Type II [(3)H]
estradiol binding site
ligands including
luteolin (a naturally occurring
bioflavonoid) and synthetic compounds such as
2,6-bis((3-methoxy-4-hydroxyphenyl)methylene)cyclohexanone (
BMHPC) inhibit normal and malignant prostate cell (PC-3, LNCaP, DU-145) proliferation in vitro and in vivo. Type II sites represent a binding domain on
histone H4 possibly involved in an epigenetic mechanism for controlling gene transcription. Treatment of PC-3 human
prostate cancer cells with
luteolin or
BMHPC modulated the expression of a number of genes in the
epidermal growth factor receptor signaling pathway (EGFRSP) and cell cycle pathway (CCP). Pronounced stimulation (400-2000% of control) of c-FOS and p21
RNA expression was observed, suggesting that these were primary sites of action. Both compounds also caused irreversible G2/M arrest (p<0.001).
siRNA's for c-FOS or p21 reduced the
RNA expression of their respective targets by 85-95%, with minimal effects on cell proliferation. Furthermore, neither
siRNA alone (single knockdown), or in combination (double knockdown), blocked
luteolin or
BMHPC inhibition of PC-3 cell proliferation. Thus, although c-FOS and p21 are known to modulate the expression of genes in the ESGRSP (EGFR, SOS, GRB2, JNK1, MKK4, RasGAP) and CCP (CCNA2, CCNE2, CDC25A, CDKN1A, CDKN1B, p27, PLK1) involved in the regulation of cell proliferation by
luteolin and
BMHPC, the c-FOS and p21
siRNA knockdown studies reported here suggest that c-FOS and p21 may be secondary bystanders in the overall response to these
ligands in the regulation of PC-3 cell proliferation.