Anti-inflammatory activities of Ptaeroxylon obliquum leaf extracts, fractions and bioactive compounds

Abstract

Ptaeroxylon obliquum (Thunb.) Radlk is traditionally used to treat inflammation and related symptoms such as arthritis, rheumatism, fever, pain and headache. The aim of this study was to determine the anti-inflammatory activities by investigating different potential mechanisms of action of the crude extracts, fractions and isolated compounds from P. obliquum leaves from different localities in South Africa. The anti-inflammatory activity of P. obliquum acetone leaf extracts, aqueous extracts (hot and cold), fractions and isolated compounds (obliquumol, O-methylalloptaeroxylin and a mixture of lupeol and β-amyrin) was determined by investigating the following: the nitric oxide (NO) inhibition in lipopolysaccharide (LPS)-induced RAW 264.7 macrophages, soybean 15-lipoxygenase (15-LOX) inhibition, as well as cyclooxygenase (COX)-2 enzyme inhibition using an ELISA method. The effects of the extracts on the pro-inflammatory cytokines, interleukin (IL)-1β and TNF-α, as well as the anti-inflammatory cytokine (IL-10) production, were also determined using ELISA assays. All acetone extracts demonstrated greater 15-LOX inhibitory activity than aqueous extracts and the positive control, with IC50 values ranging from 5.7 µg/mL to 10.4 µg/mL. Obliquumol had the highest level of 15-LOX inhibition with an IC50 of 3.66 g/mL. The isolated compounds, fractions, and extracts from P. obliquum all inhibited NO production in a dose-dependent manner. The acetone extracts reduced NO with percentages as high as 95 % and 102 % with extracts prepared from plants collected from Walter Sisulu Botanic Gardens and Nelspruit, respectively, at the lowest tested concentration (1.6 µg/mL). The high proportion of NO was not caused by toxicity of the acetone extracts to the cells, as was demonstrated by cell viability of >100 % RAW 264.7 macrophages. The extracts and the compounds reversed the LPS-induced COX-2 significantly (p < 0.001) in a dose dependent manner in comparison with the positive control, quercetin. The LPS-induced pro-inflammatory cytokines that were assessed in the study were reduced by the extracts and compounds. The acetone extract prepared from plant material collected at Walter Sisulu Botanic Gardens and obliquumol significantly inhibited the LPS-induced TNF-α levels (p < 0.027 and p < 0.008, respectively). The acetone extracts, fractions and isolated compounds of P. obliquum had good anti-inflammatory activity, thus supporting its medicinal use against inflammation.

HIGHLIGHTS • Obliquumol strongly inhibited 15-LOX with an IC50 of 3.66 µg/mL. • Ptaeroxylon obliquum extracts and compounds significantly reduced LPS-induced COX-2. • Obliquumol significantly inhibited LPS-induced TNF-α levels. • The acetone extracts reduced NO production by close to 100 % at 1.6 µg/mL.