COLLABORATIVE ROLES FOR C-JUN N-TERMINAL KINASE, C-JUN, SERUM RESPONSE FACTOR, AND SP1 IN CALCIUM-REGULATED MYOCARDIAL GENE-EXPRESSION

Electrical stimulation of contractions (pacing) of primary neonatal ra t ventricular myocytes increases intracellular calcium and activates a hypertrophic growth program that includes expression of the cardiac-s pecific gene, atrial natriuretic factor (ANF). To investigate the mech anism whereby pacing increases ANF, pacing was tested for its ability to regulate mitogen-activated protein kinase family members, ANF promo ter activity, and the trans-activation domain of the transcription fac tor, Sp1. Pacing and the calcium channel agonist BAYK 8644 activated c -Jun N-terminal kinase (JNK) but not extracellular signal-regulated ki nase. Pacing stimulated ANF-promoter activity approximately 10-fold. F urthermore, transfection with an expression vector for c-Jun, a substr ate for JNK, also activated the ANF promoter, and the combination of p acing and c-Jun was synergystic, consistent with roles for JNK and c-J un in calcium-activated ANF expression. Proximal serum response factor and Sp1 binding sites were required for the effects of pacing or c-Ju n on the ANF promoter. Pacing and c-Jun activated a GAL4-Sp1 fusion pr otein by 3- and 12-fold, respectively, whereas the two stimuli togethe r activated GAL4-Sp1 synergistically, similar to their effect on the A NF promoter. Transfection with an expression vector for c-Fos inhibite d the effects of c-Jun, suggesting that c-Jun acts independently of AP -1. These results demonstrate an interaction between c-Jun and Sp1 and are consistent with a novel mechanism of calcium-mediated transcripti onal activation involving the collaborative actions of JNK, c-Jun, ser um response factor, and Sp1.