Function Of miR-34a Research Articles

MicroRNA-26a negatively regulates toll-like receptor 3 expression of rat macrophages and ameliorates pristane induced arthritis in rats

IntroductionAbnormal toll-like receptor (TLR)3 signaling plays an indispensable role in pathogenesis of both experimental and human rheumatoid arthritis, and microRNAs (miRNAs) might orchestrate this signaling pathway. This study was performed to determine the relationship between miR-26a and TLR3 in rat macrophages and to observe effects of miR-26a mimic on pristane induced arthritis (PIA) in rats.MethodsDual luciferase reporter assay was used to validate the direct interaction between miR-26a (a candidate miRNA to target tlr3 mRNA) and tlr3 3′UTR. MiR-26a regulation on TLR3 gene expression was determined using RT-qPCR and Western blotting after miR-26a mimics and inhibitors were transfected into rat macrophage line NR8383 cells. Poly I:C (TLR3 ligand) was used to trigger TLR3 activation, and mRNA expression of its downstream cytokines interferon (ifn)-β and tumor necrosis factor (tnf)-α was accordingly detected to determine the regulation of TLR3 signaling. Expressions of TLR3 and miR-26a were detected during rat bone marrow derived macrophage (BMDM) induction, in pristane stimulated NR8383 cells and spleens from methotrexate (MTX) treated PIA rats. A miR-26a mimic was administrated intraperitoneally to PIA rats, and arthritis severity was evaluated by macroscopic or microscopic observations.ResultsDirect target relationship between miR-26a and tlr3 mRNA in rats was confirmed. Modifications of miR-26a function by transfection of miR-26a mimics and inhibitors exhibited corresponding repression and augmentation of TLR3 and its signaling downstream cytokine expressions in NR8383 cells. The alteration of miR-26a expression was negatively related with TLR3 expression during BMDM induction, in pristane-primed NR8383 cells and PIA rat spleens. Moreover, both abnormal expressions were rescued in MTX treated arthritis rat spleens. The miR-26a mimic treatment displayed the depression of TLR3 expression and ameliorated the disease severity in the rats with pristane induced arthritis.ConclusionsMiR-26a negatively regulates TLR3 signaling via targeting of TLR3 itself in rat macrophages, and this finding provides a novel insight into abnormal TLR3 overexpression during experimental arthritis.

Dysregulation of MicroRNA-34a Expression in Colorectal Cancer Inhibits the Phosphorylation of FAK Via VEGF

MicroRNAs (miRs) are small non-coding RNAs that play important roles in cancer development where they can act as oncogenes or as tumor-suppressors. MiR-34a is a tumor suppressor that is frequently downregulated in a number of tumor types. However, little is known about the role of miR-34a in colorectal cancer (CRC). This study aims to show the dysregulation of miR-34a in CRC and to characterize the function and mechanism of miR-34a in CRC cell lines. The expression of miR-34a was detected using real-time PCR on CRC tissues and adjacent non-tumorous tissues. The ELISA was used to assess vascular endothelial growth factor (VEGF). The focal adhesion kinase (FAK) and phosphorylated FAK Y397 (pY397FAK) were measured by Western blot. The functions of miR-34a in vivo were measured by migration, invasion, CCK-8 assay and flow cytometry. MiR-34a was significantly downregulated and pY397FAK was upregulated in CRC cancer tissues. It plays an important role in inhibiting migration and invasion and in increasing apoptosis of CRC cells. Bioinformatic analysis suggested that VEGF may be a target of miR-34a, and this hypothesis was proved by ELISA and RT-PCR. The level of pY397FAK that could be activated by VEGF was downregulated in miR-34a overexpression CRC cell lines. The phosphorylation of FAK at 397 sites in miR-34a-stable cell lines was completely rescued by extra VEGF treatment. MiR-34a is frequently downregulated in CRC and modulates the phosphorylation of FAK by negatively regulating VEGF.

