Premature Expression of FOXO1 in Developing Mouse Pituitary Results in Anterior Lobe Hypoplasia

Overview of chromatin regulatory processes during surface ectodermal development and homeostasis

The ectoderm is the outermost of the three germ layers of the early embryo that arise during gastrulation. Once the germ layers are established, the complex interplay of cellular proliferation, differentiation, and migration results in organogenesis. The ectoderm is the progenitor of both the surface ectoderm and the neural ectoderm. Notably, the surface ectoderm develops into the epidermis and its associated appendages, nails, external exocrine glands, olfactory epithelium, and the anterior pituitary. Specification, development, and homeostasis of these organs demand a tightly orchestrated gene expression program that is often dictated by epigenetic regulation. In this review, we discuss the recent discoveries that have highlighted the importance of chromatin regulatory mechanisms mediated by transcription factors, histone and DNA modifications that aid in the development of surface ectodermal organs and maintain their homeostasis post-development.

FOXO Transcription Factors Are Required for Normal Somatotrope Function and Growth

Understanding the molecular mechanisms underlying pituitary organogenesis and function is essential for improving therapeutics and molecular diagnoses for hypopituitarism. We previously found that deletion of the forkhead factor, Foxo1, in the pituitary gland early in development delays somatotrope differentiation. While these mice grow normally, they have reduced growth hormone expression and free serum insulin-like growth factor-1 (IGF1) levels, suggesting a defect in somatotrope function. FOXO factors show functional redundancy in other tissues, so we deleted both Foxo1 and its closely related family member, Foxo3, from the primordial pituitary. We find that this results in a significant reduction in growth. Consistent with this, male and female mice in which both genes have been deleted in the pituitary gland (dKO) exhibit reduced pituitary growth hormone expression and serum IGF1 levels. Expression of the somatotrope differentiation factor, Neurod4, is reduced in these mice. This suggests a mechanism underlying proper somatotrope function is the regulation of Neurod4 expression by FOXO factors. Additionally, dKO mice have reduced Lhb expression and females also have reduced Fshb and Prl expression. These studies reveal FOXO transcription factors as important regulators of pituitary gland function.

MiR-1299 promotes the synthesis and secretion of prolactin by inhibiting FOXO1 expression in drug-resistant prolactinomas

Prolactinoma is a clinically common intracranial tumor. When serum prolactin levels are not controlled despite administration of a dopamine agonist, the condition is referred to as drug-resistant prolactinoma. The mechanism underlying persistent prolactin secretion in drug-resistant prolactinoma remains unclear. MicroRNAs play an important role in tumorigenesis and development as well as chemotherapeutic resistance. This study was conducted to investigate the mechanism by which miRNA regulates prolactin secretion in drug-resistant prolactinoma. We first found that miR-1299 was elevated in drug-resistant prolactinoma and inhibited FOXO1 in a targeted manner through miRNA sequencing and luciferase assays. We then confirmed that FOXO1 binds to the promoter of the prolactin gene to inhibit its expression through chromatin immunoprecipitation-quantitative PCR and cytological experiments. Finally, inhibition or overexpression of miR-1299 in primary tumor cells confirmed that drug-resistant prolactinoma promoted prolactin secretion by promoting miR-1299 expression and reducing intracellular FOXO1. These results indicate that FOXO1 and miR-1299 are potential therapeutic targets for drug-resistant prolactinoma as well as other pituitary diseases.