Histone H1 and Chromosomal Protein HMGN2 Regulate Prolactin-induced STAT5 Transcription Factor Recruitment and Function in Breast Cancer Cells

Abstract

Edited by John M. DenuThe hormone prolactin (PRL) contributes to breast cancer pathogenesis through various signaling pathways, one of the most notable being the JAK2/signal transducer and activator of transcription 5 (STAT5) pathway. PRL-induced activation of the transcription factor STAT5 results in the up-regulation of numerous genes implicated in breast cancer pathogenesis. However, the molecular mechanisms that enable STAT5 to access the promoters of these genes are not well understood. Here, we show that PRL signaling induces chromatin decompaction at promoter DNA, corresponding with STAT5 binding. The chromatin-modifying protein high mobility group nucleosomal binding domain 2 (HMGN2) specifically promotes STAT5 accessibility at promoter DNA by facilitating the dissociation of the linker histone H1 in response to PRL. Knockdown of H1 rescues the decrease in PRL-induced transcription following HMGN2 knockdown, and it does so by allowing increased STAT5 recruitment. Moreover, H1 and STAT5 are shown to function antagonistically in regulating PRL-induced transcription as well as breast cancer cell biology. While reduced STAT5 activation results in decreased PRL-induced transcription and cell proliferation, knockdown of H1 rescues both of these effects. Taken together, we elucidate a novel mechanism whereby the linker histone H1 prevents STAT5 binding at promoter DNA, and the PRL-induced dissociation of H1 mediated by HMGN2 is necessary to allow full STAT5 recruitment and promote the biological effects of PRL signaling.In the mammary gland, signals from the polypeptide hormone prolactin (PRL) are essential for normal development (1-4), and these physiological effects are mediated through the homologous transcription factors signal transducer and activator of transcription 5a and 5b (referred to as STAT5) (5). PRL signals by binding to the PRL receptor (PRLR), 2 a transmembrane receptor of the cytokine receptor superfamily. Binding of PRL to the PRLR results in the activation of STAT5 via phosphorylation by the tyrosine kinase Janus kinase 2 (JAK2) (6 -8). Phosphorylated STAT5 dimerizes, and the active STAT5 dimers translocate to the nucleus. In the nucleus, STAT5 recognizes and binds to consensus elements in the DNA, which induces the expression of STAT5 target genes (9).PRL-induced STAT5 activation results in the up-regulation of many pro-proliferative genes (10 -12). Although essential for proper mammary gland development, aberrant PRL signaling and STAT5 activation also contribute to breast cancer pathogenesis. Transgenic mice that overexpress PRL in the mammary epithelium develop mammary tumors, which can be either estrogen receptor-positive or -negative (13). In epidemiologic studies, women with elevated serum PRL are at an increased risk of developing breast cancer (14 -17). PRL also enhances the proliferation, motility, and survival of breast cancer cells (18 -20). Similarly, overexpression of STAT5 in the mammary gland of transgenic mice results in mammary tumor development (21), whereas hemizygous los...

Trisomy of a Down Syndrome Critical Region Globally Amplifies Transcription via HMGN1 Overexpression

“…The HMGN family of proteins may have a more general role in cancer. The closely related HMGN2 enhances STAT5 function and chromatin accessibility downstream of the prolactin receptor in breast cancer cells ( Schauwecker et al, 2017 ), similar to what we hypothesized for HMGN1 in proB-ALL cells. Also, of potential therapeutic relevance, overexpression of HMGN family members is associated with relative chemoresistance whereas knockdown enhances chemosensitivity ( He et al, 2015 ; Yang et al, 2014 ).…”

HMGN1 and 2 remodel core and linker histone tail domains within chromatin

“…The mechanism by which HMGN1 and HMGN2 alter chromatin architecture remains unclear ( 25 , 29 , 47 ). To determine whether HMGN1 or HMGN2 alters formation of higher order tertiary chromatin structures, we employed self-association assays, which assess inter-fiber interactions in chromatin ( 2 , 48 , 49 ).…”

Biological Functions of HMGN Chromosomal Proteins

“…HMGN2 was also shown to promote breast cancer growth in response to prolactin. In this system, HMGN2 reduces the binding of linker histone H1 to chromatin regulatory regions, thereby enhancing STAT5 accessibility to promoter DNA, ultimately leading to increased proliferation of breast cancer cells [54,55]. Further analysis showed that the K2 residue of HMGN2 was deacetylated in primary breast tumors but highly acetylated in normal human breast tissue, implying that targeting HMGN2 deacetylation may be a viable treatment for this type of breast cancer [55].…”