The Loss of miR-26a-Mediated Post-Transcriptional Regulation of Cyclin E2 in Pancreatic Cancer Cell Proliferation and Decreased Patient Survival

BackgroundmiR-26a plays a critical role in tumorigenesis, either as a tumor suppressor or as an oncogenic miRNA, depending on different tumor types. However, the function of miR-26a in pancreatic cancer has not been clearly elucidated. The present study was designed to determine the roles of miR-26a in pancreatic cancer and its association with the survival of patients with pancreatic cancer.MethodsThe expression of miR-26a was examined in 15 pairs of pancreatic duct adenocarcinoma (PDAC) and their adjacent benign pancreatic tissues (ABPT), by qRT-PCR. The results were confirmed by in situ hybridization using two panels of 106 PDACs and their ABPT microarray. The association of miR-26a expression with overall survival was determined. The proliferation and cell cycle distribution of Capan-2, SW-1990, and Panc-1 cells, transfected with miR-26a mimics or a miR-26a inhibitor, were assessed using the Cell Counting Kit-8 assay and flow cytometry, respectively. The cell tumorigenicity was evaluated via murine xenograft experiments. Cyclin D2, E2, EZH2, and PCNA levels were analyzed by Western blot and immunohistochemistry.ResultsmiR-26a was expressed in the cytoplasm of pancreatic ductal epithelial cells, whereas its expression was significantly downregulated in PDAC tissues compared with that of ABPT. Patients with low miR-26a expression had a significantly shorter survival than those with high miR-26a expression. The in vitro and in vivo assays showed that overexpression of miR-26a resulted in cell cycle arrest, inhibited cell proliferation, and decreased tumor growth, which was associated with cyclin E2 downregulation.ConclusionsmiR-26a is an important suppressor of pancreatic ductal carcinoma, and can prove to be a novel prognostic factor and therapeutic target for pancreatic cancer treatment.

MiR-26a enhances miRNA biogenesis by targeting Lin28B and Zcchc11 to suppress tumor growth and metastasis

Human cancers often exhibit attenuated microRNA (miRNA) biogenesis and global underexpression of miRNAs; thus, targeting the miRNA biogenesis pathway represents a novel strategy for cancer therapy. Here, we report that miR-26a enhances miRNA biogenesis, which acts as a common mechanism partially accounting for miR-26a function in diverse cancers including melanoma, prostate and liver cancer. miR-26a was broadly reduced in multiple cancers, and overexpression of miR-26a significantly suppressed tumor growth and metastasis both in vitro and in vivo, including melanoma, prostate and liver cancers. Notably, miR-26a overexpression was accompanied by global upregulation of miRNAs, especially let-7, and let-7 expression was concordant with miR-26a expression in cancer cell lines, xenograft tumors and normal human tissues, underscoring their biological relevance. We showed that miR-26a directly targeted Lin28B and Zcchc11-two critical repressors of let-7 maturation. Furthermore, we have demonstrated that Zcchc11 promoted tumor growth and metastasis, and it was prominently overexpressed in human cancers. Our findings thus provide a novel mechanism by which a miRNA acts as a modulator of miRNA biogenesis. These results also define a role of the miR-26a and Zcchc11 in tumorigenesis and metastasis and have implications to develop new strategies for cancer therapy.

TP53-independent Function of miR-34a via HDAC1 and p21CIP1/WAF1

The tumor suppressor, microRNA-34 (miR-34), a transcriptional target of TP53, functions in a positive feedback loop to activate TP53. Although miR-34 can inhibit cancer cells carrying TP53 mutations, this feedback to TP53 may be a prerequisite for full miR-34 function and may restrict its therapeutic application to patients with intact TP53. To investigate the functional relationships between TP53 and miR-34, and that of other TP53-regulated miRNAs including miR-215/192, we have used a panel of isogenic cancer cell lines that differ only with respect to their endogenous TP53 status. miR-34-induced inhibition of cancer cell growth is the same in TP53-positive and TP53-negative cells. In contrast, miR-215/192 functions through TP53. In the absence of TP53, miR-34, but not miR-215/192, is sufficient to induce an upregulation of the cell cycle-dependent kinase inhibitor p21(CIP1/WAF1). We identify histone deacetylase 1 (HDAC1) as a direct target of miR-34 and demonstrate that repression of HDAC1 leads to an induction of p21(CIP1/WAF1) and mimics the miR-34 cellular phenotype. Depletion of p21(CIP1/WAF1) specifically interferes with the ability of miR-34 to inhibit cancer cell proliferation. The data suggest that miR-34 controls a tumor suppressor pathway previously reserved for TP53 and provides an attractive therapeutic strategy for cancer patients irrespective of TP53 status.

