Abstract 169: c-fos and Caspase 8 Mediate Palmitate-induced Cardiomyocyte Apoptosis

Abstract

Background: We previously demonstrated that p38α activation mediates palmitate (PA)-induced cardiomyocyte apoptosis, and that specific p38α siRNA markedly and dose-dependently attenuates apoptosis rates in the presence of PA. We tested the hypothesis that c-fos and caspase 8 would be up-regulated concurrently with p38a. Methods and Results: Immortalized human adult ventricular cardiomyocytes ( AC16 cells) were exposed to high physiological levels of PA. The p38α-dependent pathway was evaluated using p38α siRNA knockdown. PA dose-dependently increased transcription of an AP-1 factor, c-fos, in AC16 cardiomyocytes (control:0.34±0.11, 150 μM PA:0.83±0.19, 300 μM PA:1.58±0.45, n=5, p<0.05). c-fos protein expression increased dose-dependently with PA treatment (control:0±0%, 150 μM PA:26%±28%, 300 μM PA:258%±103%, n=4, p<0.05). Phospho-c-fos levels increased dose-dependently as well (control:0±0%, 150 μM PA:73%±41%, 300 μM PA:302%±62%, n=4, p<0.05). Transcription of a putative downstream effector, caspase 8, also increased (control:0.94±0.10, 150 μM PA:1.40±0.10, 300 μM PA:1.42±0.11, n=3, p<0.05) but protein levels did not. p38α knockdown dose-dependently blocked the PA-induced increase in phospho-c-fos protein level (control siRNA:0±0%, 300 μM PA:206±47%, 300 μM PA+30 pmol siRNA:98±31%, 300 μM PA+60 pmol siRNA:58±40%, 300 μM PA+120 pmol siRNA:-21±19%, n=4, p<0.05), without a clear reduction in total c-fos. Interestingly, caspase 8 level was also dose-dependently reduced by p38α knockdown (control siRNA:0±0%, 300 μM PA:-25±3%, 300 μM PA+30 pmol p38α siRNA:-38±13%, 300 μM PA+60 pmol p38α siRNA:-54±7%, 300 μM PA+120 pmol p38α siRNA:-46±11%, n=3, p<0.05). Conclusions: This study in cardiomyocytes demonstrated that 1) PA dose-dependently increases gene expression of c-fos and protein levels of total c-fos and phospho-c-fos, 2) PA increases transcription of caspase 8, and 3) reduced p38α expression dose-dependently attenuates PA-induced increases in c-fos and phospho-c-fos protein levels, and decreases caspase 8. Our results suggest that p38α activated by PA up-regulates the downstream transcription factor, c-fos, and maintains caspase 8 level. These molecules may underlie PA-induced cardiomyocyte apoptosis.