Cardiac cell fate control by the imidazoline I1 receptor/nischarin : application in cardiac pathology

Moxonidine, an antihypertensive sympatholytic imidazoline compound, reduces blood pressure by selective activation of non-adrenergic imidazoline I1-receptors (also known as nischarin) in brainstem medulla. Moxonidine prevents left ventricular hypertrophy development in hypertensive rats, associated with reduced cardiac DNA synthesis and early transient increase in DNA fragmentation. It is likely that the anti-proliferative and apoptotic effects occur in fibroblasts, as cardiomyocyte apoptosis may deteriorate cardiac function. The effects also occurred to sub-hypotensive doses, suggesting a blood-pressure-independent mechanism and pointing to a local cardiac action.  Imidazoline I1-receptors have been identified in cardiac tissues, and their ex vivo activation by moxonidine stimulates ANP release, demonstrating that cardiac imidazoline I1-receptors are functional without the contribution of the central nervous system.   Based on the above studies and on i) the role of natriuretic peptides in inhibition of myocardial cell apoptosis and ii) studies linking imidazoline I1-receptors to the maintenance of the extracellular matrix and PC12 cell survival, we propose that apart from centrally-mediated sympatholytic function, imidazoline I1-receptors in the heart may control cell growth and death. Activation of imidazoline receptors may delay the progression of cardiac pathologies into heart failure by inhibition of maladaptive proliferative signalling and downstream apoptotic pathways.   In order to test this hypothesis studies were performed to: 1. Explore the in vivo effects of moxonidine on cardiac structure and function in SHR and cardiomyopathic hamsters.  2. Define the pathways involved in the observed changes following moxonidine treatment, specifically, on inflammatory markers and pathways involved in LVH and cardiac cell survival/death (MAPK and Akt). 3. Explore in vitro the effect of imidazoline I1-receptor activation by moxonidine, on cell survival by over-expressing nischarin in HEK293 cells, to circumvent the lack of specific imidazoline I1-receptor agonists and antagonists. 4. Investigate in vitro, imidazoline I1-receptor localization (cardiomyocytes and fibroblasts), regulation and implication in cell growth/death in response to cardiac remodelling-associated stimuli: norepinephrine, cytokines (IL-1β, TNF-α), and oxidants (H2O2). The studies reveal that hypotensive and sub-hypotensive concentrations of moxonidine improve cardiac structure and performance in SHR by mechanisms that involve inhibition of cytokines, p38MAPK, and Akt signalling pathways. In cardiomyopathic hamsters moxonidine improves cardiac performance, in association with differential inflammatory/anti-inflammatory responses that culminate in attenuated cardiomyocyte death and fibrosis and altered collagen type expression. HEK293 cells, transfected with nischarin cDNA, show increased viability/proliferation in response to moxonidine. The overall survival response is associated with moxonidine’s inhibition of ERK, JNK, and p38MAPK. Nischarin also opposes the reduced cell viability in response to oxidative stimuli (TNF-α, IL-1β and H2O2), with differential responses to moxonidine.  Furthermore, the imidazoline I1-receptor is expressed in cardiac fibroblasts and myocytes and its activation inhibits norepinephrine-induced cardiomyocyte death and fibroblast proliferation, through differential effects on MAPKs and Akt. Moxonidine/imidazoline I1-receptor protects cardiomyocytes and facilitates elimination of myofibroblasts in inflammatory conditions, through opposite effects on JNK, p38MAPK and iNOS activity.  These studies emphasize the potential importance of imidazoline I1-receptor/nischarin as an anti-hypertrophic and anti-fibrotic target. Identification of the cardio-protective mechanisms of cardiac nischarin could result in specifically-tailored cell/gene-driven nischarin treatments, which could be important for patients with heart disease. Also, while the antihypertensive action of centrally acting compounds is appreciated, new cardiac-selective I1-receptor agonists may confer additional benefit.

Keyphrases: Moxonidine, Hypertension, Cardiomyocyte, HEK293, Cytokines, Imidazoline I1-receptor, Left ventricular hypertrophy, Heart failure, Fibroblast, Norepinephrine