XIAP and Bcl-xL are critical anti-apoptotic molecules that directly inhibit caspases and block mitochondrial membrane permeabilization, respectively. In addition to preventing apoptosis, both XIAP and Bcl-xL can be generated by cap-independent translation via the utilization of an IRES in the 5'-UTR of their mRNAs. In recent years it has been shown that activation of S6K2 induces the translational upregulation of these two apoptotic regulators. Here I have determined that activation of S6K2 enhances IRES-mediated translation of XIAP and Bcl-xL by inducing the degradation of PDCD4, which I have identified as a novel regulator of XIAP and Bcl-xL IRES elements. Furthermore, I have shown that PDCD4 is a positive modulator of the Apaf-1 IRES element. The concurrent regulation of XIAP, Bcl-xL and Apaf-1 by PDCD4 suggests a model in which the level of PDCD4 expression alters the apoptotic threshold by specifically impacting IRES-mediated translation of the XIAP, Bcl-xL and Apaf-1 mRNAs.

Programmed Cell Death 4 (PDCD4) is a known tumour suppressor, lost in carcinomas of the breast, prostate, colon, lung and ovary. This study found significantly reduced levels of PDCD4 mRNA and protein in both primary patient oral squamous cell carcinomas (OSCCs) and OSCC cell lines. Moreover, lower PDCD4 mRNA levels were significantly correlated with nodal metastasis (P=0.019). To determine the functional significance of PDCD4 down-regulation in OSCC we asked whether PDCD4 played a role in invasion. In fact, over-expression of PDCD4 decreased invasion of OSCC lines. We then sought to determine a mechanism for PDCD4 down-regulation in OSCC. Previous studies in breast and colon carcinomas suggested that reduced PDCD4 expression was due to over-expression of miR-21. Interestingly, miR-21 was inversely correlated to PDCD4 mRNA (P=0.002) and PDCD4 protein (P<0.001) levels in OSCC patient samples. Moreover, we found that miR-21 directly regulated PDCD4 protein expression in OSCC cell lines. This is the first report in OSCC that demonstrates that PDCD4 is down-regulated by miR-21 and may play a role in OSCC invasion.

Programmed Cell Death 4 (PDCD4) is a known tumour suppressor, lost in carcinomas of the breast, prostate, colon, lung and ovary. This study found significantly reduced levels of PDCD4 mRNA and protein in both primary patient oral squamous cell carcinomas (OSCCs) and OSCC cell lines. Moreover, lower PDCD4 mRNA levels were significantly correlated with nodal metastasis (P=0.019). To determine the functional significance of PDCD4 down-regulation in OSCC we asked whether PDCD4 played a role in invasion. In fact, over-expression of PDCD4 decreased invasion of OSCC lines. We then sought to determine a mechanism for PDCD4 down-regulation in OSCC. Previous studies in breast and colon carcinomas suggested that reduced PDCD4 expression was due to over-expression of miR-21. Interestingly, miR-21 was inversely correlated to PDCD4 mRNA (P=0.002) and PDCD4 protein (P<0.001) levels in OSCC patient samples. Moreover, we found that miR-21 directly regulated PDCD4 protein expression in OSCC cell lines. This is the first report in OSCC that demonstrates that PDCD4 is down-regulated by miR-21 and may play a role in OSCC invasion.

XIAP and Bcl-xL are critical anti-apoptotic molecules that directly inhibit caspases and block mitochondrial membrane permeabilization, respectively. In addition to preventing apoptosis, both XIAP and Bcl-xL can be generated by cap-independent translation via the utilization of an IRES in the 5'-UTR of their mRNAs. In recent years it has been shown that activation of S6K2 induces the translational upregulation of these two apoptotic regulators. Here I have determined that activation of S6K2 enhances IRES-mediated translation of XIAP and Bcl-xL by inducing the degradation of PDCD4, which I have identified as a novel regulator of XIAP and Bcl-xL IRES elements. Furthermore, I have shown that PDCD4 is a positive modulator of the Apaf-1 IRES element. The concurrent regulation of XIAP, Bcl-xL and Apaf-1 by PDCD4 suggests a model in which the level of PDCD4 expression alters the apoptotic threshold by specifically impacting IRES-mediated translation of the XIAP, Bcl-xL and Apaf-1 mRNAs.

