A new diketosteroid, (E)-stigmasta-24(28)-en-3,6-dione (1), along with three known steroids
(2–4) was isolated from marine alga Tydemania expeditionis collected in China Sea. Their
structures were elucidated by extensive spectroscopic methods. Comparison of the chemical
constituents revealed significant diversity among different locations. The biological activities
of 1, 3 and 4 were evaluated on the prostate cancer cell lines and androgen receptor.
Compound 1 exhibited moderate inhibitory activities against the prostate cancer cells DU145,
PC3 and LNCaP with IC50 values of 31.27±1.50, 40.59±3.10 and 19.80±3.84 μM,
respectively. Compound 3 showed more potent activities with IC50 values of 12.38±2.47,
2.14±0.33 and 1.38±0.07 μM, respectively. However, compound 4 showed only weak
inhibitory activities on LNCaP cells and was inactive on DU145 and PC3 cells. A competitive
binding assay showed that compound 1 exhibited significant affinity to the androgen receptor
with an IC50 value of 7.19±0.45 μM, while 3 and 4 were inactive. The fact that the inhibitory
properties of 1 and 3 against the prostate cancer cells were inconsistent with their affinities to
the androgen receptor suggested that there might be other mechanism of action involved in
the cytotoxic activity.