Inhibition of Adipogenesis by the c-Myc Oncoprotein

Heath, Victoria J (1997) Inhibition of Adipogenesis by the c-Myc Oncoprotein. PhD thesis, University of Glasgow.

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Enforced expression of c-myc in many cell types is associated with inhibition of terminal differentiation. However, the mechanism by which this occurs remains unclear. In order to address this issue we therefore exploited the ability of c-Myc to block differentiation in the 3T3-L1 cell line, a well characterised in vitro model of adipogenesis. Analysis of 3T3-L1 lines constitutively expressing an avian c-myc transgene revealed an inability to undergo morphological changes or accumulate cytoplasmic triglyceride, which was dependent upon the integrity of the Myc leucine zipper. In order to define the point at which adipogenesis was inhibited we analysed patterns of gene expression during the differentiation programme. This study demonstrated that the Myc block was associated with repression of the late stage markers C/EBPalpha, PPARgamma2, aP2 and SCD1. C/EBPa and PPARgamma2 are key transcriptional regulators of adipogenesis that co-ordinate the expression of genes required for lipid metabolism, including aP2 and SCD1. Hence, repression of these factors provides a molecular basis for the inability of Myc-expressing cell lines to undergo morphological differentiation. Interestingly, a 3T3-L1 clone that had spontaneously lost the ability to differentiate displayed a similar profile of gene expression. However, subtle differences indicated that this block occurred via a different mechanism. Surprisingly, low levels of c-Myc were sufficient to inhibit 3T3-L1 differentiation and this was not associated with transformation or the capacity to undergo apoptosis. Additionally, the continued presence of c-Myc did not alter the ability of the 3T3-L1 line to undergo a number of defined cell cycle events during the early phases of the differentiation programme. Hence, inhibition of adipogenesis was unlikely to reflect altered cell cycle control. It had been proposed that c-Myc inhibits adipogenesis by preventing entry into a differentiation-specific growth arrest (GD) that is both irreversible and a prerequisite for terminal differentiation. However, we were unable to characterise the Gd state by either conventional criteria or by analysis of factors known to mediate cell cycle withdrawal in other differentiation systems. Indeed, irreversible cell cycle exit was only apparent in mature adipocytes, suggesting this was a consequence of terminal differentiation rather than the driving force. Thus, c-Myc did not inhibit adipogenesis by precluding entry into Go- Finally, treatment with 10% foetal calf serum (PCS) was sufficient to rescue Myc-mediated inhibition of adipogenesis but had no effect on the differentiation- defective clone. Abrogation of the Myc block was associated with restoration of the late stage markers, accounting for the ability of the Myc-expressing lines to accumulate lipid in the presence of PCS. Whilst the active component(s) present in PCS have yet to be fully defined, it is likely that growth hormone may be partly responsible for this phenomenon. We therefore conclude that c-Myc inhibits adipogenesis by repressing the expression of master transcription factors. However, this effect is dependent upon external factors, since FCS rescues the phenotype.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Sam Crunch
Keywords: Molecular biology, Developmental biology
Date of Award: 1997
Depositing User: Enlighten Team
Unique ID: glathesis:1997-75307
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 21:15
Last Modified: 19 Nov 2019 21:15

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