Elevated microRNA‐34a in obesity reduces NAD+ levels and SIRT1 activity by directly targeting NAMPT

SIRT1 is an NAD(+)-dependent deacetylase that is implicated in prevention of many age-related diseases including metabolic disorders. As SIRT1 deacetylase activity is dependent on NAD(+) levels and the development of compounds that directly activate SIRT1 has been controversial, indirectly activating SIRT1 through enhancing NAD(+) bioavailability has received increasing attention. NAD(+) levels are reduced in obesity and the aged, but the underlying mechanisms remain unclear. We recently showed that hepatic microRNA-34a (miR-34a), which is elevated in obesity, directly targets and decreases SIRT1 expression. Here, we further show that miR-34a reduces NAD(+) levels and SIRT1 activity by targeting NAMPT, the rate-limiting enzyme for NAD(+) biosynthesis. A functional binding site for miR-34a is present in the 3' UTR of NAMPT mRNA. Hepatic overexpression of miR-34a reduced NAMPT/NAD(+) levels, increased acetylation of the SIRT1 target transcriptional regulators, PGC-1α, SREBP-1c, FXR, and NF-κB, and resulted in obesity-mimetic outcomes. The decreased NAMPT/NAD(+) levels were independent of miR-34a effects on SIRT1 levels as they were also observed in SIRT1 liver-specific knockout mice. Further, the miR-34a-mediated decreases were reversed by treatment with the NAD(+) intermediate, nicotinamide mononucleotide. Conversely, antagonism of miR-34a in diet-induced obese mice restored NAMPT/NAD(+) levels and alleviated steatosis, inflammation, and glucose intolerance. Anti-miR-34a-mediated increases in NAD(+) levels were attenuated when NAMPT was downregulated. Our findings reveal a novel function of miR-34a in reducing both SIRT1 expression and activity in obesity. The miR-34a/NAMPT axis presents a potential target for treating obesity- and aging-related diseases involving SIRT1 dysfunction like steatosis and type 2 diabetes.

A miR-34a-SIRT6 axis in the squamous cell differentiation network

Squamous cell carcinomas (SCCs) are highly heterogeneous tumours, resulting from deranged expression of genes involved in squamous cell differentiation. Here we report that microRNA-34a (miR-34a) functions as a novel node in the squamous cell differentiation network, with SIRT6 as a critical target. miR-34a expression increases with keratinocyte differentiation, while it is suppressed in skin and oral SCCs, SCC cell lines, and aberrantly differentiating primary human keratinocytes (HKCs). Expression of this miRNA is restored in SCC cells, in parallel with differentiation, by reversion of genomic DNA methylation or wild-type p53 expression. In normal HKCs, the pro-differentiation effects of increased p53 activity or UVB exposure are miR-34a-dependent, and increased miR-34a levels are sufficient to induce differentiation of these cells both in vitro and in vivo. SIRT6, a sirtuin family member not previously connected with miR-34a function, is a direct target of this miRNA in HKCs, and SIRT6 down-modulation is sufficient to reproduce the miR-34a pro-differentiation effects. The findings are of likely biological significance, as SIRT6 is oppositely expressed to miR-34a in normal keratinocytes and keratinocyte-derived tumours.

MiR-26a promotes neurite outgrowth by repressing PTEN expression

A multitude of studies have reported that microRNAs (miRNAs) are important in neuronal development. As a key regulator altering the expression of certain mRNAs, miR-26a has been demonstrated to play a role in the central nervous system (CNS). In the current study, the function of miR-26a in neuronal development was investigated. The overexpression of miR-26a was hypothesized to significantly enhance synaptic plasticity and regulate neuronal morphogenesis. The number and distribution of neurites was markedly increased by miR-26a. In addition, inhibition of miR-26a function attenuated neuronal outgrowth. Furthermore, phosphatase and tensin homolog (PTEN) was identified as a direct target of miR-26a in this process via a luciferase reporter assay. The growth of neurites was consistently suppressed by PTEN overexpression. Therefore, our study demonstrated that miR-26a promoted neurite outgrowth via the suppression of PTEN expression, indicating that miR-26a is important in neuronal development and morphogenesis. miR-26a has the potential to serve as a therapeutic target for patients with Alzheimer's disease (AD).

