1Universitätsklinikum Münster Institut für Pharmakologie und Toxikologie Münster, Deutschland
Question
Expression of PR72, a regulatory B’’ subunit of protein phosphatase 2A (PP2A), was found to be upregulated in failing human hearts. This observation is consistent with findings from both our and other research groups. Recently, our group had reported a hypercontractile phenotype that was associated with mild cardiac hypertrophy in PR72-overexpressing mice (TG) (Herting et al., Front. Cardiovasc. Med., 2023). Thus, upregulation of PR72 may represent an attempt of failing human hearts to compensate functionally. To further test this hypothesis, we studied the structural and functional effects of PR72 overexpression in a heart failure model of chronic β-adrenergic stimulation.
Methods
Mice with heart-directed PR72 overexpression (TG) and wild-type littermates (WT) were treated with isoproterenol (ISO) for seven days (30 mg/kg body weight × day). As a vehicle control, mice were also treated with NaCl. We measured relative heart weights, cardiomyocyte diameters, cardiac fibrosis, and mRNA levels of hypertrophic marker proteins. Ca2+ transients and sarcomere length (SL) shortening were measured in isolated ventricular myocytes, electrically stimulated at 0.5 Hz.
Results
Chronic ISO administration increased relative heart weights, cardiac fibrosis and mRNA levels of marker proteins (ACTA1, COL1A1, COL1A3, NPPA, MYH7) in TG and WT mice vs. vehicle controls (n=6-8, resp., P<0.05), while diameters of cardiomyocytes were not affected. Remarkably, TG hearts were 12 % heavier than WT under long-term ISO stimulation (5.38±0.27 vs. 4.8±0.16 mg/g, resp., n=8, P<0.05). This was accompanied by a 65 % higher extent of cardiac fibrosis (4.33±1.25 vs. 2.63±0.7 %, resp., n=6-8, P<0.05) and by almost 70 % increased mRNA levels of ACTA1 in TG vs. WT hearts (resp., n=8, P<0.05). Only in TG mice, chronic ISO-stimulation resulted in increased relative lung weights (TG ISO vs. TG NaCl: 6.54±0.73 vs. 5.01±0.66 mg/g, resp., P<0.05).
Chronic ISO vs. NaCl administration led only in TG cardiomyocytes to reduced SL shortening (TG ISO vs. TG NaCl: 68.6±47 vs. 100±58.4 %, resp., n=60 cells/6 hearts, P<0.05), but unchanged Ca2+ transient peak amplitudes, suggesting a reduced myofilamental Ca2+ sensitivity. Ca2+ decay was prolonged in ISO-treated TG vs. WT cells (resp., n=60/6, P<0.05) and SL re-lengthening was prolonged only in ISO- vs. NaCl-treated TG cells (TG ISO vs. TG NaCl: 148.2±89.1 vs. 100±55.4 %, resp., n=60/6, P<0.05).
Under subsequent acute β-adrenergic stimulation, long-term ISO-treated cells of both genotypes showed reduced SL shortening vs. NaCl controls (resp., n=60/6, P<0.05). Ca2+ decay was decelerated in ISO- vs. NaCl-treated cells with a prolonged Ca2+ decay in ISO-treated TG vs. WT cells (resp., n=60/6, P<0.05). SL re-lengthening remained prolonged only in ISO- vs. NaCl-treated TG cells (resp., n=60/6, P<0.05).
Conclusion
In summary, chronic β-adrenergic stimulation resulted in a more pronounced deteriorated cardiac phenotype and an attenuated myocyte contractility in TG mice. Hence, PR72 overexpression might intensify cardiac decompensation, induced by chronic β-adrenergic stimulation. Consistently, subsequent acute application of catecholamines led to a more impaired Ca2+ decay in TG vs. WT cardiomyocytes, indicating β-adrenergic desensitization under PR72 overexpression.