Inhibition of Early Growth Response 1 by a Specific siRNA Modulates the Expression of p21Waf1/Cip1 and JNK-1 Proteins in Prostate PC-3 and LNCaP Carcinoma Cell Lines

This study showed that blocking of Egr-1 expres- sion by a small interfering RNA (siRNA-Egr-1) increased the activity of p21Waf1/Cip1 and decreased the expression of JNK protein. In contrast, siRNA silencing of p21 or JNK-1 with siRNAs was unable to decrease the expression of Egr-1. The p21Waf1/Cip1 protein (hereafter, p21) and the c-Jun N-terminal kinase (JNK) are two well-characterized cell modulators that play a crucial role in cell differentiation, senescence, and apoptosis. Here, we report that transcription of the p21Waf1/Cip1 and JNK-1 genes is affected by inhibition of the early growth response-1 (Egr-1) in response to a small interfering RNA [siRNA)-Egr-1] in LNCaP and PC-3 prostate carcinoma cell lines. The expression levels of protein were determined by western blotting, and apoptosis was measured by propidium iodide staining and flow cytometric analysis. Inhibition of Egr-1, p21 and JNK-1 was carried out by siRNAs. LNCaP and PC-3 cells exhibited readily detectable Egr-1, JNK and p21, even in low serum medium without the addition of other exogenous agents. The expression of Egr-1, p21 and JNK was strongly increased after treatment of the cells with TPA, tumor necrosis factor- (TNF- ) or arsenite. Suppression of Egr-1 expression by siRNA abrogated the ability of TPA to induce Egr-1 and JNK-1 activities, moderately increasing the p21 activity and abrogating the anti-apoptotic effect of Egr-1 observed in the prostate cancer cell lines. Moreover, blockade of p21 and JNK was unable to decrease the activity of Egr-1, while siRNA against p21 abrogated the pro-apoptotic effect of p21. The results demonstrated that Egr-1 acts as a key player in prostate tumor cell growth and survival, while p21 plays a key pro-apoptotic role in LNCaP and PC-3 prostate carcinoma cell lines.