Recent work from this laboratory both in rat primary cardiomyocytes and in ventricular tissue of transgenic mouse models of induced
hypertrophy has identified two Ca(2+)/
calmodulin-dependent nuclear signaling cascades. The first 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-sensitive genes in the rat ventricular cardiomyocyte. We have documented that over-expression of an active form of
CaM kinase II silenced transcriptional induction of
hypertrophy-sensitive genes. The purpose of this study was to generate an inducible
CaM kinase II expression system and correlate its expression with the silencing of hypertrophic-sensitive reporters. A truncated form of
CaM KII,
CaM KII (1-290) was subcloned downstream and proximal to a promoter under transcriptional control (induction) of the
tetracycline-regulated
transcription factor, tet-
TransActivator (tTA).
Hypertrophy-sensitive reporter activity in primary cardiomyocytes was silenced when tet-inducible
CaM KII was co-expressed with plasmids harboring active forms of CaN, CaM KI or CaM KIV. For instance, induced
CaM KII expression silenced CaN,
CaM kinase I, or
CaM kinase IV driven
ANF reporter activity 4.9-, 2.9-, and 6.9-fold below their maximal values, respectively. Myocyte exposure to
doxycycline (DOX) blocked tTA-driven
CaM KII expression and restored CaN/CaM KI or CaN/CaM KIV driven reporter activation. This study demonstrates, for the first time, that active
CaM KII silences Ca(2+)-sensitive nuclear signaling cascades for transcriptional up-regulation of cardiomyocyte
hypertrophy.