The POU4F2/Brn-3b transcription factor is required for the hypertrophic response to angiotensin II in the heart

Abstract

Adult hearts respond to increased workload such as prolonged stress or injury, by undergoing hypertrophic growth. During this process, the early adaptive responses are important for maintaining cardiac output whereas at later stages, pathological responses such as cardiomyocyte apoptosis and fibrosis cause adverse remodelling, that can progress to heart failure. Yet the factors that control transition from adaptive responses to pathological remodelling in the heart are not well understood. Here we describe the POU4F2/Brn-3b transcription factor (TF) as a novel regulator of adaptive hypertrophic responses in adult hearts since Brn-3b mRNA and protein are increased in angiotensin-II (AngII) treated mouse hearts with concomitant hypertrophic changes [increased heart weight:body weight (HW:BW) ratio]. These effects occur specifically in cardiomyocytes because Brn-3b expression is increased in AngII-treated primary cultures of neonatal rat ventricular myocytes (NRVM) or foetal heart-derived H9c2 cells, which undergo characteristic sarcomeric re-organisation seen in hypertrophic myocytes and express hypertrophic markers, ANP/βMHC. The Brn-3b promoter is activated by known hypertrophic signalling pathways e.g. p42/p44 mitogen-activated protein kinase (MAPK/ERK1/2) or calcineurin (via NFAT). Brn-3b target genes, e.g. cyclin D1, GLUT4 and Bax, are increased at different stages following AngII treatment, supporting distinct roles in cardiac responses to stress. Furthermore, hearts from male Brn-3b KO mutant mice display contractile dysfunction at baseline but also attenuated hypertrophic responses to AngII treatment. Hearts from AngII-treated male Brn-3b KO mice develop further contractile dysfunction linked to extensive fibrosis/remodelling. Moreover, known Brn-3b target genes, e.g. GLUT4, are reduced in AngII-treated Brn-3b KO hearts, suggesting that Brn-3b and its target genes are important in driving adaptive hypertrophic responses in stressed heart.

Linking metabolic dysfunction with cardiovascular diseases: Brn-3b/POU4F2 transcription factor in cardiometabolic tissues in health and disease

“…Many observations that link the loss of Brn-3b with metabolic dysfunction and adverse cardiac responses to stress arise from studies using the global Brn-3b KO mice . However, these are strongly supported by additional data using either wild-type mice or in-vitro models [18][19][20]47 . For example, while the first evidence showing an association between a loss of Brn-3b and metabolic dysfunction (hyperglycaemia and insulin resistance) has been identified using constitutive Brn-3b KO mutants in vitro studies have confirmed that Brn-3b, is itself, regulated by key metabolites such as glucose, insulin and fatty acids, which are dysregulated in obesity and T2D 20 .…”

“…Moreover, early studies using constitutive Brn-3b knockout (KO) mice revealed essential roles for Brn-3b in maintaining survival and specification of retinal ganglion cells (RGCs), since homozygous Brn-3b KO mutants are blind, due to loss of~70% of RGCs, postnatally 24,25,30,35 . More recent studies using these mutants have also highlighted essential and previously unknown roles for Brn-3b in regulating genes that control the fate and function of metabolic, vascular and cardiac cells 18,20 (see later).…”

“…Brn-3b is a tissue-specific TF [18][19][20][21][22][23][24][25] belonging to the class IV POU (Pit-1; Oct-1; Unc-86) sub-family of homeodomain TFs, and is characterised by a highly conserved POU domain that forms a helix-turn-helix (HTH) DNA binding structure 26 . The POU domain consists of a 60aa homeodomain, POU-HD , with strong similarity to homeobox proteins and 74-82aa POU-specific domain (POU s ), which is unique to POU proteins (Fig.…”

“…However, subsequent studies have shown Brn-3b expression in other diverse tissues including reproductive tract tissue (testis, ovary and breast epithelium) 22,33,34,[38][39][40][41][42][43][44] , peripheral blood mononuclear cells (PBMC) 23,45,46 , metabolic tissue (adipocytes, skeletal muscle and liver) 20 , cardiovascular tissues e.g. cardiomyocytes 18,19,21,39,47 and vascular smooth muscle cells (VSMC) (Fig. 2).…”

“…3 ). This interaction between Brn-3b and p53 is also important in controlling the gene expression and fate of cardiomyocytes, in both developing and adult hearts 18 , 57 . The mechanisms associated with controlling effects in cardiomyocytes will be discussed in more detail later, but it is noteworthy that ectopic increases in Brn-3b in the developing heart can induce apoptosis if co-expressed with p53 and thereby contribute to congenital heart defects 19 .…”