Hormonal Regulation of MicroRNA Biogenesis

MicroRNA in cardiometabolic health and disease: The perspectives of sex, gender and personalised medicine

BACKGROUND

Personalized medicine represents a novel and integrative approach that focuses on an individual's genetics and epigenetics, precision medicine, lifestyle and exposures as key players of health status and disease phenotypes.

METHODS

In this narrative review, we aim to carefully discuss the current knowledge on gender disparities in cardiometabolic diseases, and we consider the sex- specific expression of miRNAs and their role as promising tool in precision medicine.

RESULTS

Personalised medicine overcomes the restricted care of patient based on a binomial sex approach, by enriching itself with a holistic and dynamic gender integration. Recognized as a major worldwide health emergency, cardiometabolic disorders continue to rise, impacting on health systems and requiring more effective and targeted strategies. Several sex and gender drivers might affect the onset and progression of cardiometabolic disorders in males and females at multiple levels. In this respect, distinct contribution of genetic and epigenetic mechanisms, molecular and physiological pathways, sex hormones, visceral fat and subcutaneous fat and lifestyle lead to differences in disease burden and outcomes in males and females.

CONCLUSIONS

Sex and gender play a pivotal role in precision medicine because the influence the physiology of each individual and the way they interact with environment from intrauterine life.

Potential Regulation of miRNA-29 and miRNA-9 by Estrogens in Neurodegenerative Disorders: An Insightful Perspective

Finding a link between a hormone and microRNAs (miRNAs) is of great importance since it enables the adjustment of genetic composition or cellular functions without needing gene-level interventions. The dicer-mediated cleavage of precursor miRNAs is an interface link between miRNA and its regulators; any disruption in this process can affect neurogenesis. Besides, the hormonal regulation of miRNAs can occur at the molecular and cellular levels, both directly, through binding to the promoter elements of miRNAs, and indirectly, via regulation of the signaling effects of the post-transcriptional processing proteins. Estrogenic hormones have many roles in regulating miRNAs in the brain. This review discusses miRNAs, their detailed biogenesis, activities, and both the general and estrogen-dependent regulations. Additionally, we highlight the relationship between miR-29, miR-9, and estrogens in the nervous system. Such a relationship could be a possible etiological route for developing various neurodegenerative disorders.

Comparative transcriptome analysis of brain and gonad reveals reproduction-related miRNAs in the giant prawn, Macrobrachium rosenbergii

Macrobrachium rosenbergii (M. rosenbergii), as a species of common prawn, is a delicacy that is consumed all over the world. By interacting with the target gene 3′-untranslated region (3'-UTR), microRNAs (miRNAs) regulate its expression and ultimately participate in the regulation of reproductive development. However, research focusing on miRNA regulation during gonadal development in M. rosenbergii received very little attention. To explore the association between miRNA and reproduction, we performed RNA sequencing (RNA-seq) on brain and gonad organs in male and female M. rosenbergii. A total of 494 miRNAs were obtained in RNA-seq, including 31 and 59 differentially expressed (DE) miRNAs in the brain and gonads, respectively. Furthermore, 9 DE miRNAs were randomly selected from the brain and gonads, and qRT-PCR was conducted to validate the results of RNA-seq. Interestingly, dpu-miR-133 was found to be substantially expressed in the male brain and testis but poorly expressed in the female brain, ovary, and other organs. Analysis of dpu-miR-133 by Targetscan and MiRanda predicted to target 5-HT1. Furthermore, the dual-luciferase reporter assay manifested that dpu-miR-133 can combine with 5-HT1. Overall, our research work provides basic data for further study on the miRNA-mediated regulation of brain, gonad, and reproductive development of study M. rosenbergii.

<em>miR-19a</em> targeting <em>CLCA4</em> to regulate the proliferation, migration, and invasion of colorectal cancer cells

The role of miR-19a in colorectal cancer (CRC), a devastating disease with high mortality and morbidity, remains controversial. In the present study, we show that the level of miR-19a is significantly higher in clinical CRC tissue samples than in paracancerous tissue samples, and significantly higher in CRC cells lines HT29, SW480, and CaCO2 than in the normal human colon mucosal epithelial cell line NCM460. miR-19a mimics and inhibitors were synthesized and validated. Overexpression of miR-19a mimics significantly promoted, while miR-19a inhibitors inhibited, the proliferation, survival, migration, and invasion of SW480 and CaCO2 CRC cells. Furthermore, mRNA and protein levels of chloride channel accessory 4 (CLCA4) were lower in CRC cells and tissues. Bioinformatics and a luciferase reporter assay confirmed that CLCA4 was a miR-19a target. Further, miR-19a inhibition increased CLCA4 expression. The inhibitory effect of miR-19a on cell growth, survival, migration, and invasion was reversed by knockdown of CLCA4 expression. The data demonstrated that the miR-19a/CLCA4 axis modulates phospho-activation of the PI3K/AKT pathway in CRC cells. In conclusion, our results revealed that miR-19a overexpression decreases CLCA4 levels to promote CRC oncogenesis, suggesting that miR-19a inhibitors have potential applications for future therapeutic of CRC.

