PTP1B Inhibitory and Cytotoxic C-24 Epimers of Δ-24-hydroxy Stigmastane-type Steroids from the Brown Alga Dictyopteris Undulata Holmes

Ten stigmastane-type steroids bearing unusual Δ-24-hydroxy side chains, dictyopterisins A-J, including three pairs of C-24 epimers, dictyopterisins B/C, F/G, and I/J, were isolated from the brown alga Dictyopteris undulata Holmes, together with two previously reported analogues, (24S)- and (24R)-saringosterol. Their structures were elucidated on the basis of extensive spectroscopic analysis, with their absolute configurations at the stereogenic center C-24 of the side chain being assigned by a direct comparison of H NMR data with those of related known compounds. The absolute configurations of the steroidal nuclei of dictyopterisins A, B, and H were determined using the modified Mosher's method. The mixture of dictyopterisins D and E and dictyopterisin I exhibited promising PTP1B inhibitory activities with IC values of 1.88 and 3.47 μM, respectively, comparable to the positive control oleanolic acid (IC, 2.78 μM). In addition, the mixture of dictyopterisins D and E and dictyopterisins F-J displayed significant cytotoxicities against the human cancer cell lines HL-60 (IC from 1.02 to 2.70 μM) and A-549 (IC from 1.35 to 2.85 μM).