Sai Murali Krishna Pulukuri

Epigenetic Inactivation of the Tissue Inhibitor of Metalloproteinase-2 (TIMP-2) Gene in Human Prostate Tumors

Gene silencing via CpG island methylation in the promoter region is one of the mechanisms by which tumor suppressor genes are inactivated in human cancers. Previous studies have shown that the tissue inhibitors of metalloproteinases (TIMP)-2 gene, which is an endogenous inhibitor of matrix metalloproteinases involved in cell invasion and tumorigenesis, is downregulated or silenced in a variety of human cancer cell lines. Here, we investigated the mechanism underlying TIMP-2 expression in prostate cancer cell lines and primary prostate tumor samples. We observed a strong correlation between promoter hypermethylation and lost expression of TIMP-2 gene, which was supported by other results demonstrating that promoter demethylation by 5-aza-2′-deoxycytidine and trichostatin A reactivated TIMP-2 and restored its expression in TIMP-2-silenced metastatic prostate cell lines. These results were further substantiated by a chromatin immunoprecipitation assay, showing the preferential binding of MeCP2 to methylated CpG island in TIMP-2-silenced metastatic prostate cell lines. In vitro Matrigel invasion assays showed that re-expression of TIMP-2 after a combined treatment with 5-aza and trichostatin-A in metastatic prostate cells resulted in a significant reduction of tumor cell invasion. Furthermore, CpG methylation of TIMP-2 promoter was also shown in primary prostate tumors that expressed decreased TIMP-2 protein levels. These results suggest that the downregulation of the TIMP-2 gene is associated with promoter methylation and that this may play an important role in prostate cancer progression during the invasive and metastatic stages of the disease.

Demethylation-Linked Activation of Urokinase Plasminogen Activator Is Involved in Progression of Prostate Cancer

Increased expression of urokinase plasminogen activator (uPA) has been reported in various malignancies including prostate cancer. However, the mechanism by which uPA is abnormally expressed in prostate cancer remains elusive. Here, we show that uPA is aberrantly expressed in a high percentage of human prostate cancer tissues but rarely expressed either in tumor-matched nonneoplastic adjacent tissues or benign prostatic hyperplasia samples. This aberrant expression is associated with cancer-linked demethylation of the uPA promoter. Furthermore, treatment with demethylation inhibitor S-adenosylmethionine or stable expression of uPA short hairpin RNA significantly inhibits uPA expression and tumor cell invasion in vitro and tumor growth and incidence of lung metastasis in vivo. Collectively, these findings strongly suggest that DNA demethylation is a common mechanism underlying the abnormal expression of uPA and is a critical contributing factor to the malignant progression of human prostate tumors.

Small Interfering RNA–Directed Reversal of Urokinase Plasminogen Activator Demethylation Inhibits Prostate Tumor Growth and Metastasis

Recent studies have shown that small interfering RNA (siRNA) silences genes at the transcriptional level in human cells. However, the therapeutic potential of siRNA-mediated transcriptional gene silencing remains unclear. Here, we show that siRNA targeted to the urokinase plasminogen activator (uPA) promoter induced epigenetic transcriptional silencing in human prostate cancer cells. This silencing resulted in a dramatic reduction of tumor cell invasion and angiogenesis in vitro. Furthermore, the results from a bioluminescence tumor/metastasis model showed that the silencing of uPA significantly inhibits prostate tumor growth and the incidence of lung metastasis. Our findings represent a potentially powerful new approach to not only epigenetic silencing of metastasis or growth-promoting genes as a cancer therapy, but also as a means to shed light on how aberrant de novo methylation during cancer progression might be targeted to specific sequences.

Inhibition of Histone Deacetylase Activity Promotes Invasion of Human Cancer Cells through Activation of Urokinase Plasminogen Activator

Histone acetylation plays an important role in chromatin remodeling and gene expression. The molecular mechanisms involved in differential regulation of urokinase plasminogen activator (uPA) gene expression are not fully understood. In this study, we investigated whether histone deacetylation was involved in repression of uPA expression in human cancer cells. Induction of uPA expression by histone deacetylase (HDAC) inhibitors trichostatin A (TSA), sodium butyrate, and scriptaid was observed in all three different types of human cancer cells examined. Chromatin immunoprecipitation assays showed that the induction of uPA expression by TSA was accompanied by a remarkable increase of acetylation of histones H3 and H4, which are associated with the uPA promoter region in human cancer cells. These results were further substantiated by the findings of a restriction enzyme accessibility assay and TSA-stimulated uPA promoter activity through the inhibition of HDAC activity. In vitro Matrigel invasion assays showed that induction of uPA expression by HDAC inhibitors in human cancer cells resulted in a significant increase of cancer cell invasion. Furthermore, HDAC1 knockdown by small interference RNA stimulated uPA expression and cancer cell invasion. In conclusion, this study demonstrates the important role of histone modifications in regulating uPA gene expression and raises a possibility that the use of HDAC inhibitors in patients as cancer therapy may paradoxically establish metastasis through up-regulation or reactivation of uPA.