Epigenetic Regulations of Microglia/Macrophage Polarization in Ischemic Stroke

Ischemic stroke is one of the leading causes of death and disability worldwide. Microglia/macrophages (MMs)-mediated neuroinflammation contributes significantly to the pathological process of ischemic brain injury. Microglia, serving as resident innate immune cells in the central nervous system, undergo pro-inflammatory phenotype or anti-inflammatory phenotype in response to the microenvironmental changes after cerebral ischemia. Emerging evidence suggests that epigenetics modifications, reversible modifications of the phenotype without changing the DNA sequence, could play a pivotal role in regulation of MM polarization. However, the knowledge of the mechanism of epigenetic regulations of MM polarization after cerebral ischemia is still limited. In this review, we present the recent advances in the mechanisms of epigenetics involved in regulating MM polarization, including histone modification, non-coding RNA, and DNA methylation. In addition, we discuss the potential of epigenetic-mediated MM polarization as diagnostic and therapeutic targets for ischemic stroke. It is valuable to identify the underlying mechanisms between epigenetics and MM polarization, which may provide a promising treatment strategy for neuronal damage after cerebral ischemia.

MiR-423-5p may regulate ovarian response to ovulation induction via CSF1

BACKGROUND

We have previously shown that hsa-miR-423-5p expression in ovarian granulosa cells is decreased in high ovarian response populations. The objective of the present study was to find the target gene and mechanism for miR-423-5p involved in ovarian response regulation.

METHODS

(a) TargetScan was used to predict the target gene of hsa-miR-423-5p. (b) A model for hsa-miR-423-5p overexpression or inhibition was constructed by transfecting KGN cells with lentivirus. CSF1 mRNA and protein expression and luciferase activity were measured. (c) The cell cycles of control and lentivirus treated KGN cells were analyzed. Western blot was used to measure the expression of CDKN1A in KGN cells. (d) The concentration of E₂ in KGN cell culture medium were measured.

RESULTS

(a) TargetScan revealed that the 3' un-translated region of CSF1 matched 11 bases at the 5' end of miR-423-5p, making it a likely target gene. (b) Overexpression or inhibition of miR-423-5p were associated with respective decreases or increases in CSF1 expression (both mRNA and protein) (p < 0.05) and luciferase activity (p < 0.05). (c) When miR-423-5p expression increased, the number of G0/G1 phase cells and the expression of CDKN1A protein increased while estradiol concentrations in the cell culture solution decreased (p < 0.05). However, when miR-423-5p expression decreased, the number of S phase cells increased and E2 concentrations increased while the expression of CDKN1A protein decreased (p < 0.05).

CONCLUSIONS

Colony stimulating factor 1 is a target gene of miR-423-5p and that it may regulate ovarian response to ovulation induction by affecting granulosa cells proliferation and estrogen secretion.

Estrogen Reverses HDAC Inhibitor-Mediated Repression of Aicda and Class-Switching in Antibody and Autoantibody Responses by Downregulation of miR-26a

