The Underlying Biology of Health Disparities

National Institute On Aging

Investigators

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Abstract

We have continued our work on inflammation in health disparities focusing on c-reactive protein (CRP) molecular modulation. Even though CRP plays a central role in aging, inflammation and age-related disease, most of the molecular mechanisms that regulate CRP expression are not known. Given the fact that the CRP 3'UTR contains AU-rich elements; we hypothesized that post-transcriptional regulatory factors bind and regulate CRP expression. Our data strongly suggest that the RNA binding protein HuR enhances CRP expression by associating with specific positive regulatory elements in the CRP 3UTR to regulate CRP mRNA stability (Kim Y, et al MCB 2015). Silencing HuR decreased CRP mRNA and protein levels indicating that HuR positively regulates CRP expression. These data and other experimental data in the manuscript suggest that HuR enhances CRP expression at least in part by stabilizing CRP mRNA. Previous data have shown that IL-6 upregulates CRP levels. We hypothesized that HuR may contribute to the increased CRP mRNA expression in response to IL-6. HuR silencing significantly decreased CRP mRNA and protein levels in the presence of IL-6. In contrast, HuR mRNA and protein levels were not affected by IL-6 treatment. These findings indicate that IL-6-mediated upregulation of CRP is dependent on HuR. In addition to HuR, it is likely that miRNAs also regulate CRP. Using TargetScan to test this possibility, we found that the CRP mRNA contains one predicted miR-637 site in its 3UTR. Decreasing the levels of endogenous miR-637 by transfection with a miR-637 antagomir (anti-miR-637) increased both CRP mRNA and CRP protein expression levels. These results and other included in our publication suggest that miR-637 reduces the expression levels of CRP by interacting with the CRP 3UTR. We tested whether HuR and miR-637 may affect CRP levels in vivo. HuR mRNA levels were higher and miR-637 levels were lower in individuals with high hs-CRP, consistent with our in vitro data that HuR positively and miR-637 negatively regulated CRP expression. Unraveling the mechanisms that modulate CRP expression provides insight for understanding the factors that contribute to the low-grade inflammatory state of aging and will undoubtedly shed light on age-related chronic diseases and the relevance of inflammatory risk factors in populations at risk for health disparities. Preventing cancer and promoting health span and longevity are two long sought after goals in clinical medicine. The ideal agent would be one that not only treats an age-associated disease but also enhances health span and life span. In the HANDLS cohort almost 21% of the population is either diabetic or pre-diabetic. Metformin, an oral hypoglycemic agent, has been used for decades to treat type 2 diabetes mellitus and is widely prescribed within the HANDLS cohort. Recent data has shown that diabetics that take metformin have lower incidence of cancer and mice treated with metformin have lengthened life span. However, there are gaps in our knowledge about the underlying mechanisms attributed to the therapeutic effects of metformin. Previously, a group reported that chronic metformin treatment increases both health span and lifespan in mice. Health span and life span are central to the disparities seen in minorities and the poor. Since many HANDLS participants take metformin, we were anxious to examine the possible biologic mechanisms responsible for this finding in rodents especially in view of our previous and contemporaneous work in aging and microRNAs. In cancer cells, metformin upregulates DICER1 expression, a key enzyme that processes microRNAs (miRNAs). We found a significant decrease in DICER1 mRNA levels with human age. Next, we examined whether changes in DICER1 expression occur in mice chronically treated with metformin (Noren Hooten N et al 2016 Aging Cell). In response to metformin and dietary caloric restriction, there is an upregulation in Dicer1 mRNA levels in the livers of treated mice compared to control mice on standard diet. DICER1 mRNA expression was examined in HANDLS participants who were either euglycemic normal controls (1), diabetics treated with either metformin (2) or sulfonylurea (3) hypoglycemic agents. Diabetics treated with metformin had significantly higher levels of DICER1 mRNA compared to non-diabetics. Given that chronic metformin treatment in mice and humans increased DICER1 levels, we hypothesized that metformin may alter post-transcriptional processes that would affect the stability and/or turnover of Dicer1 mRNA. We have found that DICER1 levels are upregulated by metformin treatment in both mice and humans in part through a post-transcriptional mechanism involving the RNA binding protein, AUF1. Upregulation of DICER1 by metformin leads to higher levels of a subset of miRNAs that are important for regulating senescence and aging-associated pathways. Furthermore, metformin treatment inhibits cellular senescence through a DICER1-dependent mechanism. Although the actions of metformin have been extensively studied for decades, the underlying mechanisms remain uncertain. This new insight will hopefully aid both in identifying novel targets for development of agents that could be important in cancer prevention and increasing healthspan. Other ongoing work in the laboratory focuses examining other potential biomarkers of aging and chronic disease that are relevant to health disparities including extracellular vesicles, the role of miRNAs in hypertension as well as genetic phenotypes associated with increased cardiovascular risk.