Introduction:The intestine exhibits a diurnal rhythm in apoptosis, cued by feeding and peaking during fasting. The pathways regulating this rhythm have not been elucidated. p53 upregulates pro-apoptotic Bax and Bak, and downregulates anti-apoptotic Bcl-w and Bcl-XL, culminating in caspase-3 activation, in other models of apoptosis and proliferation. We hypothesized that diurnal rhythmicity in apoptosis was similarly regulated.
Methods: Rats were acclimatized to a 12:12 hour light/dark cycle and jejunal mucosal scrapings harvested at 6 hourly intervals, beginning at ZT0 (Zeitgeber Time 0, corresponding to 7am and “lights-on”, n=6 to7 rats per time). mRNA expression of p53, Bcl-XL, Bcl-w, Bax, Bak and caspase-3 was determined using real-time PCR and statistical significance using ANOVA. Results:p53 mRNA expression peaked at ZT0, the termination of the normal nocturnal feeding period (p<0.05 vs. ZT18). Bak similarly peaked at ZT0 (p<0.0001 vs. ZT6, ZT12 and ZT18), however, no significant difference in expression was noted for Bax. In contrast, Bcl-w and Bcl-XL expressions were significantly higher during the feeding period with a peak at ZT18 (p<0.005 vs. ZT0 and ZT12). Bax and Bak expression were more than 3-fold higher than Bcl-XL and Bcl-w at ZT0 (p<0.0005 vs. ZT12 and ZT18). Caspase-3 expression was significantly higher during the lights-on period at ZT0 and ZT6 (p<0.05 vs. ZT12).
Conclusion: Our results identify a diurnal rhythm in the expression of components of the p53-mediated apoptotic pathway. Pro-apoptotic genes exhibited peak expression in the light period, when rats consume little food. Our findings suggest that diurnal rhythmicity in apoptosis may occur via a p53-mediated caspase-3 dependent pathway. Further understanding of the regulation of these apoptotic rhythms may allow the development of improved chemotherapeutic regimens with reduced gastrointestinal side-effects.