Highlights d Insulin-driven AKT phosphorylation, but not downstream signaling, is promoted by RAS d RAS action on AKT phosphorylation depends on PI3Ka d Insulin signaling in hepatocytes is driven by redundant PI3Ka and PI3Kb activities d Compound and not single-isoform inhibition of PI3Ka and PI3Kb causes hyperglycemia

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In a number of organisms, introns affect expression of the gene in which they are contained. Our previous studies revealed that the 5′-UTR intron of human ubiquitin C (UbC) gene is responsible for the boost of reporter gene expression and is able to bind, in vitro, Yin Yang 1 (YY1) trans-acting factor. In this work, we demonstrate that intact YY1 binding sequences are required for maximal promoter activity and YY1 silencing causes downregulation of luciferase mRNA levels. However, YY1 motifs fail to enhance gene expression when the intron is moved upstream of the proximal promoter, excluding the typical enhancer hypothesis and supporting a context-dependent action, like intron-mediated enhancement (IME). Yet, almost no expression is seen in the construct containing an unspliceable version of UbC intron, indicating that splicing is essential for promoter activity. Moreover, mutagenesis of YY1 binding sites and YY1 knockdown negatively affect UbC intron removal from both endogenous and reporter transcripts. Modulation of splicing efficiency by YY1 cis-elements and protein factor may thus be part of the mechanism(s) by which YY1 controls UbC promoter activity. Our data highlight the first evidence of the involvement of a sequence-specific DNA binding factor in IME.

The promoter of the polyubiquitin C gene (UBC) contains putative heat shock elements (HSEs) which are thought to mediate UBC induction upon stress. However, the mapping and the functional characterization of the cis-acting determinants for its up-regulation have not yet been addressed. In this study, the sequence encompassing 916 nucleotides upstream of the transcription start site of the human UBC gene has been dissected by in silico, in vitro and in vivo approaches. The information derived from this analysis was used to study the functional role and the interplay of the identified HSEs in mediating the transcriptional activation of the UBC gene under conditions of proteotoxic stress, induced by the proteasome inhibitor MG132. Here we demonstrate that at least three HSEs, with different configurations, exist in the UBC promoter: two distal, residing within nucleotides -841/-817 and -715/-691, and one proximal to the transcription start site (nt -100/-65). All of them are bound by transcription factors belonging to the heat shock factor (HSF) family, as determined by bandshift, supershift and ChIP analyses. Site-directed mutagenesis of reporter constructs demonstrated that while the distal elements are involved in the up-regulation of UBC in response to proteasome inhibition, the proximal one appears rather to function as negative regulator of the stress-induced transcriptional activity. This is the first evidence that an HSE may exert a negative role on the transcription driven by other HSE motifs on the same gene promoter, highlighting a new level of complexity in the regulation of HSFs and in the control of ubiquitin levels.

Ubiquitin C (UbC) is one of the four genes encoding for ubiquitin in the mammalian genome. It has been described as the most responsive gene to cellular treats such as UV irradiation, heat shock, oxidative stress and translational impairment; it was also re- ported to contribute to maintaining ubiquitin steady state levels under physiological conditions. Despite the bulk of knowledge concerning its function, little is known about the molecular mechanisms modulating UbC expression. Here we review the state of the art of UbC structure, function and transcriptional regula- tion. Starting from the first evidences which circum- scribed the genomic region, pointing out both basic promoter marks (such as transcription start site and TATA-like element), and transcript structure (exon- intron boundaries) we go through more detailed mo- lecular studies performed by Marinovic in 2002 and by Bianchi et al. in 2009 and 2013. Herein, the key players orchestrating UbC gene basal activity are underlined