CpG island promoter methylation and silencing of 14-3-3σ gene expression in LNCaP and Tramp-C1 prostate cancer cell lines is associated with methyl-CpG-binding protein MBD2

14-3-3σ proteins regulate numerous cellular processes that are important to cancer development. One of its biological roles involves G2 cell-cycle arrest following DNA damage. It has also been reported that the loss of 14-3-3σ expression via CpG methylation may contribute to malignant transformation by impairing the G2 cell-cycle checkpoint function, thereby allowing an accumulation of genetic defects. However, how the CpG methylation-dependent silencing mechanism works in relation to promoter methylation associated with methyl-CpG-binding proteins (MeCPs) is still unclear. To better understand the mechanism, we first examined the methylation status of the 14-3-3σ promoter-associated CpG islands and 14-3-3σ gene expression in a subset of prostate cancer cell lines using methylation-specific PCR (MSP), an HhaI-based DNA methylation assay, and reverse transcription–PCR (RT–PCR). We found that the 14-3-3σ expression is lost in LNCaP and Tramp-C1 prostate cancer cell lines and that this expression is restored after treatment with epigenetic silencing modifiers 5-aza-2′-deoxycytidine (5-aza) and trichostatin A (TSA). These results imply transcriptional silencing via promoter-associated CpG methylation. Chromatin immunoprecipitation analysis revealed that methyl-CpG-binding protein 2 (MBD2) is associated preferentially to the methylated CpG island in the 14-3-3σ promoter in LNCaP and Tramp-C1 cells but not in 14-3-3σ-expressing PC3 and DU145 cells, which contain an unmethylated CpG island in the 14-3-3σ promoter region. The 14-3-3σ gene silencing because of CpG methylation correlates with binding of MBD2. In addition, the activation of 14-3-3σ gene expression by a combination of 5-aza and TSA also involves the release of the MBD2 from the 14-3-3σ promoter-methylated CpG island in LNCaP and Tramp-C1 cells. Furthermore, MBD2 knockdown by siRNA stimulated 14-3-3σ expression in LNCaP cells. We also investigated whether the loss of 14-3-3σ expression in LNCaP and Tramp-C1 cells affects cell proliferation by MTT assays. Interestingly, we observed that 14-3-3σ-inactivated LNCaP and Tramp-C1 cells had markedly decreased cell proliferation and protein expression of proliferation cell nuclear antigen (PCNA) after restoration of 14-3-3σ expression with 5-aza and TSA treatment. On the other hand, the same treatment did not significantly affect 14-3-3σ-active PC3 and DU145 cells, which normally express 14-3-3σ. Finally, 14-3-3σ knockdown by siRNA resulted in increased proliferation in PC3 and DU145 cells. These findings suggest that the transcriptional silencing of the 14-3-3σ gene is caused by promoter CpG island methylation associated with MBD2, and that this may play an important role in prostate cancer progression during the invasive and metastatic stages of the disease.

Frequent loss of cystatin E/M expression implicated in the progression of prostate cancer

Cystatin E/M (CST6) is a natural inhibitor of lysosomal cysteine proteases. Recent studies have shown that experimental manipulation of CST6 expression alters the metastatic behavior of human breast cancer cells. However, the association of CST6 with prostate cancer invasion and progression remains unclear. Here, we show that CST6 is robustly expressed in normal human prostate epithelium, whereas its expression is downregulated in metastatic prostate cell lines and prostate tumor tissues. Treatment of metastatic prostate cell lines with the histone deacetylase inhibitor trichostatin A resulted in significant induction of CST6 mRNA levels and increased CST6 protein expression, indicating that epigenetic silencing may play a role in the loss of CST6 expression observed in prostate cancer. CST6 overexpression in human prostate cancer cells significantly reduced in vitro cell proliferation and matrigel invasion. Furthermore, the results from a bioluminescence tumor/metastasis model showed that the overexpression of CST6 significantly inhibits tumor growth and the incidence of lung metastasis. These results suggest that the downregulation of the CST6 gene is associated with promoter histone modifications and that this association plays an important role in prostate cancer progression during the invasive and metastatic stages of the disease.

