Cytoplasmic and nuclear STAT3 in GH-stimulated fibroblasts of children with idiopathic short stature

Investigation of the role of β-TrCP in growth hormone transduction defect (GHTD)

Background Growth hormone(GH) and epidermal growth factor (EGF) stimulate cell growth and differentiation, and crosstalking between their signaling pathways is important for normal cellular development. Growth hormone transduction defect (GHTD) is characterized by excessive GH receptor (GHR) degradation, due to over-expression of the E3 ubiquitin ligase, cytokine inducible SH2-containing protein (CIS). GH induction of GHTD fibroblasts after silencing of messenger RNA (mRNA) CIS (siCIS) or with higher doses of GH restores normal GH signaling. β-Transducing-repeat-containing protein (β-TrCP), another E3 ubiquitin ligase, also plays a role in GHR endocytosis. We studied the role of β-TrCP in the regulation of the GH/GHR and EGF/EGF receptor (EGFR) pathways in normal and GHTD fibroblasts. Materials and methods Fibroblast cultures were developed from gingival biopsies of a GHTD (P) and a control child (C). Protein expression and cellular localization of β-TrCP were studied by Western immunoblotting and immunofluorescence, respectively, after: (1) GH 200 μg/L human GH (hGH) induction, either with or without silence CIS (siCIS), and (2) inductions with 200 μg/L GH or 1000 μg/L GH or 50 ng/mL EGF. Results After induction with: (1) GH200/siCIS, the protein expression and cytoplasmic-membrane localization of β-TrCP were increased in the patient, (2) GH200 in the control and GH1000 in the patient, the protein and cytoplasmic-membrane localization of β-TrCP were increased and (3) EGF, the protein expression and cytoplasmic-membrane localization of β-TrCP were increased in both the control and the patient. Conclusions (1) β-TrCP appears to be part of the negative regulatory mechanism of the GH/GHR and EGF/EGFR pathways. (2) There appears to be a negative correlation between β-TrCP and CIS. (3) In the control and GHTD patient, β-TrCP increases when CIS is suppressed, possibly as a compensatory inhibitor of the GH/GHR pathway.

Growth hormone signaling in fibroblasts from newborn boys and prepubertal boys

BACKGROUND/AIM

Responsiveness to GH in target cells is mediated by its receptor, which activates the Janus kinase-2 (JAK2) and STAT5 (signal transducers and activators of transcription 5) leading to the expression of IGF-1 and IGFALS. The aim of this study was to compare the GH signaling pathway in newborns and prepubertal boys.

SUBJECTS AND METHODS

We determined the GHR protein content and the effect of stimulation with recombinant human GH (rhGH; 200ng/mL) on JAK2 and STAT5 phosphorylation in skin fibroblast cultures obtained from newborns and prepubertal boys. The transcript levels of IGFALS and IGF-I, were also studied and compared after 16h or 24h of stimulation with GH in both study groups.

RESULTS

Newborn infants showed less GHR protein than the prepubertal boys. After rhGH stimulation, JAK2 and STAT5 phosphorylation was absent in skin fibroblasts from newborns, but was clearly detectable in prepubertal boys. After 16h of treatment with rhGH, IGFALS and IGF-I transcript levels increased in the prepubertal boys when compared to baseline. In newborns, however, we did not observe a response after 16 and 24h of rhGH stimulation.

CONCLUSION

The significant attenuation of the GH signaling pathway observed in fibroblasts from newborn boys appears to be related to a reduction in GHR content and lack of phosphorylation of JAK2 and STAT5 in response to rhGH. This might impair STAT5 dimer formation, leading to a reduction in the transcript levels of IGFALS and IGF-I during the newborn period.

Effects of thyroid hormone on the GH signal transduction pathway

BACKGROUND/AIM

The importance of thyroid hormone on growth and development in children is well recognized. In addition, linear growth is highly dependent on the response of peripheral tissues to growth hormone, a process known as GH sensitivity, but little is known about the possible effects of T4 on this process.

METHODS

We determined the effect of stimulation with recombinant human GH (rhGH; 200 ng/mL) alone or in combination with two different concentrations of T4 (250 nM and 500 nM for 24 h) on JAK2 and STAT5 activation in skin fibroblast cultures obtained from prepubertal boys with normal height.

RESULTS

JAK2 and STAT5 were activated under co-incubation with T4 (at both concentrations) and rhGH in the non-nuclear fraction of the fibroblasts. In addition, after 24h of co-incubation with rhGH and T4 (500 nM), we observed an increase in phospho-STAT5 in the nuclear fraction, when compared to GH and T4 stimulation alone. This effect was not observed when the fibroblasts were co-incubated with GH and the lower concentration of T4 (250 nM).

CONCLUSION

Combined stimulation with GH and T4 at a concentration of 500 nM increases synergistically nuclear phospho-STAT5 in skin fibroblasts, which may amplify tissue sensitivity to GH. These findings may help to explain the effect of T4 administration on growth velocity in some children with idiopathic short stature.