Estrogen contributes to females' strong antibody response to microbial vaccines and proneness to autoimmunity, particularly antibody-mediated systemic autoimmunity, in females. We have hypothesized that this is due to estrogen-mediated potentiation of class switch DNA recombination (CSR) and somatic hypermutation (SHM). As we have shown, estrogen boosts AID expression, which is critical for both CSR and SHM, through upregulation of HoxC4, which together with NF-κB critically mediates Aicda (AID gene) promoter activation. We contend here that additional regulation of Aicda expression by estrogen occurs through epigenetic mechanisms. As we have shown, histone deacetylase inhibitors (HDIs) short-chain fatty acid (SCFA) butyrate and propionate as well as the pharmacologic HDI valproic acid upregulate miRNAs that silence AID expression, thereby modulating specific antibody responses in C57BL/6 mice and autoantibody responses in lupus-prone MRL/Faslpr/lpr mice. Here, using constitutive knockout Esr1-/- mice and B cells as well as conditional knockout Aicdacre/creEsr1flox/flox mice and B cells, we showed that the HDI-mediated downregulation of Aicda expression as well as the maturation of antibody and autoantibody responses is reversed by estrogen and enhanced by deletion of ERα or E2 inhibition. Estrogen's reversion of HDI-mediated inhibition of Aicda and CSR in antibody and autoantibody responses occurred through downregulation of B cell miR-26a, which, as we showed, targets Aicda mRNA 3'UTR. miR-26a was significantly upregulated by HDIs. Accordingly, enforced expression of miR-26a reduced Aicda expression and CSR, while miR-26a-sponges (competitive inhibitors of miR-26a) increased Aicda expression and CSR. Thus, our findings show that estrogen reverses the HDI-mediated downregulation of AID expression and CSR through selective modulation of miR-26a. They also provide mechanistic insights into the immunomodulatory activity of this hormone and a proof-of-principle for using combined ER inhibitor-HDI as a potential therapeutic approach.

Transcriptional suppression of the miR-15/16 family by c-Myc in malignant pleural mesothelioma

MicroRNA downregulation is frequent in malignant pleural mesothelioma (MPM), but the mechanisms responsible for loss of miR-15/16 and miR-193a are yet to be elucidated and were investigated in this study. Copy Number Variation (CNV) of microRNA-coding genes was analyzed in MPM cells by digital droplet PCR (ddPCR) and revealed heterozygous loss of miR-193a and miR-15a/16-1, but no change in miR-15b/16-2. Epigenetic control of microRNA expression was inferred following decitabine and Trichostatin A (TSA) treatment which did not substantially affect microRNA expression. Knockdown of c-Myc expression led to upregulation of SMC4, miR-15b and 16, and to a lesser extent DLEU2 and miR-15a, whereas c-Myc overexpression repressed microRNA expression. Chromatin immunoprecipitation (ChIP) assays confirmed the interaction of c-Myc with the DLEU2 and SMC4 promoters. Tumor microRNA expression was determined in samples from MPM patients, with samples of pleura from cardiac surgery patients used as controls. In tumor samples, a strong correlation was observed between the expression of miR-15b and 16 (R²=0.793), but not miR-15a and 16. Our data suggest that in MPM, the downregulation of miR-15/16 is due to transcriptional repression by c-Myc, primarily via control of the miR-15b/16-2 locus, while miR-193a-3p loss is due to genomic deletion.

MicroRNA Expression is Associated with Sepsis Disorders in Critically Ill Polytrauma Patients

A critically ill polytrauma patient is one of the most complex cases to be admitted to the intensive care unit, due to both the primary traumatic complications and the secondary post-traumatic interactions. From a molecular, genetic, and epigenetic point of view, numerous biochemical interactions are responsible for the deterioration of the clinical status of a patient, and increased mortality rates. From a molecular viewpoint, microRNAs are one of the most complex macromolecular systems due to the numerous modular reactions and interactions that they are involved in. Regarding the expression and activity of microRNAs in sepsis, their usefulness has reached new levels of significance. MicroRNAs can be used both as an early biomarker for sepsis, and as a therapeutic target because of their ability to block the complex reactions involved in the initiation, maintenance, and augmentation of the clinical status.

Circulating small non-coding RNAs associated with age, sex, smoking, body mass and physical activity

Small non-coding RNAs (sncRNA) are regulators of cell functions and circulating sncRNAs from the majority of RNA classes are potential non-invasive biomarkers. Understanding how common traits influence ncRNA expression is essential for assessing their biomarker potential. In this study, we identify associations between sncRNA expression and common traits (sex, age, self-reported smoking, body mass, self-reported physical activity). We used RNAseq data from 526 serum samples from the Janus Serum Bank and traits from health examination surveys. Ageing showed the strongest association with sncRNA expression, both in terms of statistical significance and number of RNAs, regardless of RNA class. piRNAs were abundant in the serum samples and they were associated to sex. Interestingly, smoking cessation generally restored RNA expression to non-smoking levels, although for some sncRNAs smoking-related expression levels persisted. Pathway analysis suggests that smoking-related sncRNAs target the cholinergic synapses and may therefore potentially play a role in smoking addiction. Our results show that common traits influence circulating sncRNA expression. It is clear that sncRNA biomarker analyses should be adjusted for age and sex. In addition, for specific sncRNAs, analyses should also be adjusted for body mass, smoking, physical activity and technical factors.

