| Summary: | Thesis (MSc)--Stellenbosch University, 2012. === ENGLISH ABSTRACT: Physical and psychological stressors trigger activation of the hypothalamo-pituitary-adrenocortical
(HPA) axis that leads to enhanced secretion of glucocorticoids e.g. cortisol. Moreover, chronic activation
of this pathway may elevate oxidative stress that is linked to the onset of insulin resistance and
cardiovascular diseases (CVD). Our laboratory previously found that oxidative stress increases flux
through metabolic circuits such as the hexosamine biosynthetic pathway (HBP), in effect increasing its
modification of target proteins post-transcriptionally with O-GlcNAc moeities. This in turn may alter
protein function and contribute to the onset of myocardial insulin resistance and impaired contractile
function. Since the underlying mechanisms linking chronic stress to cardiometabolic pathophysiology
are poorly understood, we hypothesised that cortisol elicits myocardial oxidative stress, HBP activation,
and decreased glucose uptake (due to attenuated glucose transport functionality) with detrimental
outcomes, i.e. insulin resistance and apoptosis. To investigate this hypothesis we established an in vitro model using HL-1 cardiomyocytes, with which
we evaluated the degree of O-GlcNAcylation and oxidative stress in response to a range of time-dose
treatments with dexamethasone (synthetic glucocorticoid). Glucose transporter 4 (GLUT4) translocation
to the sarcolemma was also assessed. In agreement with the literature, results suggest that GLUT4
translocation is significantly decreased subsequent to dexamethasone treatment. Although no significant differences were observed with regards to oxidative stress or O-GlcNAcylation, the data show that
dexamethasone increased the latter with a maximal effect after two hours exposure to the 10-6 M dose.
Although our results were not conclusive, the data suggest a potential novel link between dexamethasone
exposure, HBP activation and decreased GLUT4 translocation. Based on our findings we propose
that detrimental effects of chronic stress on the heart may be mediated by increased HBP flux. Given
that glucocorticoid excess and GLUT4 dysregulation have been associated with insulin resistance (and
related metabolic derangements and diseases), these results provide new targets for potential therapeutic
agents. === AFRIKAANSE OPSOMMING: Fisiese sowel as psigologiese stressors veroorsaak die aktivering van die hipotalamiese-hipo seale-bynier
(HHB) pad wat lei tot die verhoogde sekresie van glukokortikoïede soos kortisol. Kroniese aktivering van
hierdie pad kan ook oksidatiewe stres verhoog wat weer tot insulienweerstandigheid en kardiovaskulêre
siektes (KVS) kan lei. Navorsing uit ons laboratorium het voorheen bewys dat oksidatiewe stres 'n toename
in vloei deur metaboliese paaie soos die heksoamine biosintetiese pad (HBP) kan veroorsaak deur
die modi sering van teikenproteïene met O-GlcNAc motiewe. Dit kan weer proteïen funksie verander
en bydra tot die ontstaan van miokardiale insulienweerstandigheid en verswakte kontraktiele funksie.
Die onderliggende meganismes wat kroniese stres aan kardiometaboliese pato siologie verbind word
nog nie goed verstaan nie, daarom is ons hipotese dat kortisol miokardiale oksidatiewe stres veroorsaak,
die HBP pad aktiveer, en glukose opname verminder (deur die funksionele onderdrukking van
glukose transport), wat nadelige uitkomste soos insulienweerstandigheid en apoptose tot gevolg kan hê.
Om hierdie hipotese te ondersoek, is 'n in vitro model van HL-1 kardiomiosiete gebruik waarmee
die graad van O-GlcNAsilering en oksidatiewe stres in reaksie op 'n reeks tyd-konsentrasie behandelings
met deksametasoon (sintetiese glukokortikoïed), bepaal is. Glukose transporter 4 (GLUT4)
translokasie na die sarkolemma is ook geasseseer. In ooreenstemming met die literatuur, is GLUT4
translokasie insiggewend onderdruk tydens deksometasoon behandeling. Alhoewel geen insiggewende
verskille rakende oksidatiewe stres en O-GlcNAsilering gevind is nie, het ons data aangedui dat laasgenoemde
deur deksametasoon vermeerder het na twee ure van blootstelling aan die 10-6 M konsentrasie.
Alhoewel ons resultate geen afdoende bewys lewer nie, stel dit wel voor dat daar 'n potensiële verbintenis
tussen deksametasoon behandeling en 'n afname in GLUT4 translokasie is. Gebasseer op ons
bevindings, stel ons voor dat die nadelige e ekte van kroniese stres op die hart bemiddel kan word
deur 'n toename in vloei deur die HBP. Gegewe dat 'n oormaat glukokortikoïede en GLUT4 wanregulering
geassosieer is met insulien weerstandigheid (en verbandhoudende metaboliese veranderinge en
siektes), verskaf hierdie resultate nuwe teikens vir potensiële terapeutiese ingrepe.
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