and 0.39±0.03 μM, respectively. Acetylshikonin also inhibited the O₂ consumption in neutrophils in response to fMLP/CB as well as to PMA.

3. Acetylshikonin did not scavenge the generated O₂^.− in the xanthine-xanthine oxidase system or during dihydroxyfumaric acid (DHF) autoxidation but, on the contrary, acetylshikonin enhanced the O₂^.− generation in these cell-free oxygen radical generating systems.

4. Acetylshikonin inhibited the formation of inositol trisphosphate (IP₃) (39.0±7.8% inhibition at 10 μM, P<0.05) in neutrophils in response to fMLP.

5. Both the neutrophil cytosolic protein kinase C (PKC) activity and the PMA-induced PKC associated with the membrane were unaffected by acetylshikonin.

6. Acetylshikonin did not affect the porcine heart protein kinase A (PKA) activity. Upon exposure to acetylshikonin, the cellular cyclic AMP level was decreased in neutrophils in response to fMLP.

7. The cellular formation of phosphatidic acid (PA) and, in the presence of ethanol, phosphatidylethanol (PEt) induced by fMLP/CB were inhibited by acetylshikonin (60.1±7.3 and 63.2±10.5% inhibition, respectively, at 10 μM, both P<0.05). Moreover, acetylshikonin attenuated the fMLP/CB-induced protein tyrosine phosphorylation (about 90% inhibition at 1 μM).

8. In PMA-activated neutrophil particulate NADPH oxidase preparations, acetylshikonin did not inhibit, but enhanced, the O₂^.− generation in the presence of NADPH. However, acetylshikonin decreased the membrane associated p47^phox in PMA-activated neutrophils (about 60% inhibition at 1 μM).

9. Collectively, these results suggest that the attenuation of protein tyrosine phosphorylation and a failure in the assembly of a functional NADPH oxidase complex probably contribute predominantly to the inhibition of respiratory burst in neutrophils by acetylshikonin. In contrast, the blockade of phospholipase C (PLC) and phospholipase D (PLD) pathways play only a minor role in this respect.

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