The MKK6-p38 MAPK pathway prolongs the cardiac contractile calcium transient, downregulates SERCA2, and activates NF-AT

Objective: Our goal was to determine if the MKK6-p38 MAPK pathway regulates cardiac intracellular calcium ([Ca²⁺]_i). We also tested if MKK6 might influence expression of SERCA2, a calcium regulatory molecule involved in relaxation, and the activity of nuclear factor of activated T-cells (NF-AT), a calcium-regulated transcription factor that participates in pathological responses to pressure-overload. Methods: Neonatal rat ventricular myocytes were transfected with MKK6(Glu), an activator of p38 MAPK. Green fluorescent protein (GFP) was used as transfection marker and [Ca²⁺]_i was evaluated via indo-1. SERCA2 expression was assayed via Northern and Western techniques. The activity of the rat SERCA2 gene promoter and NF-AT-dependent gene expression were monitored with reporter genes. Myocyte contractility was regulated by electrical pacing. Results: MKK6(Glu) prolonged decay of the contractile calcium transients, downregulated SERCA2 expression, and reduced the activity of the rat SERCA2 gene promoter. Diastolic [Ca²⁺]_i in myocytes pacing at 1-2 Hz was dramatically increased by MKK6(Glu). NF-AT-dependent gene expression was activated by MKK6(Glu) and by pacing of contractions in a synergistic manner. Overexpression of SERCA2 mitigated the effects of MKK6(Glu) on [Ca²⁺]_i and NF-AT. Conclusions: The MKK6(Glu)-p38 MAPK pathway prolongs the decay phase of the cardiac contractile calcium by downregulating SERCA2, increasing diastolic [Ca²⁺]_i which activates NF-AT. The ability of SERCA2 over-expression to reduce NF-AT activity represents a potential novel therapeutic effect of SERCA2 that should be further considered in the development of cardiac gene therapy strategies.