| Relaxation of the ventricular myocardium is highly dependent on the reuptake of cytosolic calcium into the sarcoplasmic reticulum(SR). Patients with cardiac hypertrophy and heart failure display abnormally slowed muscle relaxation, which is associated with the down regulation of the sarcoplasmic reticulum calcium ATPase 2a(SERCA 2a), the ATP-dependent, SR pump. Previous data has shown a decrease in SERCA 2a mRNA levels during hypertrophy, identifying a level of transcriptional control. Further investigation of the SERCA 2a message has led us to believe there is an element of post-transcriptional control mediated by the 3′UTR Sequence analysis of the SERCA 2a message has identified a 609bp 3′untranslated region(3 ′UTR) containing potential AU rich elements(AREs). AREs identified within the 3′UTR of genes such as c-myc and TNF-α bind cellular proteins that regulate the message stability of the mRNA transcripts. Therefore we studied the proposed element of post-transcriptional control found within the 3′UTR of SERCA 2a during cardiomyocyte hypertrophy. We have found the 3′UTR of SERCA 2a to be capable of directly binding several cellular proteins, and this protein binding is enhanced during PMA and ET-1 stimulation. One of the proteins directly interacting with the 3′UTR element was identified as AUF-1, a known destabilizing factor. Upon stimulation with PMA and ET-1, the levels of AUF-1 binding to the 3′UTR of SERCA 2a increases, and this protein:RNA association occurs mainly within the nucleus. However, a known 3′UTR stabilizing factor, HuR, was not found to bind directly to the SERCA 2a 3′UTR, but does associate with AUF-1 predominantly in the cytoplasm of cardiomyocytes. We have determined that the AUF-1 found in the AUF-1:HuR complexes within the cytoplasm becomes phosphorylated, but the AUF-1 interacting with the 3′UTR of SERCA 2a is not phosphorylated. We propose that AUF-1 and HuR play unique roles within cardiomyocytes, and the post-transcriptional signaling pathway of SERCA 2a is complex. Therefore we conclude that the 3′UTR of SERCA 2a functions as a cis-acting destabilizing element that post-transcriptionally downregulates mRNA levels resulting in decreased SERCA 2a pumps during cardiomyocyte hypertrophy. |