MicroRNA-34a Sensitizes Lung Cancer Cell Lines to DDP Treatment Independent of p53 Status

miR-34a was identified as one of the downregulated microRNAs (miRNAs) in human lung cancer. However, the precise biological role of miR-34a in p53 deficient lung cancer cell lines remains largely elusive. In the present study, we aimed to identify the role of miR-34a in the regulation of lung cancer cell proliferation. Using quantitative RT-PCR analysis, we found that miR-34a was highly upregulated in the p53 wild-type A549 human lung cancer cell line when treated with the DNA damaging agent adriamycin (ADR), but not in the SBC-5 cells harboring mutated p53. Transient introduction of miR-34a into A549 and SBC-5 cell lines caused complete suppression of cell proliferation and induced the cell cycle arrested at the G(1) phase. When we knockdown the miR-34a downstream target--Sitr1--using the small-interfering RNA, there was also a cell growth inhibition in both cell lines though not as much as miR-34a did. Moreover, we demonstrated that pretransfection of miR-34a could increase the sensitivity of both lung cancer cell lines to cisplatin (DDP), and this could be reverted by the miR-34a inhibitor. Moreover, when cells pretreated with siR-Sirt1, they are more sensitive to DDP than the control pretreated cells as well. We thus hypothesize the miR-34a/Sirt1 cascade involved with p53-independent functions. Overall, in this study, we found the proliferation inhibition function of miR-34a in vitro in lung cancer cell lines is p53 independent, and also demonstrated the combination therapeutic potential of miR-34a and DDP in lung cancer cell lines.

MicroRNA-34 suppresses breast cancer invasion and metastasis by directly targeting Fra-1

MicroRNAs have key roles in tumor metastasis. Here, we describe the regulation and function of miR-34a and miR-34c (miR-34a/c) in breast cancer metastasis. Expression analysis verified that miR-34a/c expression is significantly decreased in metastatic breast cancer cells and human primary breast tumors with lymph node metastases. Overexpression of miR-34a/c could inhibit breast cancer cell migration and invasion in vitro and distal pulmonary metastasis in vivo. Further studies revealed that Fos-related antigen 1 (Fra-1 or Fosl1) is a downstream target of miR-34a/c as miR-34a/c bound directly to the 3'untranslated region of Fra-1, subsequently reducing both the mRNA and protein levels of Fra-1. Silencing of Fra-1 recapitulated the effects of miR-34a/c overexpression, whereas enforced expression of Fra-1 reverses the suppressive effects of miR-34a/c. Moreover, significant downregulation of miR-34a in metastatic breast cancer tissues was found to be inversely correlated with Fra-1 expression. Our results demonstrate that miR-34a/c functions as a metastasis suppressor to regulate breast cancer migration and invasion through targeting Fra-1 oncogene and suggest a therapeutic application of miR-34 in breast cancer.

Altered miRNA and gene expression in acute myeloid leukemia with complex karyotype identify networks of prognostic relevance

Recently, the p53-miR-34a network has been identified to have an important role in tumorigenesis. As in acute myeloid leukemia with complex karyotype (CK-AML) TP53 alterations are the most common known molecular lesion, we further analyzed the p53-miR-34a axis in a large cohort of CK-AML with known TP53 status (TP53(altered), n=57; TP53(unaltered), n=31; altered indicates loss and/or mutation of TP53). Profiling microRNA (miRNA) expression delineated TP53 alteration-associated miRNA profiles, and identified miR-34a and miR-100 as the most significantly down- and upregulated miRNA, respectively. Moreover, we found a distinct miR-34a expression-linked gene expression profile enriched for genes belonging to p53-associated pathways, and implicated in cell cycle progression or apoptosis. Clinically, low miR-34a expression and TP53 alterations predicted for chemotherapy resistance and inferior outcome. Notably, in TP53(unaltered) CK-AML, high miR-34a expression predicted for inferior overall survival (OS), whereas in TP53(biallelic altered) CK-AML, high miR-34a expression pointed to better OS. Thus, detailed molecular profiling links impaired p53 to decreased miR-34a expression, but also identifies p53-independent miR-34a induction mechanisms as shown in TP53(biallelic altered) cell lines treated with 15-deoxy-Δ(12,14)-prostaglandin. An improved understanding of this mechanism might provide novel therapeutic options to restore miR-34a function and thereby induce cell cycle arrest and apoptosis in TP53(altered) CK-AML.