XIAP and Bcl-xL are critical anti-apoptotic molecules that directly inhibit caspases and block mitochondrial membrane permeabilization, respectively. In addition to preventing apoptosis, both XIAP and Bcl-xL can be generated by cap-independent translation via the utilization of an IRES in the 5'-UTR of their mRNAs. In recent years it has been shown that activation of S6K2 induces the translational upregulation of these two apoptotic regulators. Here I have determined that activation of S6K2 enhances IRES-mediated translation of XIAP and Bcl-xL by inducing the degradation of PDCD4, which I have identified as a novel regulator of XIAP and Bcl-xL IRES elements. Furthermore, I have shown that PDCD4 is a positive modulator of the Apaf-1 IRES element. The concurrent regulation of XIAP, Bcl-xL and Apaf-1 by PDCD4 suggests a model in which the level of PDCD4 expression alters the apoptotic threshold by specifically impacting IRES-mediated translation of the XIAP, Bcl-xL and Apaf-1 mRNAs.

XIAP and Bcl-xL are critical anti-apoptotic molecules that directly inhibit caspases and block mitochondrial membrane permeabilization, respectively. In addition to preventing apoptosis, both XIAP and Bcl-xL can be generated by cap-independent translation via the utilization of an IRES in the 5'-UTR of their mRNAs. In recent years it has been shown that activation of S6K2 induces the translational upregulation of these two apoptotic regulators. Here I have determined that activation of S6K2 enhances IRES-mediated translation of XIAP and Bcl-xL by inducing the degradation of PDCD4, which I have identified as a novel regulator of XIAP and Bcl-xL IRES elements. Furthermore, I have shown that PDCD4 is a positive modulator of the Apaf-1 IRES element. The concurrent regulation of XIAP, Bcl-xL and Apaf-1 by PDCD4 suggests a model in which the level of PDCD4 expression alters the apoptotic threshold by specifically impacting IRES-mediated translation of the XIAP, Bcl-xL and Apaf-1 mRNAs.

Internal ribosome entry site (IRES)-mediated translation is critical for the cell’s ability to respond to stress. Understanding how RNA binding proteins (IRES trans-acting factors; ITAFs) regulate IRESes is crucial to elucidating the mechanism of alternative translation initiation. Furthermore, determining how these ITAFs are regulated is central to understanding their functions in diseased states. I have identified the tumour suppressor programmed cell death 4 (PDCD4) as a novel ITAF of the XIAP and Bcl-xL IRES elements. I demonstrate that under normal conditions, PDCD4 acts to inhibit translation from these IRES elements by preventing formation of the 48S translation initiation complex. Furthermore, I show that in response to treatment with the pro-survival fibroblast growthfactor-2 (FGF-2), S6 kinase 2 (S6K2) phosphorylates PDCD4 leading to its degradation and the subsequent de-repression of XIAP and Bcl-xL translation. Importantly, I demonstrate the clinical significance of this regulation in glioblastoma multiforme (GBM) tumours where the loss of PDCD4 expression correlates with an increase in Bcl-xL protein and poor patient outcome. Additionally, re-expression of PDCD4 down-regulates Bcl-xL and decreases cell viability, and direct inhibition of Bcl-xL by a small molecule antagonist ABT-737 sensitizes GBM cells to the chemotherapeutic doxorubicin. Finally, I demonstrate that PDCD4 can be regulated at multiple levels. Importantly, I identify the RNA binding protein HuR as a regulator of microRNA (miR) -21 induced silencing of PDCD4. I show that HuR can bind the PDCD4 3'UTR and prevent miR-21 binding, and that a loss of PDCD4 expression following H2O2 treatment is mediated via miR-21. These results provide novel insight into the role of PDCD4 as a tumour suppressor and highlight the importance of ITAFs in cancer progression.

