The small RNA molecules of about 19-22 nucleotides in length, aptly called microRNAs, perform the task of gene regulation in the cell. Interestingly, till the early nineties very little was known about them but eventually, the microRNAs have become forefront in the area of research. The huge number of microRNAs plus each one of them targeting a vast number of related as well as unrelated genes makes them very interesting molecules to study. To add to the mystery of miRNAs is the fact that the same miRNA can have antagonizing role in two different cell types i.e. in one cell type; the miRNA promotes proliferation whereas in another cell type the same miRNA inhibits proliferation. Another remarkable aspect of the microRNAs is that many of them exist in clusters. In humans alone, out of 721 microRNAs known, 247 of them occur in 64 clusters at an inter-miRNA distance of less than 5000bp. The reason for this clustering of miRNAs is not fully understood but since the miRNA clusters are evolutionary conserved, their significance cannot be ruled out. The objective of this review is to summarize the recent progress on the functional characterization of miR-23a~27a~24-2 cluster in humans in relation to various health and diseased conditions and to highlight the cooperative effects of the miRNAs of this cluster.

miRNAs have emerged as important players in the regulation of gene expression and their deregulation is a common feature in a variety of diseases, especially cancer. Currently, many efforts are focused on studying miRNA expression patterns, as well as miRNA target validation. Here, we show that the over expression of miR-23a∼27a∼24-2 cluster in HEK293T cells induces apoptosis by caspase-dependent as well as caspase-independent pathway as proved by the annexin assay, caspase activation, release of cytochrome-c and AIF (apoptosis inducing factor) from mitochondria. Furthermore, the over expressed cluster modulates the expression of a number of genes involved in apoptosis including FADD (Fas Associated protein with Death Domain). Bioinformatically, FADD is predicted to be the target of hsa-miR-27a and interestingly, FADD protein was found to be up regulated consistent with very less expression of hsa-miR-27a in HEK293T cells. This effect was direct, as hsa-miR-27a negatively regulated the expression of FADD 3′UTR based reporter construct. Moreover, we also showed that over expression of miR-23a∼27a∼24-2 sensitized HEK293T cells to TNF-α cytotoxicity. Taken together, our study demonstrates that enhanced TNF-α induced apoptosis in HEK293T cells by over expression of miR-23a∼27a∼24-2 cluster provides new insights in the development of novel therapeutics for cancer.

miRNAs and DNA methylation are both critical regulators of gene expression. Aberration in miRNA expression or DNA methylation is a causal factor for numerous pathological conditions. DNA methylation can inhibit the transcription of miRNAs, just like coding genes, by methylating the CpG islands in the promoter regions of miRNAs. Conversely, certain miRNAs can directly target DNA methyltransferases and bring about their inhibition, thereby affecting the whole genome methylation pattern. Recently, methylation patterns have also been revealed in mRNA. Surprisingly, the two most commonly studied methylation states in mRNA (m6A and m5C) are found to be enriched in 3'-UTRs (untranslated regions), the target site for the majority of miRNAs. Whereas m5C is reported to stabilise mRNA, m6A has a destabilising effect on mRNA. However, the effect of mRNA methylation on its interaction with miRNAs is largely unexplored. The review highlights the complex interplay between microRNA and methylation at DNA and mRNA level.

Previously, we had reported that overexpression of miR-23a~27a~24-2 cluster induces caspase-dependent and -independent apoptosis via JNK in human embryonic kidney (HEK293T) cells. Herein, we describe the molecular mechanism(s) responsible for miR-23a~27a~24-2 cluster induced apoptosis. Gene expression profiling was used to characterize the transcriptional response to miR-23a~27a~24-2 cluster overexpression in HEK293T cells. The microarray analysis gave 1,025 differentially expressed genes and analysis of the gene expression data with Ingenuity Pathway Analysis (IPA) software revealed p53 signaling, oxidative stress response and mitochondrial dysfunction among the top processes being affected. This data substantiates our previous study where we had reported increase of ROS and the release of proapoptotic factors such as cytochrome c (cyt c) and apoptosis-inducing factor (AIF) from the mitochondria to cytosol. Additionally, components of ER stress-mediated apoptosis pathway i.e., C/EBP homologous protein (CHOP/DDIT3/GADD153) and TRIB3, an Akt inhibitor were found to be significantly enriched. Also, the enhanced expression of ATF3 and ATF4 was observed at RNA as well as protein level in miR-23a~27a~24-2 cluster overexpressed HEK293T cells. Induction of BIM appeared to be specific, because ER stress caused only a minor change in the expression of the related BH3-only proteins BID or PUMA. The fact that miR-23a~27a~24-2 cluster triggered endoplasmic reticulum stress (ER stress) was further established by the increase in cytosolic calcium levels after overexpression of this cluster in HEK293T cells. These findings were also supported by PANTHER analysis wherein biological process categories of apoptosis and stress response were enriched. Taken together, this work underlines the role of ER stress in miR-23a~27a~24-2 cluster mediated apoptosis in HEK293T cells. Since the detailed knowledge of this cluster induced apoptosis has now been elucidated, the in vivo study of this cluster would help evaluate the prospect of its use as a therapeutic in diseases known to occur because of deregulation of apoptosis.

Metastatic cancer cells generally cannot be eradicated using traditional surgical or chemoradiotherapeutic strategies, and disease recurrence is extremely common following treatment. On the other hand, therapies employing stem cells are showing increasing promise in the treatment of cancer. Stem cells can function as novel delivery platforms by homing to and targeting both primary and metastatic tumor foci. Stem cells engineered to stably express various cytotoxic agents decrease tumor volumes and extend survival in preclinical animal models. They have also been employed as virus and nanoparticle carriers to enhance primary therapeutic efficacies and relieve treatment side effects. Additionally, stem cells can be applied in regenerative medicine, immunotherapy, cancer stem cell-targeted therapy, and anticancer drug screening applications. However, while using stem cells to treat human cancers appears technically feasible, challenges such as treatment durability and tumorigenesis necessitate further study to improve therapeutic performance and applicability. This review focuses on recent progress toward stem cell-based cancer treatments, and summarizes treatment advantages, opportunities, and shortcomings, potentially helping to refine future trials and facilitate the translation from experimental to clinical studies.