Abstract 680: Loss of miR-34a as measured by quantitative in situ hybridization on a tissue microarray is associated with poor outcome in breast cancer

Abstract Introduction: MicroRNAs (miRNAs) have emerged as key regulators in the pathogenesis of cancers as either oncogenes or tumor suppressors. It has been shown that the loss of miR-34a leads to tumor progression and metastasis using cell lines and a limited number of patient samples by RT-PCR. Lack of a robust, reproducible and quantitative method has limited large scale analysis and assessment of miR-34a as tumor suppressor marker in cancer. Herein, we have developed and validated a method for the quantitative analysis of miRNA expression by in situ hybridization (qISH) allowing for the direct assessment of tumor epithelial expression of miR34a in breast cancer. Method: Expression level of miR-34a was measured in a retrospective breast cancer (n=461) cohort with 20 year follow-up using quantitative immunofluorescence (AQUA) technology for ISH in a tissue microarray (TMA) format. The assay was performed in two-fold redundancy with a 40 case index TMA for reproducibility and standardization. Averaged AQUA scores for miR34a were correlated with clinical and pathological characteristics and 10 year disease-free survival in this cohort. Results: Since miR-34a is a tumor suppressor, we determined the threshold at which no specific hybridization was seen (AQUA score =16.89). Reproducibility between two different builds (cores) of the TMA was R*2 = 0.59. Using overall survival as an endpoint in Kaplan Meier analysis with the threshold of expression as the cutpoint, the group with loss of miR-34a had significantly worse survival (log rank p = 0.0264). Cox univariate analysis for miR-34a showed hazard ratio of 1.53 (95% CI: 1.05-2.22, p=0.0276). Cox multivariate analysis including age, nuclear grade, nodal status, ER, PR and Her2 showed miR34a is not an independent marker (p=0.0908). An independent breast cancer cohort will be used to validate this result. Conclusions: The microRNA miR-34a has been proposed to be a tumor suppressor using mechanistic and functional data. This result, showing loss of miR-34a is associated with worse outcome in a large breast cancer cohort provides clinical evidence of the tumor suppressor function of miR-34a. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 680. doi:1538-7445.AM2012-680

MiR-34a Regulates Mouse Neural Stem Cell Differentiation

BackgroundMicroRNAs (miRNAs or miRs) participate in the regulation of several biological processes, including cell differentiation. Recently, miR-34a has been implicated in the differentiation of monocyte-derived dendritic cells, human erythroleukemia cells, and mouse embryonic stem cells. In addition, members of the miR-34 family have been identified as direct p53 targets. However, the function of miR-34a in the control of the differentiation program of specific neural cell types remains largely unknown. Here, we investigated the role of miR-34a in regulating mouse neural stem (NS) cell differentiation.Methodology/Principal FindingsmiR-34a overexpression increased postmitotic neurons and neurite elongation of mouse NS cells, whereas anti-miR-34a had the opposite effect. SIRT1 was identified as a target of miR-34a, which may mediate the effect of miR-34a on neurite elongation. In addition, acetylation of p53 (Lys 379) and p53-DNA binding activity were increased and cell death unchanged after miR-34a overexpression, thus reinforcing the role of p53 during neural differentiation. Interestingly, in conditions where SIRT1 was activated by pharmacologic treatment with resveratrol, miR-34a promoted astrocytic differentiation, through a SIRT1-independent mechanism.ConclusionsOur results provide new insight into the molecular mechanisms by which miR-34a modulates neural differentiation, suggesting that miR-34a is required for proper neuronal differentiation, in part, by targeting SIRT1 and modulating p53 activity.

MicroRNA, SND1, and alterations in translational regulation in colon carcinogenesis

Post-transcriptional regulation of gene expression by microRNA (miRNA) has recently attracted major interest in relation to its involvement in cancer development. miRNA is a member of small non-coding RNA, consists of 22–24 nucleotides and regulates expression of target mRNA species in a post-transcriptional manner by being incorporated with RNA-induced silencing complex (RISC). Staphylococcal nuclease homology domain containing 1 (SND1), a component of RISC, is frequently up-regulated in human colon cancers and also chemically induced colon cancers in animals. We here showed that SDN1 is involved in miRNA-mediated gene suppression and overexpression of SND1 in colon cancer cells causes down-regulation of APC without altering APC mRNA levels. As for the miRNA expression profile in human colon cancer, miR-34a was among the list of down-regulated miRNA. Expression of miR-34a is tightly regulated by p53, and ectopic expression of miR-34a in colon cancer cells causes remarkable reduction of cell proliferation and induces senescence-like phenotypes. MiR-34a also participates in the positive feedback loop of the p53 tumor suppressor network. This circuitry mechanism for p53 activation is of interest in understanding the tumor suppressive function of miR-34a in colon carcinogenesis. miRNA should also be considered as novel anti-cancer agents in tumor suppressive therapeutic applications.

