No abstract

Pituitary gonadotropin hormones are regulated by gonadotropin-releasing hormone (GnRH) via MAPK signaling pathways that stimulate gene transcription of the common α-subunit () and the hormone-specific β-subunits of gonadotropin. We have reported previously that GnRH-induced activities at these genes include various histone modifications, but we did not examine histone phosphorylation. This modification adds a negative charge to residues of the histone tails that interact with the negatively charged DNA, is associated with closed chromatin during mitosis, but is increased at certain genes for transcriptional activation. Thus, the functions of this modification are unclear. We initially hypothesized that GnRH might induce phosphorylation of Ser-10 in histone 3 (H3S10p) as part of its regulation of gonadotropin gene expression, possibly involving cross-talk with H3K9 acetylation. We found that GnRH increases the levels of both modifications around the gene transcriptional start site and that JNK inhibition dramatically reduces H3S10p levels. However, this modification had only a minor effect on expression and no effect on H3K9ac. GnRH also increased H3S28p and H3K27ac levels and also those of activated mitogen- and stress-activated protein kinase 1 (MSK1). MSK1 inhibition dramatically reduced H3S28p levels in untreated and GnRH-treated cells and also affected H3K27ac levels. Although not affecting basal expression, MSK1/2 inhibition repressed GnRH activation of expression. Moreover, ChIP analysis revealed that GnRH-activated MSK1 targets the first nucleosome just downstream from the TSS. Given that the elongating RNA polymerase II (RNAPII) stalls at this well positioned nucleosome, GnRH-induced H3S28p, possibly in association with H3K27ac, would facilitate the progression of RNAPII.

Gonadotropin-releasing hormone (GnRH) stimulates the expression of multiple genes in the pituitary gonadotropes, most notably to induce synthesis of the gonadotropins, luteinizing hormone (LH), and follicle-stimulating hormone (FSH), but also to ensure the appropriate functioning of these cells at the center of the mammalian reproductive endocrine axis. Aside from the activation of gene-specific transcription factors, GnRH stimulates through its membrane-bound receptor, alterations in the chromatin that facilitate transcription of its target genes. These include changes in the histone and DNA modifications, nucleosome positioning, and chromatin packaging at the regulatory regions of each gene. The requirements for each of these events vary according to the DNA sequence which determines the basal chromatin packaging at the regulatory regions. Despite considerable progress in this field in recent years, we are only beginning to understand some of the complexities involved in the role and regulation of this chromatin structure, including new modifications, extensive cross talk, histone variants, and the actions of distal enhancers and non-coding RNAs. This short review aims to integrate the latest findings on GnRH-induced alterations in the chromatin of its target genes, which indicate multiple and diverse actions. Understanding these processes is illuminating not only in the context of the activation of these hormones during the reproductive life span but may also reveal how aberrant epigenetic regulation of these genes leads to sub-fertility.

Oxytocin (OT) and vasopressin (VP) are neurohypophyseal hormones with potent stimulatory actions on the uterus. In order to determine whether these hormones may have a paracrine action on the uterus, OT and VP gene expression was studied in myometrium from pregnant rats at gestational ages of 14 and 20 days, and from ovariectomized animals treated with oestradiol and progesterone. OT and VP mRNA concentrations were measured using real-time quantitative reverse transcription-PCR, and OT-and VP-like immunoreactivities were determined using RIA. OT mRNA was detected in the uterus from pregnant rats, but did not differ between the groups of different gestational ages. Oestradiol significantly (P!0 . 05) stimulated OT gene expression in ovariectomized rats. Progesterone alone was without effect on OT mRNA concentrations, but significantly (P!0 . 05) reduced the oestradiol-induced OT mRNA accumulation. The OT-like immunoreactivity in an extract of myometrium from pregnant rats was eluted from a reverse-phase HPLC column with a retention time identical to that of synthetic OT. Neither VP mRNA nor VP-like immunoreactivity was detected in the myometrium from pregnant or ovariectomized rats. The study demonstrates steroid-dependent expression of the OT gene in the rat uterus and processing of uterine preprooxytocin to the mature nonapeptide. The data support the theory that this peptide may act in a paracrine pathway. No evidence was found for the presence of VP in the uterus so that, if the hormone is involved in a stimulatory action on this tissue, it probably acts via an endocrine mechanism.