**3. miRNA may be biomarkers for oxidative stress**

miRNAs are highly conserved regulatory molecules expressed in eukaryotic cells. They are short noncoding RNAs that regulate gene expression by binding to target mRNAs, which leads to reduced protein synthesis and sometimes decreased steady-state mRNA levels(Ambros, 2004; Carthew, 2006; Carthew and Sontheimer, 2009; Li and Carthew, 2005). Although hundreds of miRNAs have been identified, much less is known about their biological function. There is evidence that miRNAs affect pathways fundamental to metabolic control in higher organisms such as adipocyte and skeletal muscle differentiation. Also, some miRNAs are implicated in lipid, amino acid, and glucose homeostasis. Thus miRNA abnormalities may contribute to common metabolic diseases and there may be novel therapeutic opportunities based on miRNA targeting. Indeed, the expression of certain genes can depend more on levels of regulatory miRNAs than on levels of mRNAs. miRNAs act through a mechanism similar to that of short interfering RNAs (siRNA). The expression of miRNA target genes can be fine-tuned in animals by altering the concentrations or identities of miRNAs within cells.

There are many evidences indicated that miRNAs are able to modulate the cellular response to oxidative stress both in vitro and in vivo(Babar et al., 2008). We hypothesize that miRNAs may be biomarkers for oxidative stress due to following several reasons. First, since miRNA are post-transcriptional gene regulators, they may be able to function as 'quick responders' to oxidative stress. For example, upon exposure to stress, miRNA may rapidly localize to P-bodies or stress granules to regulate key genes involved in the oxidative stress response. After the stress is mitigated, miRNA inhibition may be promptly abated, allowing commencement of translation and expeditive restoration of cells back to their normal state. Second, since miRNA regulate numerous targets, they have the capacity to powerfully and efficiently coordinate a stress response involving numerous genes. Third, owing to their small size and high stability, miRNA may be less susceptible to certain types of stress, such as genotoxic insults. Hence, their ability to modulate stress response would be less likely to be compromised under high-stress condition. Given the recent development in the field of miRNA research, we predict that miRNA will be promised biomarkers for oxidative stress.
