Fibroblast Growth Factor Receptor 3 (FGFR3) is one of four high-affinity receptors for canonical FGF ligands. It acts in many tissues and plays a special role in skeletal development, especially post-embryonic bone growth, where it inhibits chondrocyte proliferation and differentiation. Gain of function mutations cause the most common forms of dwarfism in humans, and they are also detected in cancer. Triggered by ligand binding or in some cases mutation, FGFR3 activation involves dimerization of receptor monomers, phosphorylation of specific tyrosine residues in the receptor's kinase domain and in the tightly linked scaffold protein Fibroblast Receptor Factor Substrate 2 (FRS2). Signaling molecules recruited to these phosphorylation sites propagate signals through cascades that are subject to modulation. Signal output is also regulated by the fate of the receptor and the interval between its activation and degradation. Trafficking pathways have been identified for both lysosomal and proteasomal degradation, as well as, an alternative fate that involves intramembrane cleavage that produces an intracellular domain fragment capable of nuclear transport and potential function.

Glucocorticoid (GC) treatment of rat or chicken embryonic pituitary (CEP) cells induces premature production of growth hormone (GH). GC induction of the GH gene requires ongoing protein synthesis, and the GH genes lack a canonical GC response element (GRE). To characterize cis-acting elements and identify trans-acting proteins involved in this process, we characterized the regulation of a luciferase reporter containing a fragment of the chicken GH gene (-1727/+48) in embryonic day 11 CEP cells. Corticosterone (Cort) increased luciferase activity and mRNA expression, and mRNA induction was blocked by protein synthesis inhibition. Through deletion analysis, we identified a GC-responsive region (GCRR) at -1045 to -954. The GCRR includes an ETS-1 binding site and a degenerate GRE (dGRE) half site. Nuclear proteins, including ETS-1, bound to a GCRR probe in electrophoretic mobility shift assays, and Cort regulated protein binding. Using chromatin immunoprecipitation, we found that ETS-1 and GC receptor (GR) were associated with the GCRR in CEP cells, and Cort increased GR recruitment to the GCRR. Mutation of the ETS-1 site or dGRE site in the -1045/+48 GH reporter abolished Cort responsiveness. We conclude that GC regulation of the GH gene during development requires cis-acting elements in the GCRR and involves ETS-1 and GR binding to these elements. Similar ETS-1 elements/dGREs are located in the 5'-flanking regions of GH genes in mammals, including rodents and humans. This is the first study to demonstrate involvement of ETS-1 in GC regulation of the GH gene during embryonic development in any species, enhancing our understanding of GH regulation in vertebrates.

Within the anterior pituitary gland, glucocorticoids such as corticosterone (CORT) provide negative feedback to inhibit adrenocorticotropic hormone secretion and act to regulate production of other hormones including growth hormone (GH). The ontogeny of GH production during chicken embryonic and rat fetal development is controlled by glucocorticoids. The present study was conducted to characterize effects of glucocorticoids on gene expression within embryonic pituitary cells and to identify genes that are rapidly and directly regulated by glucocorticoids. Chicken embryonic pituitary cells were cultured with CORT for 1.5, 3, 6, 12, and 24 h in the absence and presence of cycloheximide (CHX) to inhibit protein synthesis. RNA was analyzed with custom microarrays containing 14,053 chicken cDNAs, and results for selected genes were confirmed by quantitative reverse transcription real-time PCR (qRT-PCR). Levels of GH mRNA were maximally induced by 6 h of CORT treatment, and this response was blocked by CHX. Expression of 396 genes was affected by CORT, and of these, mRNA levels for 46 genes were induced or repressed within 6 h. Pathway analysis of genes regulated by CORT in the absence of CHX revealed networks of genes associated with endocrine system development and cellular development. Eleven genes that were induced within 6 h in the absence and presence of CHX were identified, and eight were confirmed by qRT-PCR. The expression profiles and canonical pathways defined in this study will be useful for future analyses of glucocorticoid action and regulation of pituitary function.

Affecting approximately 250 000 individuals worldwide, achondroplasia (Ach) represents a family of skeletal dysplasias. Although inherited as an autosomal dominant trait, it results most often from a new mutation to unaffected parents. Virtually all patients have the same mutation in the gene that codes for the receptor tyrosine kinase, FGFR3. The mutation exaggerates the receptor's inhibitory functions on bone growth resulting in characteristic clinical features. Similar mutations of FGFR3 cause other members of this disease family. Our understanding of how mutant FGFR3 antagonises bone growth remains incomplete, and delineating the relevant molecular details has proved challenging. Nevertheless, new insights into the FGFR3 biology, that is, downstream FGFR3 signals are modulated by the paracrine hormone CNP, FGFR3 is processed to a fragment with potential nuclear function and FGFR3 signal output is influenced by the molecular chaperone Hsp90, have advanced the field and provided a glimpse into future growth stimulating treatments for Ach. Key Concepts: Achondroplasia is the prototype of a ‘family’ of human skeletal dysplasias characterised by dwarfism and characteristic craniofacial manifestations. Achondroplasia results from gain of function mutations of the tyrosine kinase‐mediated receptor, FGFR3, which is an important negative regulator of linear bone growth. Although achondroplasia can be inherited as an autosomal dominant trait, it most often results from de‐novo mutations to nonachondroplastic parents. The new mutations display a paternal age effect. Virtually all patients with achondroplasia have the same FGFR3 mutation. Other mutations of FGFR3 that enhance its function cause similar but more or less severe dwarfing conditions, such as thanatophoric dysplasia and hypochondroplasia. FGFR3 acts through ‘signals’ it transmits to the cellular machinery that regulates the proliferation and other behaviours of cells responsible for bone growth – chondrocytes that occupy the growth plates of growing bones. The predominant signalling output involves tyrosine kinase cascades collectively referred to as the MAPK signalling pathway. FGFR3 receptors bearing the achondroplasia mutation exhibit exaggerated MAPK signalling when investigated in both cell culture and mouse models of achondroplasia. The major goal in developing treatments for achondroplasia has been to safely reduce the output of FGFR3 signals to or towards normal. Strategies have ranged from blocking receptor activation to inhibiting FGFR3 tyrosine kinase activity to shortening the survival of actively signalling receptor to antagonising signals downstream of the receptor. These strategies are based on understanding the relevant molecular events. The most promising strategy at present involves administration of the peptide, CNP, which antagonises MAPK signals initiated by FGFR3.