|Ph.D Student||Barac Yaron|
|Subject||The Long Term Effects of Fas Activation in Cardiac Myocytes:|
A Key Role for the 1,4,5-IP3 Pathway
|Department||Department of Medicine||Supervisor||Professor Emeritus Ofer Binah|
|Full Thesis text|
Cardiovascular disorders and diseases and their associated complications are a principal cause for morbidity and mortality worldwide. Arrhythmias are a common, serious and often fatal complication of many forms of heart diseases.
The Fas/Fas Ligand system plays an important role in various myocardial diseases. However, recent studies have indicated that healthy cardiomyocytes are apoptosis-resistant following Fas activation. Moreover long-term effect of Fas activation induces hypertrophy. Additionally, Dimmeler's group has found that lpr mice which are knockout for Fas receptor do not develop heart hypertrophy in response to pressure overload. Cardiac hypertrophy is a compensatory response to increased mechanical load. Since Fas receptor activation is an important component in hypertrophy induced by pressure- and volume-overload, deciphering the underlying signaling pathways is of prime importance. Based on our previous work showing that in mice and in neonatal rats ventricular myocytes (NRVM) - the electrophysiological disturbances and diastolic [Ca2+]i-rise caused by 3 hrs of Fas activation are dependent on the Fas - phospholipase C (PLC) - 1,4,5-inositol trisphosphate (1,4,5-IP3) - sarcoplasmic reticulum (SR) [Ca2+]i release pathway, we tested the hypothesis that this pathway is also critical for Fas-mediated hypertrophy. Using the model of NRVM we first confirmed that Fas activation causes an increase in hypertrophic markers. Indeed, in these cultures we found that increased nuclei surface area, atrial natriuretic peptide and connexin43 (Cx43) mRNA, the protein levels of total Cx43 and non-phosphorylated Cx43 and sarcomeric actin, all indicating hypertrophy. Concomitantly, Fas activation decreased mRNA of SERCA-2a, the ryanodine receptor (RyR) and nuclear IP3R3. Further, Fas activation caused NFAT nuclear translocation. The hypertrophy was abolished by U73122, xestospongin C (blockers of the 1,4,5- IP3 pathway), genistein and by the PI3K blocker LY294002. In complementary experiments we measured conduction velocity changes that were caused due to alternation in Cx43 protein.
Due to importance of the Fas receptor in myocardial pathologies and our abovementioned findings, we measured the levels of sFas in the blood of congestive heart failure patients as well as in the blood of myocardial infarct patients. We found significant changes in Fas receptor level indicating the importance of the Fas system in the myocardial pathology progression.
In conclusion, Fas-mediated hypertrophy is dependent on the 1,4,5-IP3 pathway, which is functionally interconnected to the PI3K/AKT/GSK3β pathway. Both pathways act in concert to cause NFAT nuclear translocation and subsequent hypertrophy.