In conclusion we show that HBP is

In conclusion, we show that HBP1 is phosphorylated on three sites, which control HBP1 transcriptional activity and glioblastoma cell proliferation. This phosphorylation largely depends on AKT, although other kinases may also be involved. In a previous study, we had shown that the PI3K-AKT pathway repressed HBP1 transcription by inhibiting FOXO binding to the HBP1 gene promoter [8]. Altogether, our work is consistent with a model in which AKT regulates HBP1 by two different mechanisms, both at post-translational and transcriptional levels. The following are the supplementary data related to this article.
Acknowledgements
Introduction The serine/threonine kinase Akt has a high level of evolutionary conservation and plays a key role in the regulation of cellular growth and metabolism [1]. There are three distinct Akt isoforms (Akt1, 2, and 3) that are the products of distinct genes, but are highly related, exhibiting >80% protein identity and sharing the same structural organization [2]. Akt isoforms are activated by insulin and phosphoinositol-3 kinase (PI3K). Class 1A PI3K (PI3Kα) comprises the p110α catalytic subunit that forms functional heterodimers with various p85 subunit isoforms and mediates the activation of Akt isoforms in response to various growth factors and hormones, such as insulin, insulin-like growth factor-1 (IGF-1), and vascular endothelial growth factor (VEGF), which act via tyrosine kinase receptors. All three isoforms are expressed in the myocardium; however, Akt1 and Akt2 (Akt1/2) comprise 99% of the Akt protein in the heart [3]. Akt signaling plays an important role in the regulation of cardiac growth and metabolism [4]. Global IGF-1, human recombinant of Akt1 resulted in a smaller heart size that was proportional to body size, but with preserved cardiac contractile function [5]. Conversely, overexpression of Akt in isolated cardiomyocytes increased insulin-stimulated protein synthesis, and mice with cardiac overexpression of Akt have greater p70S6K activity and show extreme cardiac hypertrophy [6]. Furthermore, physiological cardiac hypertrophy with preserved contractility developed in the short-term and dilated cardiomyopathy developed in the chronic phase following the induction of activated Akt1 in the heart [7]. Despite the relatively high expression level of Akt2 in the heart, Akt2 knockout (KO) mice exhibited a normal heart size at baseline, as well as a normal response to IGF-1-stimulated cardiomyocyte growth in culture. Insulin-induced glucose uptake was decreased and fatty acid (FA) oxidation increased in Akt2 KO mouse cardiomyocytes [8]. These observations suggested that distinct physiological functions of cardiac Akt might be revealed only when total Akt levels are below a critical threshold in the heart. Thus, questions remain regarding the integrated roles of Akt isoforms in cardiac structure and function. Double Akt1/2 KO mice die shortly after birth [9]; therefore, it was necessary to create inducible or conditional Akt1/2 KO mice to study the roles of these proteins in the heart. We aimed to test the hypothesis that Akt1/2 isoforms are required to maintain baseline cardiac function and structure using mutant mice in which Akt2 was conditionally deleted from the hearts of Akt1−/− mice by introducing a floxed Akt2 allele and inducing recombination in adult hearts by cardiomyocyte-restricted expression of a tamoxifen-inducible Cre recombinase.
Materials and methods
Results
Discussion The present study demonstrated that cardiac-specific Akt1−/−/iAkt2 KO in mice reduces cardiac size and induces contractile dysfunction without myocardial structural remodeling, such as interstitial fibrosis, hypertrophy, and apoptosis. Moreover, Akt1−/−/iAkt2 KO hearts showed decreased expression of Cx43 and connexin-interacting protein ZO-1. These studies identify a previously unrecognized mechanism by which Akt isoforms maintain cardiac contractile function, namely gap junction protein stability.