Prostate cancer is the most commonly diagnosed cancer and the second most leading cause of cancer deaths in American men (www.cancer.org). Most prostate cancer-related deaths are due to the metastatic form of the disease. The 5-year relative survival rate in patient's diagnosed with metastatic prostate cancer is just 28%, as compared to 100% in patient's diagnosed with localized prostate cancer. This clearly indicates the lack of effective treatment available for metastatic prostate cancer. MicroRNAs (miRNAs) are small (18~23 nucleotide long) non-coding RNAs that can influence gene expression by binding to the 3'-untranslated region of coding RNAs at the post-transcriptional level. Some miRNAs has been termed as oncomirs due to their role in promoting tumor growth, invasion and metastasis. One such oncomir is microRNA-21 (miR-21) whose levels are often up-regulated in a number of cancers, including prostate cancer. MiR-21 increases the survival and invasiveness of cancer cells by suppressing its target tumor suppressor genes, namely programmed cell death 4 (PDCD4) and maspin. Thus, drugs which target miR-21 for inhibition could provide novel treatment options for metastatic prostate cancer. Resveratrol (3,5,4'-trihydroxystilbene) is a polyphenolic phytoalexin found in high quantities in various dietary sources, such as grapes, red wine, berries and peanuts. Various reports have demonstrated a significant role of resveratrol in the management of several old age diseases including cancer. The efficacy of resveratrol as an anti-cancer agent resides in its ability to interfere with cell proliferation and metastasis and enhancement of apoptosis. Resveratrol has been shown to act on several intracellular targets to exert these effects. However, the exact mechanism by which resveratrol mediates its beneficial cancer chemotherapeutic actions are not clear and is the focus of this study. Based on the reported data, we hypothesized that resveratrol mediates its anti-cancer action against metastatic prostate cancer by inhibiting the signaling pathway which involves miR-21 expression and function. To address this hypothesis, we show that resveratrol decreased cell viability, migration and invasiveness of androgen-receptor negative and highly aggressive human prostate cancer cells, PC-3M-MM2. These effects of resveratrol were associated with the inhibition of miR-21, since over-expression of miR- 21 with pre-miR-21 oligonucleotides attenuated resveratrol's effect on these cells. Additionally, resveratrol increased the expression of tumor suppressors, PDCD4 and maspin, which are negatively regulated by miR-21 and knockdown of PDCD4 by short interfering (si) RNA reversed the resveratrol's effect on prostate cancer cells. PC-3M-MM2 cells also exhibits high levels of phospho-Akt (pAkt), which were reduced by both resveratrol and LY294002, a known PI3-kinase inhibitor. MiR-21 expression in these cells appears to be dependent on Akt, as LY294002 reduced the levels of miR-21 along with a concurrent increase in PDCD4 expression. These in vitro findings were further corroborated in a severe combined immunodeficient (SCID) mouse xenograft model of prostate cancer. Oral administration of resveratrol not only inhibits the tumor growth but also decreased the incidence and number of metastatic lung lesions. These tumor- and metastatic-suppressive effects of resveratrol were associated with reduced miR-21 and pAkt, and elevated PDCD4 levels. Future investigation into the molecular mechanisms revealed that resveratrol suppressed prostate cancer growth by decreasing the levels of insulin-like growth factor-1 (IGF-1) and its receptor (R). Previous studies had associated elevated levels of serum IGF-1 with high risk of prostate cancer. IGF-1, after binding to its receptors, acts as a potent mitogen which stimulates cancer cell growth and proliferation mainly by activating Akt signaling pathway. Interestingly, this effect of resveratrol on IGF-1/IGF-1R was independent of its effect on miR-21. In summary, our data show that resveratrol exerts its anti-cancer effect on metastatic prostate cancer cells, at least in part, by targeting Akt/miR-21 pathway. These data highlight a potential molecular mechanism for resveratrol's anti-cancer action for the treatment of metastatic prostate cancer and suggest that inhibition of the IGF-1/Akt/miR-21 pathway is a rationale approach for the treatment prostate cancer metastasis.