Pterostilbene-Isothiocyanate Conjugate Suppresses Growth of Prostate Cancer Cells Irrespective of Androgen Receptor Status

Chemotherapy and anti-hormonal therapies are the most common treatments for non-organ-confined prostate cancer (PCa). However, the effectiveness of these therapies is limited, thus necessitating the development of alternative approaches. The present study focused on analyzing the role of pterostilbene (PTER)-isothiocyanate (ITC) conjugate – a novel class of hybrid compound synthesized by appending an ITC moiety on PTER backbone – in regulating the functions of androgen receptor (AR), thereby causing apoptosis of PCa cells. The conjugate molecule caused 50% growth inhibition (IC(50)) at 40±1.12 and 45±1.50 μM in AR positive (LNCaP) and negative (PC-3) cells, respectively. The reduced proliferation of PC-3 as well as LNCaP cells by conjugate correlated with accumulation of cells in G2/M phase and induction of caspase dependent apoptosis. Both PI3K/Akt and MAPK/ERK pathways played an important and differential role in conjugate-induced apoptosis of these PCa cells. While the inhibitor of Akt (A6730) or Akt-specific small interference RNA (siRNA) greatly sensitized PC-3 cells to conjugate-induced apoptosis, on the contrary, apoptosis was accelerated by inhibition of ERK (by PD98059 or ERK siRNA) in case of LNCaP cells, both ultimately culminating in the expression of cleaved caspase-3 protein. Moreover, anti-androgenic activity of the conjugate was mediated by decreased expression of AR and its co-activators (SRC-1, GRIP-1), thus interfering in their interactions with AR. All these data suggests that conjugate-induced inhibition of cell proliferation and induction of apoptosis are partly mediated by the down regulation of AR, Akt, and ERK signaling. These observations provide a rationale for devising novel therapeutic approaches for treating PCa by using conjugate alone or in combination with other therapeutics.