microRNA Regulation in Estrogen Receptor-Positive Breast Cancer and Endocrine Therapy

As de novo and acquired resistance to standard first line endocrine therapies is a growing clinical challenge for estrogen receptor-positive (ER⁺) breast cancer patients, understanding the mechanisms of resistance is critical to develop novel therapeutic strategies to prevent therapeutic resistance and improve patient outcomes. The widespread post-transcriptional regulatory role that microRNAs (miRNAs) can have on various oncogenic pathways has been well-documented. In particular, several miRNAs are reported to suppress ERα expression via direct binding with the 3’ UTR of ESR1 mRNA, which can confer resistance to estrogen/ERα-targeted therapies. In turn, estrogen/ERα activation can modulate miRNA expression, which may contribute to ER⁺ breast carcinogenesis. Given the reported oncogenic and tumor suppressor functions of miRNAs in ER⁺ breast cancer, the targeted regulation of specific miRNAs is emerging as a promising strategy to treat ER⁺ breast cancer and significantly improve patient responsiveness to endocrine therapies. In this review, we highlight the major miRNA-ER regulatory mechanisms in context with ER⁺ breast carcinogenesis, as well as the critical miRNAs that contribute to endocrine therapy resistance or sensitivity. Collectively, this comprehensive review of the current literature sheds light on the clinical applications and challenges associated with miRNA regulatory mechanisms and novel miRNA targets that may have translational value as potential therapeutics for the treatment of ER⁺ breast cancer.

DDX3X promotes the biogenesis of a subset of miRNAs and the potential roles they played in cancer development

DDX3X, located on the X-chromosome, belongs to the DEAD-box RNA helicase family and acts as a key RNA-binding protein to exert its regulatory functions in various biological processes. In this paper, knock-down the expression of DDX3X can affect a subset of miRNA expression levels, especially for miR-1, miR-141, miR-145, miR-19b, miR-20a and miR-34a. Through adopting the immunoprecipitation (IP), RNA immunoprecipitation (RIP), dual luciferase reporter assays, we illustrate that DDX3X could interact with Drosha/DGCR8 complex, elevate the processing activity of Drosha/DGCR8 complex on pri-miRNAs, and increase mature miRNA expression levels. For the studies of potential roles and biological functions of DDX3X-dependent miRNAs and their downstream target genes in multiple cancers, we use the primary data from The Cancer Genome Atlas (TCGA), Ingenuity Pathway Analysis (IPA) and several miRNA target prediction databases, to systematically analyze the expression levels of DDX3X-dependent miRNAs in almost 14 kinds of cancers versus normal tissues, and the essential biological functions for their putative downstream target genes. All these findings will provide us novel insights and directions for thoroughly exploring the regulatory mechanisms of miRNA biogenesis, and shed light on effectively searching the clinical significances and biological roles of DDX3X-dependent miRNAs and their target genes in cancer development.

The Expression of Nuclear Transcription Factor Kappa B (NF-κB) in the Case of Critically Ill Polytrauma Patients with Sepsis and Its Interactions with microRNAs

Critical polytrauma patients present a series of pathophysiological disturbances, biochemical and molecular dysfunction, which comprise to be the major cause of intensive care unit admission. In regard to molecular damage, there exists a series of factors, which all together contribute to the aggravation of the clinical status leading to increased mortality rate in these patients. One of the most important biochemical factors involved is the nuclear transcription factor B (NF-κB). Impaired NF-κB functioning is reflected on the clinical status of the patient through increased production of pro-inflammatory molecule, leading to multiple organ dysfunction syndrome. In addition to this, through microRNAs interactions, various pathophysiological as well as biochemical disturbances are produced, which altogether further reduce the patient's survival rate. In this paper, we would like to present the modifications seen in the expression of NF-κB in critically polytraumatized patients with sepsis. In additions to this, we would like to discuss the correlation between the microRNAs and its further implications in clinical status of these patients.

Estrogen repression of microRNA as a potential cause of cancer

MicroRNAs (miRNAs) are endogenous small molecules that regulate gene expression and have been implicated in the pathogenesis of many human diseases, including cancer. This review describes the results that show a global repression in miRNA expression in various tumors and cancer cell lines. Intriguingly, recent discoveries have shown a widespread downregulation of miRNA after exposure to the steroid hormone estrogen. The integration of the results suggests that estrogen-dependent repression of miRNA is a potential cause of cancer.

