Widespread effects of nicotinic acid on gene expression in insulin-sensitive tissues: implications for unwanted effects of nicotinic acid treatment

Abstract

Nicotinic acid (NA; or niacin) has been used as a hypolipidemic agent for more than 4 decades. However, the mechanisms underlying the effects of NA treatment (wanted and unwanted) are still poorly understood. In the present study, we discovered that NA infusion in rats resulted in dephosphorylation (ie, activation) of the forkhead transcription factor FOXO1 in insulin-sensitive tissues such as skeletal and cardiac muscles, liver, and adipose tissue. These NA effects were opposite to the effects of insulin to increase FOXO1 phosphorylation. To test whether NA alters gene expression in these tissues, rats were infused for 7 hours with NA (30 μmol/h) and/or insulin (5 mU/[kg min]); and gene expression was evaluated using a microarray analysis. Nicotinic acid had widespread effects on gene expression in all of the tissues studied, and the number of genes affected by NA greatly exceeded that of genes affected by insulin. A systematic (or strategic) analysis of the microarray data revealed that there were numerous genes whose expression was regulated inversely by insulin and NA in correlation with FOXO1 phosphorylation, representing potential FOXO1 target genes. We also identified a group of genes whose expression was altered by NA exclusively in adipose tissue, presumably because of stimulation of the NA receptor in this tissue. Finally, there were genes whose expression was altered by both NA and insulin, likely via lowering plasma free fatty acid levels, including lipoprotein lipase and adenosine triphosphate-binding cassette A1, which play a major role in the regulation of circulating lipids. Thus, our data suggest that NA alters gene expression in insulin-sensitive tissues by various mechanisms. Some of the NA-induced changes in gene expression are discussed as potential mechanisms underlying wanted and unwanted effects of NA treatment.

Figure 1

Representative Western blots and quantitative data showing the effects of a short (1.5 h) infusion of NA on Akt and FOXO1 phosphorylation in insulin sensitive tissues of rats in vivo. Data are means±SEM (n=4). *, P<0.05 vs. saline control.

Figure 2

Plasma glucose (A), glucose infusion rate (GINF, B), plasma insulin (C) and FFA (D) during an infusion of saline ( ), NA (30 μmol/h; ▯), insulin (5 mU/kg/min; ), or both NA and insulin ( ) for 7 h in conscious rats. Data are means± SEM (n=3 for each group).

Figure 3

Western blots showing the effects of a short (1.5 h) infusion of NA and/or insulin on Akt and FOXO1 phosphorylation in insulin sensitive tissues.

Figure 4

Representative Northern blots and quantitative data showing the effects of a 7-h NA and/or insulin (I) infusion on FOXO1 target gene (i.e., PDK4) mRNA expression in gastrocnemius muscle. Equal loading was confirmed by 18S and 28S rRNA stained by ethidium bromide (data not shown). Data are means± SEM (n=3 for each group). *, P<0.05 vs. saline.

Figure 5

Effects of NA and insulin on LITAF mRNA expression in adipocytes as determined by microarray analysis (Affymetrix) and real-time quantitative RT-PCR on pooled RNA samples using two different primer sets (Primers 1 & 2).

Figure 6

Effects of NA and/or insulin on LPL (A) and ATP-binding cassette A1, detected by microarray analysis. The Y-axis signals are the hybridization intensities in DNA microarray experiments. *, P<0.05 vs. saline control.