ESPE Abstracts

Background: epidermal growth factor (EGF) stimulates cell growth and differentiation through its receptor EGFR. Cross-talking between the GH and EGF signaling pathways is important for normal cellular development. GH transduction defect (GHTD), a clinical disorder characterized by impaired STAT3 phosphorylation due to excessive GHR degradation, is caused by over-expression of the E3 ubiquitin ligase, CIS.Induction of GHTD fibroblasts with 200 ng/ml hGH (GH200) and silencing mRNA CIS (siCIS) or high dose hGH, 1 000 ng/ml (GH1000), suppresses excess CIS and restores normal GH signalling. After rhGH treatment,GHTD children show significant ‘catch-up’ growth and normal adult height. β-transducin repeat-containing protein β-TrCP (β-TrCP), another E3 ubiquitin ligase, also plays a role in GHR endocytosis.

Objective and hypotheses: To study the role of β-TrCP in the regulation of the GH/GHR and EGF/EGFR pathways were studied in normal and GHTD cells.

Method: Fibroblast cultures were developed from gingival biopsies of a GHTD patient (P) and control (C) child. Protein expression and cellular localization of β-TrCP were studied by Western Immunoblotting and Immunofluorescence respectively, at i) the basal state and after GH200 induction, either with or without siCIS and ii) the basal state and after GH200, GH1000 or 50 ng/ml EGF inductions.

Results: i) After GH200/siCIS, the protein expression and cytoplasmic and membrane localization of β-TrCP were increased in the P. ii) After induction with GH200 in the C and GH1000 in the P (inductions of successful GH signaling), the protein expression and cytoplasmic localization of β-TrCP were increased. iii) After induction with EGF, the protein expression and cytoplasmic localization of β-TrCP were also increased in both the C and P.

Conclusion: In GHTD, β-TrCP increases, possibly as a compensatory negative regulator of the GH/GHR pathway when the excessive CIS is suppressed. β-TrCP also seems to be part of the negative regulatory mechanism of the EGF/EGFR pathway under normal conditions and in the clinical disorder, GH transduction defect.