Regulating life or death: potential role of microRNA in rescue of the corpus luteum

The role of miRNA in tissue biology has added a new level of understanding of gene regulation and function. The corpus luteum (CL) is a transitory endocrine gland; the dynamic nature of the CL makes it a candidate for regulation by miRNA. Rescue of the CL from luteolysis is essential for the maintenance of pregnancy in all eutherian mammals. Using next generation sequencing, we profiled miRNA expression in the bovine CL during maternal recognition of pregnancy. We identified 590 luteal miRNA, of which 544 were known and 46 were novel miRNAs. Fifteen (including 3 novel) miRNAs were differentially expressed between CL of pregnant vs. cyclic animals. Target analysis of the differentially expressed miRNA resulted in genes involved in regulating apoptosis and immune response, providing evidence that miRNAs regulate the intracellular pathways that lead to either luteal regression or survival.

Interplay between steroid signalling and microRNAs: implications for hormone-dependent cancers

Hormones are key drivers of cancer development. To date, interest has largely been focussed on the classical model of hormonal gene regulation, but there is increasing evidence for a role of hormone signalling pathways in post-translational regulation of gene expression. In particular, a complex and dynamic network of bi-directional interactions with microRNAs (miRs) at all stages of biogenesis and during target gene repression is emerging. miRs, which act mainly by negatively regulating gene expression through association with 3'-UTRs of mRNA species, are increasingly understood to be important in development, normal physiology and pathogenesis. Given recent demonstrations of altered miR profiles in a diverse range of cancers, their ability to function as oncogenes or tumour suppressors, and hormonal regulation of miRs, understanding mechanisms by which miRs are generated and regulated is vitally important. miRs are transcribed by RNA polymerase II and then processed in the nucleus by the Drosha-containing Microprocessor complex and in the cytoplasm by Dicer, before mature miRs are incorporated into the RNA-induced silencing complex. It is increasingly evident that multiple cellular signalling pathways converge upon the miR biogenesis cascade, adding further layers of regulatory complexity to modulate miR maturation. This review summarises recent advances in identification of novel components and regulators of the Microprocessor and Dicer complexes, with particular emphasis on the role of hormone signalling pathways in regulating their activity. Understanding hormone regulation of miR production and how this is perturbed in cancer are critical for the development of miR-based therapeutics and biomarkers.

Integrated analysis of rifampicin-induced microRNA and gene expression changes in human hepatocytes

MicroRNAs (miRNAs) post-transcriptionally regulate mRNA expression, controlling global cell function. Altered expression or function of miRNAs causes various diseases. Chemically induced changes in miRNA expression in human tissues are not fully understood. We investigated the changes in miRNA expression by rifampicin, which modulates the expression of various genes related to drug metabolism and pharmacokinetics, in human hepatocytes, and evaluated the relationship with the gene expression changes. We found that 23 miRNAs were increased (>2-fold) and 17 miRNAs were decreased (<0.5-fold) among 150 detected miRNAs, whereas 60 genes were increased and 105 genes were decreased among 22,673 detected genes upon treatment with 10 µM rifampicin. Changes in 17 intragenic miRNAs out of 40 altered miRNAs did not occur in parallel with alterations in their host genes. We searched for the target mRNAs of the miRNAs altered by rifampicin and found that the changes in expression of 16 mRNA/miRNA pairs were inversely associated. Thus, some mRNA expression altered by rifampicin may result from miRNA regulation. In conclusion, we found that rifampicin altered miRNA expression in human hepatocytes. We obtained new insight on the mechanism of the miRNA expression changes and the complicated relationship with gene transcripts.

Influence of sex differences on microRNA gene regulation in disease

Sexual dimorphism is observed in most human diseases. The difference in the physiology and genetics between sexes can contribute tremendously to the disease prevalence, severity, and outcome. Both hormonal and genetic differences between males and females can lead to differences in gene expression patterns that can influence disease risk and course. MicroRNAs have emerged as potential regulatory molecules in all organisms. They can have a broad effect on every aspect of physiology, including embryogenesis, metabolism, and growth and development. Numerous microRNAs have been identified and elucidated to play a key role in cardiovascular diseases, as well as in neurological and autoimmune disorders. This is especially important as microRNA-based tools can be exploited as beneficial therapies for disease treatment and prevention. Sex steroid hormones as well as X-linked genes can have a considerable influence on the regulation of microRNAs. However, there are very few studies highlighting the role of microRNAs in sex biased diseases. This review attempts to summarize differentially regulated microRNAs in males versus females in different diseases and calls for more attention in this underexplored area that should set the basis for more effective therapeutic strategies for sexually dimorphic diseases.

