Student Theses and Dissertations

Date of Award

1992

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

RU Laboratory

Krueger Laboratory

Abstract

Psoriasis is a hyperproliferative disease of the skin characterized histologically by epidermal and dermal hyperplasia and inflammatory cell infiltrates. While the pathogenesis of psoriasis is unknown, altered expression of cytokine pathways is likely to produce many of the phenotypic changes in the disease. The epidermal growth factor (EGF) receptor pathway is an important mediator of keratinocyte growth and both ligand and receptorcomponents of this pathway are abnormally expressed in hyperproliferative epidermis. In contrast, little is known about the function of other growth factor pathways in regulating keratinocyte growth. The purpose of this thesis was to characterize the function of three other major tyrosine kinase growth factor receptor pathways -- insulin-like growth factor I (IGF-I), fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF), in the regulation of keratinocyte growth in vitro and to characterize the expression of these receptors in normal and psoriatic skin. Growth of normal human keratinocytes in a chemically defined medium demonstrated that IGF-I, EGF, or basic FGF (bFGF) alone did not support significant keratinocyte spreading or proliferation. Considerable proliferation was observed when IGF-I was added in combination with either EGF or bFGF. In contrast, the combination of EGF with bFGF did not stimulate growth more than the addition of these factors alone. Growth synergy between IGF-I and EGF may be due in part to an IGF-I receptor mediated increase in keratinocyte EGF receptor expression. Treatment of cultured normal human keratinocytes with IGF-I increased EGF receptor binding an average of 1.8-fold in a dose and time dependent manner without altering EGF binding affinity. To analyze potential growth factor pathway interactions in vivo, sections of normal and psoriatic human epidermis were stained with IGF-I receptor, EGF receptor, and FGF receptor specific antibodies. In normal skin, plasma membrane IGF-I receptor staining localized exclusively to the proliferative basal cell compartment of the epidermis. IGF-I receptor staining was seen in both basal and suprabasal keratinocytes of lesional psoriatic skin correlating with the increased size of the proliferative cell compartment in psoriatic epidermis. In contrast, both EGF and FGF receptor expression was detected throughout the viable cell layers of the epidermis. Colocalization ofIGF-I receptors with EGF and FGF receptors in proliferating epidermal keratinocytes suggests that interactions among growth factor pathways may be necessary for keratinocyte growth activation. Since bFGF may regulate keratinocyte proliferation in skin, identification of a FGF receptor in keratinocytes was undertaken. Chemical crosslinking and immunoblotting experiments identified a single major FGF receptor protein with a molecular size of approximately 160 kd. Taken together with growth studies and immunohistochemical analysis, these data suggest that the FGF receptor protein identified may be involved in keratinocyte growth regulation. PDGF is a potent mitogenic and chemotactic factor for fibroblasts and other cell types. Analysis of PDGF receptor metabolism and immunohistochemical staining indicated that keratinocytes do not express PDGF receptors and are thus not directly responsive to this hormone. However, expression of PDGF receptors was greatly elevated in dermal fibroblasts and blood vessels of growth-activated skin and could contribute to dermal hyperproliferative changes seen in psoriasis. To begin analyzing the mechanism of action of anti-psoriatic drugs, the effects of anthralin and cyclosporin A (CSA) on keratinocyte proliferation and the EGF receptor pathway were examined. In contrast to CSA, anthralin-treated keratinocytes were more sensitive than lymphocytes to growth inhibition. CSA produced a cell cycle specific block at G1 in keratinocytes, while anthralin did not specifically block the cell cycle at any stage. While CSA did not significantly decrease expression of the EGF receptor or its ligand, transforming growth factor-alpha (TGF- α), anthralin decreased both TGF- α mRNA levels and EGF receptor binding in cultured keratinocytes. TGF- α expression remained at high levels in CSA-treated psoriasis patients suggesting that CSA does not act via direct modification of the EGF receptor pathway in vivo. These results may help explain differing response patterns to treatment with these two drugs. The findings of this thesis suggest possible mechanisms of interaction among the pathways studied that might contribute to the psoriatic phenotype. In particular, IGF-I receptor expression may define keratinocyte proliferative potential, and may, therefore, be a promising target for the development of new anti-psoriatic therapies.

Comments

A thesis submitted to the faculty of the Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy

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