Editor's note: Per the U.S. Food and Drug Administration, cosmetics are articles intended to beautify appearance and should not alter the structure or function of the human body; those that do are considered drugs. The concepts presented here blur this line, and although they reflect major advances in cosmetic science.
Follow this series including Part II on the delivery of RNAi therapeutics, and Part III on RNA activation; also hear more from the authors in our exclusive Author Commentary podcast.
If distilled down to the very basics, modern day cosmetics amount to a liquid, gel, ointment or cream solution used to deliver a mixture of molecules to the skin for beneficial effects. But what if skin care or conditions could be tackled genetically rather than biochemically? This would essentially amount to making cells in the skin do the work of the cosmetic solution. Or potentially combining an effective biochemical solution with a genetic component.
This article briefly discusses RNA interference (RNAi) technology,1 a platform that has been applied in many fields of biomedical research since its inception in the early 1990s. RNAi represents a new frontier in the skin care industry that has been actively explored by many different laboratories and companies. In fact, once a year or more, articles are published detailing the untapped potential for deploying RNAi technology for the amelioration of a variety of skin diseases and disorders.2–5
RNAi Effectors and Biogenesis
DNA has become part of the global lexicon, thanks to many crime-fighting television programs and commercially available genetic profiling. However, the public is much less familiar with its more ancient counterpart: ribonucleic acid (RNA). Many diversified forms of RNA have been discovered in recent years, of which small non-coding RNA (ncRNA) molecules are of interest to the present discussion.
Effectors: Molecules that affect RNAi are referred to as short-interfering RNAs (siRNAs) and microRNAs (miRNAs or miRs). RNAi is one of the many naturally occurring pathways in human cells that regulates gene expression and the subsequent synthesis of proteins. siRNAs and miRNAs effectively reduce the levels of certain proteins by preventing their messenger RNAs (mRNAs) from reaching the ribosomes in cells for the assembly of new proteins. These small ncRNAs achieve this by annealing to specific “recognition sequences” in the mRNA.
siRNAs require exquisite sequence complementarity, while miRNAs have greater flexibility in this regard and only need to recognize a short “seed sequence” of eight nucleotides in length to have an effect. This down-modulation of protein expression by siRNAs and miRNAs can be of considerable benefit to skin care when these small RNA effectors target the mRNAs of proteins, causing skin disorders and diseases. Alternatively, if a particular protein is beneficial in combating a skin condition, an miRNA that diminishes its cellular abundance should itself become a target for degradation.
Biogenesis: Typically, siRNAs arise from exogenous or external sources of double-stranded RNA (dsRNA) such as an invading viral pathogen, while miRNAs are generated endogenously from sequences in a host cell DNA. Indeed, miRNA genes have been identified in intergenic regions, as well as within the coding sequence in intronic and exonic segments. Moreover, multiple miRNA biogenesis pathways exist—to date, no less than five distinct pathways have been elucidated—highlighting the evolutionary significance of this form of post-transcriptional gene regulation.6, 7
In general, miRNA biogenesis can be summed up as follows (see Figure 1). The synthesis of a large single-stranded primary miRNA (pri-miRNA) in the cell nucleus is trimmed via a protein complex containing Drosha and DGCR8 to a stem loop structure called a pre-miRNA. This is exported to the cytoplasm by an exportin protein, where it undergoes a cleavage maturation by the cytoplasmic Dicer enzyme to a short 18–25 nucleotide long dsRNA molecule. Eventually, one “passenger strand” is displaced and the other “guide strand” is incorporated into RNA-induced silencing complex (RISC) to seek out mRNA molecules with sequences complementary to seed sequences in the mature miRNA.
The list of potential miRNAs that could be adapted for cosmetics is continually expanding, thanks to microRNA profiling efforts.
miRNA Modulation and Cosmetic Potential
Endogenous levels of miRNAs can typically be modulated up or down by two main approaches: miRNA (miR) replacement therapy using miRNA “mimics”; or miRNA antagonism through miRNA “antagomiRs.”
The former supplements the existing amount of a particular endogenous miRNA species by introducing a dose of a synthetic counterpart. This increases the down-regulation of that particular miRNA’s targets, resulting in a more substantial reduction in the concentration of the downstream protein target(s) (see Figure 2a).8 In many cases, commercial manufacturers of synthetic miRNA mimics have altered the chemistry through proprietary means, providing greater stability and resistance to degradation by nucleases.
An example where an miRNA mimic could benefit a skin condition is in the case of aberrant pigmentation. It has been reported the miR-218 mimic can impede melanogenesis by targeting one of the transcription factors in the melanin biosynthesis pathway—microphthalmia-associated transcription factor (MITF).9 The topical delivery of miR-218 mimics to a site of hyperpigmentation could therefore result in skin whitening.
The latter approach for diminishing and ablating the down-regulatory effect of a specific endogenous miRNA would be to introduce a synthetic complementary miRNA molecule, or antagomiR, to bind a target endogenous miRNA irreversibly so it can neither anneal to a target mRNA molecule, nor interact with the RISC complex. This, in turn, would increase the abundance of the downstream protein targeted by that miRNA (see Figure 2b).10
An example antagomiR to treat skin conditions is miR-29, which has been associated with the aging process given its ability to function as a tumor “suppressor” and coordinate the expression of the p53 tumor suppressor protein, leading to cellular senescence in response to DNA damage.11 It also has been found that miR-29 is elevated in older individuals, which leads to decreased elastin and collagen protein levels, resulting in wrinkle formation.12 Indeed, a patent has already been filed for treating wrinkles via the application of compounds that down-modulate the cellular levels of miR-29. In it, antagomiRs of both isoforms of miR-29, i.e., miR-29a and miR-29b, are specifically named as active agents capable of reducing the concentration of miR-29.13
The list of potential miRNAs that could be adapted for cosmetics is continually expanding, thanks to microRNA profiling efforts. Laboratories around the world have been extensively researching which miRNA species are increased or decreased in response to a variety of diseases and disorders, including those of the skin.
