THE ROLE OF FIBROBLAST GROWTH FACTOR-1 IN HUMAN ADIPOGENESIS

Newell, Felicity Sara (2007). THE ROLE OF FIBROBLAST GROWTH FACTOR-1 IN HUMAN ADIPOGENESIS PhD Thesis, School of Medicine, University of Queensland.

       
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Author Newell, Felicity Sara
Thesis Title THE ROLE OF FIBROBLAST GROWTH FACTOR-1 IN HUMAN ADIPOGENESIS
School, Centre or Institute School of Medicine
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Professor Johannes Prins
Abstract/Summary Obesity has developed into a world-wide epidemic highlighting the necessity for basic research into the mechanisms that govern adipose tissue growth. The accumulation of adipose tissue mass, termed adipogenesis, is the result of both an increase in the triglyceride content of mature adipocytes and the proliferation and differentiation of the adipocyte precursors, preadipocytes. The features of murine adipogenesis have been extensively studied using in vitro models of differentiation, however human adipogenesis is poorly understood due to a lack of available cell lines and the often poor differentiation capacity of primary preadipocytes. In our laboratory, we have recently demonstrated that fibroblast growth factor- 1 (FGF-1) reproducibly increases the differentiation capacity of primary human preadipocytes (phPA). FGF-1 treatment increases phPA proliferation and during this period also primes cells for subsequent differentiation. The highest levels of phPA differentiation are obtained following FGF-1 treatment during both proliferation and differentiation. However it is not known how FGF-1 mediates these actions, or whether FGF-1 has a similar effect on other human preadipocyte models. Furthermore the features of preadipocyte differentiation in response to FGF-1 treatment have not been examined. Therefore, the aims of this thesis were to further characterise the features of human adipogenesis in FGF-1 treated human preadipocytes and to investigate the mechanisms of FGF-1 action in these cells. The features of differentiation were examined in FGF-1 treated phPA and FGF-1 treated SGBS PA (a human preadipocyte strain derived from an individual with Simpson-Golabi-Behmel syndrome). These models were compared with the well-characterised murine 3T3-L1 preadipocyte cell line (3T3-L1 PA). FGF-1 upregulated the adipogenic program in phPA, with increased expression of adipocyte markers during differentiation. Moreover, phPA differentiated in the presence of FGF-1 were more insulin responsive and secreted increased levels of adiponectin. FGF-1 treatment of SGBS PA elicited similar effects on cellular proliferation and differentiation, resulting in increased expression of adipocyte genes and an increased number of functional adipocytes. For the most part the adipogenic program in phPA and SGBS PA paralleled that observed in 3T3-L1 PA. However there was no evidence of mitotic clonal expansion in phPA, a process that is postulated to be necessary for differentiation in murine models and to involve the activation of ERK1/2 (extracellular signal-regulated kinase). In this thesis it was demonstrated that FGF-1 induced robust phosphorylation of ERK1/2 in early differentiation in phPA. Furthermore, inhibition of ERK1/2 activity significantly reduced phPA differentiation demonstrating that ERK1/2 activation is necessary for human adipogenesis in the absence of mitotic clonal expansion. The work in this thesis therefore highlights the existence of species specific differences with respect to adipogenesis and demonstrates that FGF-1 treated phPA and SGBS PA represent useful in vitro models for the study of human adipogenesis. The mechanisms of FGF-1 action in phPA and SGBS PA proliferation and differentiation were subsequently examined. FGF-1 is a member of the fibroblast growth factor family and elicits many of its effects by binding to cell surface tyrosine kinase FGF receptors (FGFR) and subsequently activating downstream signal transduction cascades. In this thesis, it was demonstrated that phPA expressed four FGF receptors (FGFR1-4), and that FGFR activation was required for phPA proliferation. FGFR activation in proliferation resulted in the stimulation of a number of downstream signalling pathways including the Ras-MAPK (mitogen activated protein kinase) pathway and the PI-3 kinase (phosphatidylinositol 3-kinase) pathway. Long term FGF-1 treatment of phPA during proliferation primes the cells for differentiation. In this thesis it was demonstrated that this priming resulted in changes in gene expression in confluent phPA, with alterations in the expression of other FGFs as well as genes required for differentiation. The role of FGF-1 in phPA differentiation was also examined. It was observed that, following induction of differentiation, FGF-1 increased signalling through several signal transduction pathways (the PI-3 kinase and Ras-MAP kinase pathways) which have postulated roles in human adipogenesis. Therefore, the work presented in this thesis serves to build on our earlier novel findings of the adipogenic role of FGF-1 in phPA and provides further evidence of the requirement for FGF-1 throughout phPA development. In addition, FGF-1 treated human preadipocytes offer an attractive in vitro model to study many features of adipogenesis in a human model and provide further insights into the processes that ultimately lead to obesity.

 
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Created: Fri, 21 Nov 2008, 15:40:48 EST