Johns Hopkins University
Publication Date: April 14, 2015
It is rare in human biology in which one finds such an evolutionarily conserved, important axis such as the Lin28/Let-7. The mRNA binding protein Lin28, together with the let-7 microRNA, has been implicated by an increasingly large body of research in a host of biological processes and diseases including early development, oncogenesis, stem cell differentiation and even diabetes1.
The field of small, yet biologically essential non-coding strands of micro RNAs has exploded since Victor Ambrose’s seminal discovery of the lin4 micro RNA in C. elegans in 1993 2. Since then, there have been more than 5000 miRNAs have been discovered, many of which exhibit extreme evolutionary conservation across species3,4. MicroRNAs (miRNAs) base pair with complementary mRNAs, silencing their translation, and thus conferring selective posttranscriptional regulation in the cell. In order to initiate this highly regulated posttranscriptional silencing, the short, double-stranded, hairpin miRNA must be enzymatically cleaved to yield two single stranded RNA molecules. One of the strands aggregates with Dicer and recruits Argonaute in order to form the RNA-induced-silencing-complex (RISC). Binding of the RISC complex to the 3’ UTR of the target mRNA allows the target mRNA to be either degraded or unable to be translated, depending on the RISC-3’ UTR binding specificity. This remarkable biochemical machinery allows the cell to post transcriptionally silence mRNAs involved in virtually any cellular process5.
The let-7 are an ancient and evolutionarily conserved class of miRNAs known to be powerful regulators of a diverse group of metabolic processes in the vertebrate body. However, like nearly all other biological molecules, let-7 is regulated by specific mechanisms, in this case its biogenesis is controlled by Lin28. Lin28A and its paralog, Lin28B, are RNA-binding proteins that selectively repress the maturation of the let-7 family binding to the pre-Let-7 GGAG consensus motif, and subsequently promoting its degradation6,7. Therefore, Lin28 proteins reduce levels of mature Let-7 miRNAs, which results in a selective upregulation in the translation of Let-7-targeted mRNAs. These include programs of genes know to be involved in embryonic development, stem cell differentiation, glucose metabolism, and induce pluripotency1.
Role of the Lin28/let-7 Axis in Development
The Lin28/let-7 pathway has emerged as a powerful regulator of organismal development. Our super pathway serves as a switch, deciding whether an organism can develop past the neonatal stage. Heterochronic studies have elucidated the development time course of Lin28 and let-7 expression. Generally, Lin28 is highly expressed during the neonatal period and subsequently reduces as the organism reaches puberty. Neonatal overexpression of Lin28 leads to delayed development and reduced differentiation. Let-7, on the other hand, follows an opposite expression profile. Let-7 expression gradually increases as an organism matures, contributing to development. One study even found that neonatal treatment with estrogen and androgen, hormones known to regulate puberty, altered expression of Lin28 and let-7 during onset of puberty. Therefore, the Lin28 and let-7 work together to control development1,3.
The Lin28/let-7 Axis Involved in Regulation of Cellular Glucose Metabolism
In a seminal 2011 Cell paper, George Daley’s Harvard based lab showed that Lin28/let-7 pathway to be implicated in glucose regulation and diabetes in mice. Mice engineered to overexpress both Lin28A and Lin28B in separate trials showed reduced glucose levels compared to wild type mice. Amazingly, these Lin28 overexpression mice showed a resistance to high-fat-diet inducted diabetes and obesity; that is, when fed a diet containing 45% kcal from fat, these Lin28 overexpression mice showed better glucose metabolism than wild type mice8.
On the other hand, overexpressing let-7 in mice, much as we would expect considering the mutually antagonistic Lin28/let-7 relationship, leads to impaired glucose metabolism and high-fat-diet induced obesity. The lab went to investigate the relationship between let-7 and Type 2 diabetes, a condition characterized by widespread glucose dysregulation. Amazingly, Daley then went on to find a suite of Type 2 diabetes genes enriched for let-7 target sites. That is, let-7 miRNAs bind to a group of diabetes-related mRNAs and regulate glucose metabolism8.
