Tetracycline-inducible CaM kinase II silences hypertrophy-sensitive gene expression in rat neonate cardiomyocytes

Recent work from this laboratory both in rat primary cardiomyocytes and in ventricular tissue of transgenic mouse models of induced hypertrophy has id entified two Ca2+/calmodulin-dependent nuclear signaling cascades. The firs t involves the phosphatase calcineurin (CaN). The second is the CaM kinase kinase cascade which involves CaM kinase I and CaM kinase IV. Each of these signaling cascades strongly up-regulate transcription of hypertrophy-sensi tive genes in the rat ventricular cardiomyocyte. We have documented that ov er-expression of an active form of CaM kinase II silenced transcriptional i nduction of hypertrophy-sensitive genes. The purpose of this study was to g enerate an inducible CaM kinase II expression system and correlate its expr ession with the silencing of hypertrophic-sensitive reporters. A truncated form of CaM KII, CaM HII (1-290) was subcloned downstream and proximal to a promoter under transcriptional control (induction) of the tetracycline-reg ulated transcription factor, tet-TransActivator (tTA). Hypertrophy-sensitiv e reporter activity in primary cardiomyocytes was silenced when tet-inducib le CaM HII was co-expressed with plasmids harboring active forms of CaN, Ca M KI or CaM KIV. For instance, induced CaM KII expression silenced CaN, CaM kinase I, or CaM kinase TV driven ANF reporter activity 4.9-, 2.9-, and 6. 9-fold below their maximal values, respectively. Myocyte exposure to doxycy cline (DOX) blocked tTA-driven CaM KII expression and restored CaN/CaM KI o r CaN/CaM KIV driven reporter activation. This study demonstrates, for the first time, that active CaM KII silences Ca2+-sensitive nuclear signaling c ascades for transcriptional upregulation of cardiomyocyte hypertrophy. (C) 2000 Academic Press.