Overexpression of Protein Tyrosine Phosphatase 1B in HepG2 Cells Ameliorates Insulin-mediated Suppression of Apolipoprotein B mRNA translation via its Untranslated Regions

Background: the hepatic secretion of apolipoprotein b (apob), containing lipoproteins, is known to be regulated by insulin, and the overproduction of these atherogenic lipoproteins occurs in insulin-resistant states. protein tyrosine phosphatase 1b (ptp-1b) is a key regulator of hepatic insulin signaling and is also upregulated in insulin resistance. we aimed to investigate the role of ptp-1b in regulating apob mrna translational efficiency mediated by 5’/3’ untranslated regions (utrs) under conditions of insulin stimulation. methods: human hepatoma hepg2 cells were transfected with a vector carrying the firefly luciferase reporter gene and either a chimeric apob mrna encoding the 5’/3’ untranslated region (5’luc3’-pgl3) or a null sequence of length equivalent to apob 5’ utr (luc-pgl3). the transfected cells were then infected with adenovirus carrying the mouse ptp-1b gene (adptp1b) in the absence or presence of insulin, and the cellular luciferase activity was determined. the rna extracts from cells were transfected with constructs carrying 5’/3’ apob utr, or a null sequence was also translated in vitro in a rabbit reticulocyte translation system. results: the luciferase activity of the cells transfected with constructs containing the apob utr sequences (5’luc3’) was significantly higher than that of the control constructs carrying a null sequence (p<0.01, n=12). similar results were observed following in vitro translation studies showing a significantly higher expression of the 5’/3’ utr constructs (p<0.001, n=6). treatment with 100 nm insulin led to a significant reduction in the luciferase activity of the constructs carrying apob 5’/3’ utr (p<0.0001, n=12). the down regulation of the apob mrna translation mediated by insulin was mediated by the apob 5’/3’ utr sequences since insulin did not affect the control constructs containing a null sequence. the infection of hepg2 cells expressing 5’luc3’ or control constructs with adptp-1b attenuated the inhibitory effect of insulin and led to higher levels of luciferase activity compared to the ad?-gal infected control cells (p< 0.05, n=12). however, the activity was lower than that in the control cells infected with 5’luc3’-pgl3 but not treated with insulin (p<0.05, n=12). conclusion: our data suggest that ptp-1b can potentially modulate apob synthesis by blocking insulin-mediated inhibition of the apob mrna translation via its 5’/3’ utr sequences. we hypothesize that the ptp-1b-mediated attenuation of the insulin action can lead to the upregulation of the apob mrna translation and contribute to a lipoprotein overproduction in conditions such as insulin resistance.