For example, more than 25 miRNAs have been detected whose levels fluctuate in keratinocytes in response to UVA and UVB exposure—with some miRNAs unique to either form of UV.14 Similarly, in skin tissue, miRNA profiles have been established for atopic dermatitis, psoriasis and vitiligo.15, 16 Ongoing profiling and studies guarantee the list of miRNAs associated with various skin conditions will continue to grow.
The topical delivery of miR-218 mimics to a site of hyperpigmentation could potentiate skin whitening.
From Lab to Market
Thus far, the marriage of miRNAs with the field of cosmetics has involved two distinct strategies. In either case, the goal is the same: to catalyze specific gene pathways to produce a desired outcome.17, 18
First is the application of cosmetics containing materials designed to alter the steady state levels of endogenous miRNAs. As such, cosmetic groups have been profiling the up- and down-regulation responses of endogenous miRNAs to assess whether such materials trigger signaling pathways to induce beneficial effects.
Alternatively, albeit less successfully, is the incorporation of miRNAs themselves, i.e., mimics and/or antagomiRs, into topical applications to increase or antagonize normal levels of miRNAs in skin cells. One of the stumbling blocks has been the ability to get RNA molecules into the skin.17 Multiple patents have been filed for the development of cosmetic solutions to improve the topical delivery of miRNAs, although as of writing this manuscript, none have made it to market yet.
siRNAs and miRNAs have certain biophysical attributes that challenge their ability to penetrate skin. Both RNAi effectors have a negative charge, which presents a problem, given that cell surfaces are decorated with negatively charged molecules, and like charges do not attract. Also, although they are on the smaller size for RNA molecules, siRNAs and miRNAs are both still relatively large. Finally, the skin is covered in a variety of nucleases—enzymes that will break these RNAs to pieces. Therefore, considerable research has been focused on how to protect these RNA effectors and facilitate their penetration into the skin. This will be the focus of part II in this series, scheduled to appear in January 2018.
Considerable research has been conducted by the cosmetics industry to bring RNAi-based solutions from the laboratory to the marketplace. In fact, an in-depth review from 2013 described no less than nine different anti-aging products in various stages of development, each of which containing siRNAs, miRNA mimics and miRNA antagomiRs.18
Moreover, the number of patents filed for the utilization of RNAi in skin care continues to increase. Thus, as skin care companies continue to research RNAi-based skin solutions, the question is not if but when the first skin condition will be ameliorated through a commercialized RNAi-based topical solution.
Continue to Part II...
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All websites accessed on June 14, 2017.
- A Fire, S Xu, MK Montgomery, SA Kostas, SE Driver and CC Mello, Potent and specific genetic interference by double-stranded RNA in Caenorhabditis elegans, Nature 391 806-811 (1998)
- M Faller and F Guo, MicroRNA biogenesis: There's more than one way to skin a cat, Biochim Biophys Acta 1779 663-667 (2008)
- SY Ying, DC Chang and SL Lin, The microRNA (miRNA): Overview of the RNA genes that modulate gene function, Mol Biotechnol 38 257-268 (2008)
- AG Bader, D Brown and M Winkler, The promise of microRNA replacement therapy, Cancer Res 70 7027-7030 (2010)
- J Guo et al, MicroRNA-218 inhibits melanogenesis by directly suppressing microphthalmia-associated transcription factor expression, RNA Biol 11 732-741 (2014)
- CC Esau, Inhibition of microRNA with antisense oligonucleotides, Methods 44 55-60 (2008)
- F Rodier, J Campisi and D Bhaumik, Two faces of p53: Aging and tumor suppression, Nucleic Acids Res 35 7475-7484 (2007)
- M Takahashi, A Eda, T Fukushima and H Hohjoh, Reduction of type IV collagen by upregulated miR-29 in normal elderly mouse and klotho-deficient, senescence-model mouse, PLoS One 7:e48974 (2012)
- A Kraemer et al, UVA and UVB irradiation differentially regulate microRNA expression in human primary keratinocytes, PLoS One 8:e83392 (2013)
- E Sonkoly et al, MicroRNAs: Novel regulators involved in the pathogenesis of psoriasis? PLoS One 2:e610 (2007)
- Y Wang, K Wang, J Liang, H Yang, N Dang, X Yang and Y Kong, Differential expression analysis of miRNA in peripheral blood mononuclear cells of patients with non-segmental vitiligo, J Dermatol 42 193-197 (2015)
- B Geusens, N Sanders, T Prow, M Van Gele and J Lambert, Cutaneous short-interfering RNA therapy, Expert Opin Drug Deliv 6 1333-1349 (2009)
- P Zhang, J Chen, T Li and YY Zhu, Use of small RNA as antiaging cosmeceuticals, J Cosmet Sci 64 455-468 (2013)