Keratinocyte differentiation. Involvement of the growth hormon (GH)/insulinlike growth factor1 (IGFI) axis and IGF1 post receptor signaling

Abstract: At cellular level, little is known about the genes that regulate keratinocyte differentiation andproliferation. Psoriasis is regarded as a T-cell mediated inflammatory disease with hyperproliferativekeratinocytes. The aim of this thesis was to investigate whether the growth hormone(GH)/insulin like growth factor-1 (IGF-1) axis and IGF-1 post receptor signaling were affected inthe differentiation process of normal keratinocytes and the abnormalities seen in psoriasis. Weexplored the extracellular parameters of the GH/IGF-1 axis and one downstream target of IGF-1receptor signaling, the helix-loop-helix (HLH) protein Id1.In paper I, serum concentrations of IGF-1, IGF-binding protein-3, GH-binding protein and GHurine concentration were measured by immunoassays. GH receptor (GHR) gene expression insuction-blister roofs was measured using an RNase protection assay (RPA) or quantitative reversetranscriptase/polymerase chain reaction (RT-PCR). Our data demonstrated that normal epidermisexpressed GHRs. However, no significant difference between psoriatic patients and normal controlswas detected concerning the parameters studied.In paper II, Id1 gene expression in cultured keratinocytes and punch biopsies from normaland psoriatic involved and uninvolved skin was measured by RPA or Northern blot. Id1immunoreactivity was determined by immunofluorescence or Western blot. We found elevatedId1 expression in cells with high proliferative potential, which was downregulated in terminal differentiatedcells. Furthermore, the expression of Id1 in psoriatic involved skin was significantlyelevated compared with uninvolved skin and normal controls. The results indicated that Id1 maybe involved in the process of keratinocyte differentiation seen in normal skin and that the Id1pathway is activated in psoriatic involved skin. Furthermore, the results suggested that Id1 is likelyto have a specific dimerization partner in the skin. In paper III, we therefore searched for additionalunknown members of the HLH family by using the bioinformatics tools provided by the NCBI forhomology searches. This resulted in the cloning of a fourth member of the human achaete-scutecomplex family of genes, Hash4, which was mainly expressed in fetal skin. The function of Hash4remains to be clarified.The objectives of paper IV were to test the hypothesis that an altered regulation of the tumorsuppressor gene p16 in psoriasis was involved in a relative resistance of psoriatic plaque to transformationinto squamous cell cancer. To address this question, expression of genes involved in theIGF-1 receptor signaling through the Ras-Raf-MEK-ERK cascade were measured using real-timePCR, since candidates in this pathway are associated with the regulation of both Id1 and p16. Theresult showed that several genes in this pathway including JunB, which is an inducer of p16, wereupregulated in psoriatic involved skin. The results indicated activation of a pathway that mayprotect the keratinocytes by inducing p16 in an event of malignant stress.The results of this thesis indicate perturbation of genes involved in IGF-1 receptor signaling inpsoriatic involved skin. However, they do not support a major role for the systemic parameters of theGH/IGF-1 axis. Furthermore, the results indicate that Id1 is involved in the process of keratinocytedifferentiation seen in normal skin.