Human TRB3 is upregulated in stressed cells by the induction of translationally efficient mRNA containing a truncated 5′-UTR

Tribbles homolog 3 (TRB3) is a pseudokinase that has been implicated in the control of stress response, cell viability and metabolic processes, and has been linked to medical conditions, including insulin resistance, cardiovascular disease and diabetes. Therefore, the understanding of mechanisms that regulate TRB3 expression is of considerable importance. We have previously described the existence of several human (h) TRB3 mRNA isoforms that differ in their 5′-untranslated region (5′-UTR). In this study, we use a reverse transcription-quantitative polymerase chain reaction (RT-qPCR) system to characterize the expression levels of hTRB3 mRNA isoforms in HepG2 hepatoma cells cultured in regular medium or exposed to arsenite, and investigate the effect of hTRB3 5′-UTR variants on the efficiency of mRNA translation. The data indicate that of the hTRB3 mRNA splice variants, 1A is predominant (> 80% of molecules) in both the stressed and unstressed states, and that the remainder consists mainly of 1B4, with the variants 1B1, 1B2 and 1B3 together forming less than 1% of the population in either condition. In addition to the substantial transcriptional upregulation of all hTRB3 mRNA splice variants, the exposure of cells to arsenite results in a marked increase in the proportion of splice variant 1A molecules containing a truncated 5′-UTR. The shortened 1A 5′-UTR proved to be translationally more efficient than the untruncated 1A 5′-UTR, due to the lack of an inhibitory upstream open reading frame (uORF). Thus, increased transcription as well as altered usage of 5′-UTR variants contributes to the upregulation of hTRB3 protein synthesis in stressful conditions.