Pachymic acid isolated from Poria cocos inhibits proliferation of prostate cancer cell lines

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Poria cocos is a medicinal herb that is widely used as a sedative, diuretic, and food tonic supplement in Asian countries. Alcoholic extracts of Poria cocos contain various lanostane-type triterpenoids, which, aside from their inhibitory effect on tumor promotion, have been shown to cause moderate cytotoxicity in a number of human cancer cell lines. Unfortunately, the efficacy and mechanism of these triterpenoids in cancer treatment have not been systemically evaluated. In this study, we investigated the effect of one of these triterpenoids, pachymic acid (PA), on the cell cycle progression and proliferation of androgen-dependent (LNCaP) and -independent (DU145) human prostate cancer cells. We demonstrated that PA significantly inhibits the proliferation of these cells in a dose- and time-dependent manner. Moreover, exposure of these cells to PA induces characteristic features of apoptosis, including proteins that favor apoptosis, DNA laddering, PARP cleavage, Annexin-V staining, and caspase-3 and -9 activation. Although flow cytometry showed the cell cycle distribution was not significantly altered following PA-treatment, the expression of molecules important for G1 progression (such as cyclin D and cdk 4/6) were reduced. The levels of cell cycle regulator proteins Rb and p21 also increased in both cell lines, while p53 increased only in LNCaP prostate cells. Because DU145 prostate cells have a mutated p53, our data implies that PA influences p21 protein expression by p53-dependent and independent pathways. It is noteworthy that a normal epithelial prostate cell line (PrEC) was minimally affected by PA, even at concentrations that were highly cytotoxic to the prostate cancer cells, suggesting that PA exerts selective cytotoxic effect over both androgen-dependent and independent human prostate cancer cells. Lanostane-type triterpenoids from P. cocos inhibit phospholipase A2 (PLA2), the enzyme responsible for arachidonic acid (AA) release from membrane phospholipids. Several reports have linked AA and its COX or LOX metabolites with prostate cancer development and progression. Agents that block PLA2 may also remove potential signals that activate the pro-survival kinase, AKT. We compared whether PA mimics the effects seen with a non-specific pharmacological inhibitor of PLA2 (4-bromophenacyl bromide) . Our preliminary results showed total AKT protein was reduced and arachidonic acid production was lowered following PA-treatment of LNCaP and DU145 prostate cells. We hypothesize that PA reduces cPLA2 activity, thereby leading to reduced pro-survival activity of AKT. We are continuing to investigate whether there is a link between cPLA2, AKT, and p21 in dictating the sensitivity of prostate carcinoma cells to PA.