Small Interfering RNA-Directed Knockdown of Uracil DNA Glycosylase Induces Apoptosis and Sensitizes Human Prostate Cancer Cells to Genotoxic Stress

Uracil DNA glycosylase (UNG) is the primary enzyme responsible for removing uracil residues from DNA. Although a substantial body of evidence suggests that DNA damage plays a role in cancer cell apoptosis, the underlying mechanisms are poorly understood. In particular, very little is known about the role of base excision repair of misincorporated uracil in cell survival. To test the hypothesis that the repair of DNA damage associated with uracil misincorporation is critical for cancer cell survival, we used small interfering RNA (siRNA) to target the human UNG gene. In a dose-dependent and time-dependent manner, siRNA specifically inhibited UNG expression and modified the expression of several genes at both mRNA and protein levels. In LNCaP cells, p53, p21, and Bax protein levels increased, whereas Bcl2 levels decreased. In DU145 cells, p21 levels were elevated, although mutant p53 and Bax levels remained unchanged. In PC3 cells, UNG inhibition resulted in elevated p21 and Bax levels. In all three cell lines, UNG inhibition reduced cell proliferation, induced apoptosis, and increased cellular sensitivity to genotoxic stress. Furthermore, an in vitro cleavage experiment using uracil-containing double-stranded DNA as a template has shown that siRNA-mediated knockdown of UNG expression significantly reduced the uracil-excising activity of UNG in human prostate cancer cells, which was associated with DNA damage analyzed by comet assay. Taken together, these findings indicate that RNA interference–directed targeting of UNG is a convenient, novel tool for studying the biological role of UNG and raises the potential of its application for prostate cancer therapy. (Mol Cancer Res 2009;7(8):1285–93)

RNA Interference-Directed Knockdown of Urokinase Plasminogen Activator and Urokinase Plasminogen Activator Receptor Inhibits Prostate Cancer Cell Invasion, Survival, and Tumorigenicity In Vivo

The invasive ability of tumor cells plays a key role in prostate cancer metastasis and is a major cause of treatment failure. Urokinase plasminogen activator-(uPA) and its receptor (uPAR)-mediated signaling have been implicated in tumor cell invasion, survival, and metastasis in a variety of cancers. This study was undertaken to investigate the biological roles of uPA and uPAR in prostate cancer cell invasion and survival, and the potential of uPA and uPAR as targets for prostate cancer therapy. uPA and uPAR expression correlates with the metastatic potential of prostate cancer cells. Thus, therapies designed to inhibit uPA and uPAR expression would be beneficial. LNCaP, DU145, and PC3 are prostate cancer cell lines with low, moderate, and high metastatic potential, respectively, as demonstrated by their capacity to invade the extracellular matrix. In this study we utilized small hairpin RNAs (shRNAs), also referred to as small interfering RNAs, to target human uPA and uPAR. These small interfering RNA constructs significantly inhibited uPA and uPAR expression at both the mRNA and protein levels in the highly metastatic prostate cancer cell line PC3. Our data demonstrated that uPA-uPAR knockdown in PC3 cells resulted in a dramatic reduction of tumor cell invasion as indicated by a Matrigel invasion assay. Furthermore, simultaneous silencing of the genes for uPA and uPAR using a single plasmid construct expressing shRNAs for both uPA and uPAR significantly reduced cell viability and ultimately resulted in the induction of apoptotic cell death. RNA interference for uPA and uPAR also abrogated uPA-uPAR signaling to downstream target molecules such as ERK1/2 and Stat 3. In addition, our results demonstrated that intratumoral injection with the plasmid construct expressing shRNAs for uPA and uPAR almost completely inhibited established tumor growth and survival in an orthotopic mouse prostate cancer model. These findings uncovered evidence of a complex signaling network operating downstream of uPA-uPAR that actively advances tumor cell invasion, proliferation, and survival of prostate cancer cells. Thus, RNA interference-directed targeting of uPA and uPAR is a convenient and novel tool for studying the biological role of the uPA-uPAR system and raises the potential of its application for prostate cancer therapy.