NF‐κB is constitutively activated in most human pancreatic adenocarcinoma, which is a deadly malignancy with a 5‐year survival rate of about 5%. In this work, we investigate whether microRNAs (miRNAs) contribute to NF‐κB activation in pancreatic cancer. We demonstrate that miR‐301a down‐regulates NF‐κB‐repressing factor (Nkrf) and elevates NF‐κB activation. As NF‐κB promotes the transcription of miR‐301a, our results support a positive feedback loop as a mechanism for persistent NF‐κB activation, in which miR‐301a represses Nkrf to elevate NF‐κB activity, which in turn promotes miR‐301a transcription. Nkrf was found down‐regulated and miR‐301a up‐regulated in human pancreatic adenocarcinoma tissues. Moreover, miR‐301a inhibition or Nkrf up‐regulation in pancreatic cancer cells led to reduced NF‐κB target gene expression and attenuated xenograft tumour growth, indicating that miR‐301a overexpression contributes to NF‐κB activation. Revealing this novel mechanism of NF‐κB activation by an miRNA offers new avenues for therapeutic interventions against pancreatic cancer.

The tumor suppressor p53, encoded by the TP53 gene, is recognized as the guardian of the human genome because it regulates many downstream genes to exercise its function in cell cycle and cell death. Recent reports have revealed that several microRNAs (miRNAs) are important components of the p53 tumor suppressor network with miR-125b and miR-504 directly targeting TP53. In this report, we use a screening method to identify that two miRNAs (miR-25 and miR-30d) directly target the 3'UTR of TP53 to down-regulate p53 protein levels and reduce the expression of genes that are transcriptionally activated by p53. Correspondingly, both miR-25 and miR-30d adversely affect apoptotic cell death, cell cycle arrest, and cellular senescence. Inhibition of either miR-25 or miR-30d expression increases endogenous p53 expression and elevates cellular apoptosis in several cell lines, including one from multiple myeloma that has little TP53 mutations. Thus, beyond miR-125b and miR-504, the human TP53 gene is negatively regulated by two more miRNAs: miR-25 and miR-30d.

c-Myc dysregulation is one of the most common abnormalities found in human cancer. MicroRNAs (miRNAs) are functionally intertwined with the c-Myc network as multiple miRNAs are regulated by c-Myc, while others directly suppress c-Myc expression. In this work, we identified miR-33b as a primate-specific negative regulator of c-Myc. The human miR-33b gene is located at 17p11.2, a genomic locus frequently lost in medulloblastomas, of which a subset displays c-Myc overproduction. Through a small-scale screening with drugs approved by the US Food and Drug Administration (FDA), we found that lovastatin upregulated miR-33b expression, reduced cell proliferation and impaired c-Myc expression and function in miR-33b-positive medulloblastoma cells. In addition, a low dose of lovastatin treatment at a level comparable to approved human oral use reduced tumour growth in mice orthotopically xenografted with cells carrying miR-33b, but not with cells lacking miR-33b. This work presents a highly promising therapeutic option, using drug repurposing and a miRNA as a biomarker, against cancers that overexpress c-Myc.

The p53 protein is a transcription factor that regulates multiple cellular processes in human and other high eukaryotes including cell proliferation, differentiation, cell cycle, and metabolism. The central roles played by p53 in tumor development have drawn extensive studies on p53 activation and inactivation. The regulation of p53 and its pathway, as well as its transactivational targets is of prime importance in the understanding of tumorigenesis. Recently, microRNAs (miRNAs) have been reported to be directly transactivated by p53. Equally, p53 and components of its pathway have been shown to be targeted by miRNA thereby affecting p53 activities. In this review, we focus our discussion on the biological and pathological roles of miRNAs in the p53 pathway.

c‐Myc is a transcription factor that regulates numerous processes including cell cycle, apoptosis, cellular differentiation, cellular metabolism and genomic instability. In normal cells, expression of c‐Myc is tightly regulated by external signals such as growth factors and extracellular matrix. However in stressed and cancerous cells, c‐Myc is generally over‐expressed by chromosomal translocation, gene amplification, or stabilization of its mRNA. We hypothesize that MYC is a target of microRNA (miRNA). miRNAs are 20‐22bp short RNAs that regulate gene expression by binding target mRNA 3′‐UTR, leading to mRNA degradation or translational repression. We used a reporter construct carrying c‐Myc binding sites to screen miRNAs targeting MYC. The results demonstrated that miR‐212 (30%), miR‐203 (40%), miR‐33b (50%) and miR‐33a (50%) down‐regulated reporter activity. Next, we cloned the 3′‐UTR of MYC into a reporter vector and performed assays in cell transfected with miRNAs. We found that miR‐33a (60%), miR‐33b (50%), miR‐203 (40%) down‐regulated the expression of the reporter gene located upstream of the MYC 3′‐UTR. Finally, we performed colony formation assay with rat RK3E‐cmyc cells, which constitutively express c‐Myc. Our results showed that miR‐212 (85%), miR‐33a (50%), miR‐33b (60%) and miR‐203 (17%) reduced colony formation. We will continue to investigate whether MYC is an authentic target of these miRNAs.