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The effects of growth hormone receptor-associated ERK activation on adipocyte differentiation and function

Moftah, Souad A. M. 2012. The effects of growth hormone receptor-associated ERK activation on adipocyte differentiation and function. PhD Thesis, Cardiff University.
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Growth hormone (GH) modulates adipocyte function to promote lipolysis via a cell surface GH receptor (GHR) which activates multiple signalling cascades including STAT5 and p42/44 MAP kinase (MAPK) pathways. The growth promoting effects of GH are mediated primarily by STAT5 activation but little is known about pathways mediating the effects of GH on adipocyte function. We therefore studied the effect of GH on STAT5 and MAPK (ERK) activation in the 3T3-L1 mouse pre-adipocyte cell line during adipogenesis. Cells were plated, allowed to reach confluence and cultured in adipogenic medium containing the PPAR agonist, pioglitazone. GH induced activation (10 minutes exposure) of STAT5 and MAPK was analysed on days 0, 2, 5 and 8 during adipogenesis by phospho-specific western blotting and densitometry. During adipogenesis, GH progressively loses the ability to activate p42/44 MAPK despite elevated GHR and unaltered total ERK levels. In contrast, GH-stimulated STAT5 activation increases as 3T3-L1 differentiation proceeds. Subsequently we investigated possible explanations for the altered GHR signalling. The adapter protein p66Shc is thought to be necessary to link GHR activation to the ERK pathway. However levels of this protein, measured by western blotting and densitometry, did not decrease as 3T3-L1 cells underwent adipocyte differentiation. GHR levels increase with adipogenic differentiation of 3T3-L1 cells leading us to hypothesize that this may lead to preferential association with JAK2-STAT5. This was tested by overexpressing the GHR in 3T3-L1; similar GH-stimulated ERK pathway activation was obtained in cells transfected with the GHR vector and in those transfected with the empty vector. Finally, we have investigated whether changes in GHR signalling also occur during adipogenesis of primary pre-adipocytes from mice and various human depots. There was minimal GH-induced phosphorylation of ERK at all-time points before and during differentiation (required up to 15 days in primary cells) and no depot, either murine or human, demonstrated a reduction in p ERK, suggesting that this feature is unique to 3T3-L1. Furthermore, ERK phosphorylation may be the stimulus for mitotic clonal expansion which occurs in the cell line but not in human primaries. GH-stimulated STAT5 activation increases as human and mouse primary pre-adipocytes differentiation progresses, as in the 3T3-L1 cell-line, and may be the result of increased GHR transcript levels as differentiation proceeds. Future studies could investigate the mechanisms responsible for these similarities and differences.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Medicine
Subjects: Q Science > QP Physiology
R Medicine > R Medicine (General)
Date of First Compliant Deposit: 30 March 2016
Last Modified: 19 Mar 2016 23:17

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