MicroRNA circuits in transforming growth factor-β actions and diabetic nephropathy

Abstract

Diabetes is associated with significantly increased rates of kidney disease or diabetic nephropathy (DN), a severe microvascular complication that can lead to end-stage renal disease. End-stage renal disease needs to be treated by dialysis or kidney transplantation and also is associated with cardiovascular disease and macrovascular complications. Therefore, effective renal protection is critical to reduce the rates of mortality associated with diabetes. Although key signal transduction and gene regulation mechanisms have been identified and several drugs are currently in clinical use, the rates of DN are still escalating, suggesting the imperative need to identify new biomarkers and drug targets. The recent discovery of microRNAs (miRNAs) and their cellular functions provide an opportunity to fill these critical gaps. Because miRNAs can modulate the actions of key factors involved in DN such as transforming growth factor-β, they could be novel targets for the treatment of DN. This review covers the recent studies on the roles of miRNAs and miRNA circuits in transforming growth factor-β actions and in DN.

Figure 1. Biogenesis and Mechanism of Action of microRNAs and miR-192

MicroRNA transcripts initially originate as primary miRNAs (Pri-miRNAs) that are then processed into Pre-miRNAs by the Drosha enzyme, which are further cleaved to result in double-strand RNA duplexes. The miRNA duplexes are then unwound by the action of a second enzyme, Dicer, and the mature miR guide strand is loaded into the RNA-induced silencing complex (RISC). miRNAs in the RISC complex then guide the recognition of target RNAs to induce their downregulation depending on the type of complementarity. In the case of miR-192, transcription of pri-miRNAs is enhanced by Smad3 or p53 and processing into pre-miRNA (by Drosha) may also be enhanced by p53 and Smad3. Mature miR-192 interacts with 3′UTR of targets Zeb1/2 and leads to inhibition of translation or induces degradation of the target mRNAs in Processing bodies (P-body).

Figure 2. Mesangial fibrosis and hypertrophy caused by amplification of signal cascades triggered by key miRNAs and downstream effector miRNAs

Scheme shows amplification of signals by miRNA circuits can operate in diabetic mouse glomeruli and MMC treated with TGF-β. Cascades originating from miR-192 to miR-216a and miR-217, and from miR-192 to miR-200 family can amplify signals from a single miRNA to several downstream miRNAs. Amplified signals through miR-192 and miR-200 family increase the expression of ECM (collagens) genes via Zeb1/2 and activate Akt via Fog2/PI3K pathway. Another amplified signal through miR-192 and miR-216a/217 activates Akt via Pten and Activated Akt also enhances ECM gene expression. In addition, other key miRNAs such as miR-21 and miR-377 can also contribute to these events. Therefore, these miRNA-mediated amplified signals increase fibrosis and hypertrophy related to DN.

Figure 3. Acceleration of renal gene expression by signaling loops involving TGF-β and miRNAs

Signaling loops initiated by TGF-β accelerate downstream action and effects via miRNAs. A) TGF-β increases miR-192 expression via Smad3 or p53. TGF-β1 gene expression is itself auto-upregulated by miR-192 through Zeb1/2. B) miR-192 enhances p53 levels via inhibition of Mdm2 (inhibitor of p53). p53 increases transcription of miR-192 through p53 responsive elements on the promoter. C) miR-200 family members can further augment their own expression through inhibition of Zeb1/2. Initial expression of miR-200 family can also be induced by miR-192 through Zeb1/2. D) The promoter of the non-coding RNA RP23 (the host gene of miR-216a and miR-217) also contains E-boxes and increased miR-216a also activates its own promoter through Tsc-22 and Tfe3. miR-216a/Ybx1 pathway can amplify the signaling initiated by TGF-β and miR-192.

Figure 4. Schematic model for the use of miRNAs biomarkers for human renal disease diagnosis or early detection through miRNA profiling in biofluids, and for therapeutic purposes with chemically modified oligonucleotides targeting specific miRNAs

Circulating miRNAs in biofluids are potential diagnostic biomarkers of various human diseases in general and renal disorders in particular. Moreover, synthetic chemical inhibitors of specific miRNAs are very effective to reduce the target miRNAs and downstream signaling in animal models of human diseases ^{, , , –}. Therefore, key circulating miRNAs will be useful for early detection of renal diseases and furthermore, anti-miRNA therapies could also be developed for the treatment of acute and chronic renal disorders.