Cytotoxic, cytostatic and HIV-1 PR inhibitory activities of the soft coral Litophyton arboreum

Abstract

Bioassay-guided fractionation using different chromatographic and spectroscopic techniques in the analysis of the Red Sea soft coral Litophyton arboreum led to the isolation of nine compounds; sarcophytol M (1), alismol (2), 24-methylcholesta-5,24(28)-diene-3β-ol (3), 10-O-methyl alismoxide (4), alismoxide (5), (S)-chimyl alcohol (6), 7β-acetoxy-24-methylcholesta-5-24(28)-diene-3,19-diol (7), erythro-N-dodecanoyl-docosasphinga-(4E,8E)-dienine (8), and 24-methylcholesta-5,24 (28)-diene-3β,7β,19-triol (9). Some of the isolated compounds demonstrated potent cytotoxic- and/or cytostatic activity against HeLa and U937 cancer cell lines and inhibitory activity against HIV-1 protease (PR). Compound 7 was strongly cytotoxic against HeLa cells (CC₅₀ 4.3 ± 0.75 µM), with selectivity index of SI 8.1, which was confirmed by real time cell electronic sensing (RT-CES). Compounds 2, 7, and 8 showed strong inhibitory activity against HIV-1 PR at IC₅₀s of 7.20 ± 0.7, 4.85 ± 0.18, and 4.80 ± 0.92 µM respectively. In silico docking of most compounds presented comparable scores to that of acetyl pepstatin, a known HIV-1 PR inhibitor. Interestingly, compound 8 showed potent HIV-1 PR inhibitory activity in the absence of cytotoxicity against the cell lines used. In addition, compounds 2 and 5 demonstrated cytostatic action in HeLa cells, revealing potential use in virostatic cocktails. Taken together, data presented here suggest Litophyton arboreum to contain promising compounds for further investigation against the diseases mentioned.

Figure 1

Cytotoxicity of L. arboreum main fractions tested in U937 cells at 100 µg/mL.

Figure 2

Chemical structure of L. arboreum isolated compounds.

Figure 3

Comparison of RT-CES data of compounds 7β-acetoxy-24-methylcholesta-5-24(28)-diene-3,19-diol (7) and 24-methylcholesta-5,24(28)-diene-3β,7β,19-triol (9) showing cytotoxicity profiles in HeLa cells. The compounds were more toxic than the positive control, Actinomycin D.

Figure 4

The effect of compound 7 on the proliferation of HeLa cells using RT-CES analysis. The compound displayed a dose dependent cytotoxic tendency at the IC₅₀ obtained by in vitro viability assay, and twice that amount. The lowest concentration, half of IC₅₀ (2.5 µg/mL), displayed cytostatic effects when compared to the untreated cells grown in culture medium only. Actinomycin D (5.6 µM) was used as a positive control for cell death.

Figure 5

The effect of compound 2 at 16 μg/mL and compound 5 at 60 μg/mL in HeLa cells were compared to the untreated cells (culture media only) and the positive control Actinomycin D. The compounds demonstrated cytostatic behavior (in the presence of these compounds, cells did not proliferate, nor did they die).

Figure 6

Protease inhibition of L. arboreum isolated compounds at 100 µg/mL. Fractions are indicated by number while the known inhibitor used as positive control, acetyl pepstatin is abbreviated AP.

Figure 7

The effect of the active compound 8 on the proliferation of HeLa cells using RT-CES analysis. Cells treated with a high concentration (100 µg/mL) of compound 8 showed low level cytotoxicity compared with the untreated cells grown in culture medium only and the positive control Actinomycin D.

Figure 8

3D Stereo-diagram of the ligand “acetyl pepstatin” combined with the HIV-1 protease receptor (5HVP) active site. The green color shows the lipophilic nature of the pocket.

Figure 9

Interaction of bioactive compounds with different amino acid moieties in HIV-1 protease receptor (5HVP) active site.