In recent spotlight is the pathway’s role in one of biology’s most infamous problems: cancer.
Dual Role of Lin28/let-7 Axis in Cellular Differentiation and Cancer Cell Proliferation
Every tissue cell in our body is derived from undifferentiated stem cells. It is now well know that the Lin28/let-7 pathway regulates cell differentiation, allowing a stem cell to either differentiate or self-renew. Amazingly, embryonic stem cells engineered to be unable to produce mature miRNAs failed to differentiate, rather they engaged in repeated self-renewal. Introduction of let-7 miRNAs rescued differentiation, stopping self-renewal. It was later found that let-7s target a suite of well-known pluripotency genes, downregulating their function and allows cells to differentiate. Much as we would expect, let-7’s antagonist partner, Lin28, acts in an opposite manner, stopping differentiation and promoting stemness in cells9,10. In fact, Lin28 has been shown to be a pluripotency factor, which, when combined with other pluripotency factors such as Sox2, Oct4, and Nanog, is capable of transforming somatic cells into induced-pluripotent stem cells11.
While this process of cellular differentiation is crucial for normal development, disaster can strike if this process becomes dysregulated, as in cancer. The molecular basis of cancer as long eluded the past century’s most accomplished scientists, thus propelling cancer into a world of extreme notoriety. Recent advancement have linked the Lin28/let-7 pathway in cancer cell proliferation. Many studies have found an increase of Lin28 and, much as we would expect, a decrease of let-7 in cancerous tissue1,12. Furthermore, overexpression of let-7 in many types of cancer can inhibit growth. This effect is due to due to many cell-cycle progression and oncogenic let-7 targets: K-Ras, Cyclin D1, c-Myc, Cdc34, Hmga2, E2f2, and Lin28. Let-7 targets these genes, reducing their expression and stopping spread of cancer. On the other hand, Lin28 downregulates let-7, thus upregulating these cell-cycle progression genes and promotes cancer1.
It seems then that let-7, targets a host of genes, regulating a wide array of biological functions, and thus contributing to many disease states. Lin28 binds to let-7, preventing its maturation into mature miRNA and thus upregulating let-7 targeted genes. This pathway, already implicated in a wide array of biochemical processes, begs to be further elucidated. Basic science is already being put to clinical use: a recent study involving transfection of synthetic let-7 mimic molecules (causing widespread let-7 overexpression) induced cell cycle arrest of lung cancers12. By studying this ancient, evolutionarily conserved pathway, we could uncover more related diseases and potential novel therapeutic targets related to the axis.
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2) Lee, R. C., Feinbaum, R. L. & Ambros, V. The C. elegans heterochronic gene lin-4 encodes small RNAs with antisense complementarity to lin-14. Cell 75, 843–854 (1993).
3) Alvarez-Garcia, I. & Miska, E. A. MicroRNA functions in animal development and human disease. Development 132, 4653–4662 (2005).
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7) Nam, Y., Chen, C., Gregory, R. I., Chou, J. J. & Sliz, P. Molecular basis for interaction of let-7 MicroRNAs with Lin28. Cell 147, 1080–1091 (2011).
8) Zhu, H. et al. The Lin28/let-7 axis regulates glucose metabolism. Cell 147, 81–94 (2011).
9) Viswanathan, S. R. & Daley, G. Q. Lin28: A MicroRNA Regulator with a Macro Role. Cell 140, 445–449 (2010).
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11) Sangiao-Alvarellos, S. et al. Changes in hypothalamic expression of the Lin28/let-7 system and related MicroRNAs during postnatal maturation and after experimental manipulations of puberty. Endocrinology 154, 942–955 (2013).
12) Takamizawa, J. et al. Reduced expression of the let-7 microRNAs in human lung cancers in association with shortened postoperative survival. Cancer Res. 64, 3753–3756 (2004).
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