The impact of miR‐34a on protein output in hepatocellular carcinoma HepG2 cells

MicroRNAs are small non-coding RNA molecules that play essential roles in biological processes ranging from cell cycle to cell migration and invasion. Accumulating evidence suggests that miR-34a, as a key mediator of p53 tumor suppression, is aberrantly expressed in human cancers. In the present study, we aimed to explore the precise biological role of miR-34a and the global protein changes in HCC cell line HepG2 cells transiently transfected with miR-34a. Transfection of miR-34a into HepG2 cells caused suppression of cell proliferation, inhibition of cell migration and invasion. It also induced an accumulation of HepG2 cells in G1 phase. Among 116 protein spots with differential expression separated by 2-DE method, 34 proteins were successfully identified by MALDI-TOF/TOF analysis. Of these, 15 downregulated proteins may be downstream targets of miR-34a. Bioinformatics analysis produced a protein-protein interaction network, which revealed that the p53 signaling pathway and cell cycle pathway were two major hubs containing most of the proteins regulated by miR-34a. Cytoskeletal proteins such as LMNA, GFAP, MACF1, ALDH2, and LOC100129335 are potential targets of miR-34a. In conclusion, abrogation of miR-34a function could cause downstream molecules to switch on or off, leading to HCC development.

Differential MicroRNA-34a Expression and Tumor Suppressor Function in Retinoblastoma Cells

The role of miR-34a, a p53-regulated microRNA, in retinoblastoma (RB) was investigated. The expression of miR-34 family members in RB cells was determined by semiquantitative RT-PCR and real-time qPCR. Regulation of miR-34a expression by p53-activating compounds was determined by qPCR analysis. The tumor suppressor functions of miR-34a in RB cell lines were determined by tetrazolium-based cell growth assay and by caspase-3/7 and activated caspase-3 apoptotic activity assays. Additive growth inhibitory properties of miR-34a in combination with topotecan were determined by cell growth assay. miR-34a targets in RB cells were identified by real-time qPCR expression analysis of previously reported and GenMiR++-predicted mRNAs. Differential miR-34a and miR-34b expression was observed in RB cell lines and tumor samples. miR-34a expression could be increased in Y79 cells, but not Weri-Rb1 cells, after p53 activation. This differential regulation was not caused by genomic alterations at the miR-34a p53 binding site or mature gene. Exogenous miR-34a inhibited Y79 and Weri-Rb1 cell growth and increased apoptotic activity in Y79 cells. Increased inhibition of Y79 and Weri-Rb1 cell growth was observed with combination miR-34a and topotecan treatment. mRNA expression changes were observed in 7 of 7 previously reported and 13 of 18 GenMiR++-predicted miR-34a targets after transfection of Y79 cells with miR-34a compared with negative control microRNA. miR-34a functions as a tumor suppressor in RB cells and is a potential therapeutic target. Differential expression, regulation, and activity of miR-34a in RB cells may suggest further p53 pathway inactivation in RB.

Transactivation of miR-34a by p53 Broadly Influences Gene Expression and Promotes Apoptosis

The p53 tumor suppressor protein is a critical regulator of the cellular response to cancer-initiating insults such as genotoxic stress. In this report, we demonstrate that microRNAs (miRNAs) are important components of the p53 transcriptional network. Global miRNA expression analyses identified a cohort of miRNAs that exhibit p53-dependent upregulation following DNA damage. One such miRNA, miR-34a, is commonly deleted in human cancers and, as shown here, frequently absent in pancreatic cancer cells. Characterization of the miR-34a primary transcript and promoter demonstrates that this miRNA is directly transactivated by p53. Expression of miR-34a causes dramatic reprogramming of gene expression and promotes apoptosis. Much like the known set of p53-regulated genes, miR-34a-responsive genes are highly enriched for those that regulate cell-cycle progression, apoptosis, DNA repair, and angiogenesis. Therefore, it is likely that an important function of miR-34a is the modulation and fine-tuning of the gene expression program initiated by p53.