Sex differences in microRNA regulation of gene expression: no smoke, just miRs

Males and females differ widely in morphology, physiology, and behavior leading to disparities in many health outcomes, including sex biases in the prevalence of many neurodevelopmental disorders. However, with the exception of a relatively small number of genes on the Y chromosome, males and females share a common genome. Therefore, sexual differentiation must in large part be a product of the sex biased expression of this shared genetic substrate. microRNAs (miRs) are small non-coding RNAs involved in the post-transcriptional regulation of up to 70% of protein-coding genes. The ability of miRs to regulate such a vast amount of the genome with a high degree of specificity makes them perfectly poised to play a critical role in programming of the sexually dimorphic brain. This review describes those characteristics of miRs that make them particularly amenable to this task, and examines the influences of both the sex chromosome complement as well as gonadal hormones on their regulation. Exploring miRs in the context of sex differences in disease, particularly in sex-biased neurodevelopmental disorders, may provide novel insight into the pathophysiology and potential therapeutic targets in disease treatment and prevention.

Common genetic polymorphisms of microRNA biogenesis pathway genes and risk of breast cancer: a case-control study in Korea

Recent compelling evidence indicates that mutation, aberrant expression, and dysregulation of microRNA (miRNA) biogenesis are implicated in cancer development and progression. Based on the important role of miRNA biogenesis pathway in carcinogenesis, we hypothesized that genetic variations in this pathway genes may play a role as susceptibility factors for breast cancer. To test this hypothesis, we investigated the associations between 41 single nucleotide polymorphisms (SNPs) in 14 genes involved in miRNA biogenesis pathway and breast cancer risk in a case-control study of 559 Korean breast cancer cases and 567 controls frequency-matched by age. In all women, 3 SNPs (AGO1 rs595055, AGO2 rs3864659, and p68 rs1991401) were significantly associated with breast cancer risk. In stratified analysis by menopausal status, altered risk associations were observed for 7 SNPs in postmenopausal breast cancer. When subjects were grouped by the number of high-risk genotypes, we found a progressive increase in gene-dosage effect (P (trend) = 9.46E-7). The protective effects of AGO2 rs3864659 and HIWI rs11060845 were more pronounced in progesterone receptor-positive (PR+) cancer than in progesterone receptor-negative (PR-) cancer (odds ratio (OR), 0.50; 95% confidence interval (CI), 0.30-0.84 vs. OR, 0.94; 95% CI, 0.60-1.84; P (heterogeneity) = 0.04 and OR, 0.57; 95% CI, 0.37-0.88 vs. OR, 0.97; 95% CI, 0.65-1.44; P (heterogeneity) = 0.02, respectively), and the DROSHA rs644236 had stronger association with estrogen receptor-negative (ER-) cancer than for estrogen receptor-positive (ER+) cancer (OR, 1.39; 95% CI, 1.08-1.78 vs. OR, 1.05; 95% CI, 0.85-1.29; P (heterogeneity) = 0.04). Our results suggest that genetic variants in miRNA biogenesis pathway genes may be associated with breast cancer risk, and the modifiable effects might be different according to the menopausal status and hormone receptor status.

Stimulation of pri-miR-18a processing by hnRNP A1

Recent evidence suggests that the canonical miRNA processing pathway can b regulated by a number of positive and negative trans-acting factors. This chapter provides an overview of hnRNP A1-mediated regulation of miR-18a biogenesis. Our laboratory has recently established that the multifunctional RNA-binding protein hnRNP A1 is required for the processing of miR-18a at the nuclear of Drosha-mediated processing. By combining structural and functional analysis of RNA, we showed that hnRNP A1 regulates the processing of pri-miR-18a by binding to its terminal loop and reshaping its stem-loop structure, thus allowing for a more effective Drosha cleavage. Furthermore, we linked the event of hnRNP A1-binding to the pri-miR-18a with an unusual phylogenetic sequence conservation of its terminal loop. Bioinformatic and mutational analysis revealed that a number of pri-miRNAs have highly conserved terminal loops, which are predicted to act as landing pads for trans-acting factors influencing miRNA processing. These results underscore a previously uncharacterized role for general RNA-binding proteins as factors that facilitate the processing of specific miRNAs, revealing an additional level of complexity for the regulation of miRNA production and function.