Abstract 276: Cyclin D2 Is a Critical Mediator of Exercise-Induced Cardiac Hypertrophy
Exercise training activates a number of hypertrophic signaling pathways that can be distinct from those activated by pathologic stimuli. However, there must be some overlap in those pathways underlying myocyte cell growth. Using a number of mouse genetic models, we investigated the role of several molecules implicated in pathologic cardiac hypertrophy for their cardiac responses to exercise. We used three-month-old female transgenic mice expressing the anti-hypertrophic molecules, cardiac-specific constitutively active glycogen synthase kinase-3β (caGSK-3β), an inhibitor of Ca2+-calmodulin-dependent protein kinase (CaMKII Inh), and doubly transgenic mice expressing both caGSK-3β and CaMKII inhibition (caGSK-3β/CaMKII Inh). We also studied the exercise responsiveness of mice expressing the pro-hypertrophic cardiac-specific activated (myr)Akt. MAPK/ERK kinase kinase-1 (MEKK1) has been shown to be essential for pathologic cardiac hypertrophy and we therefore studied the requirement of MEKK1 for exercise-induced cardiac growth. Cell cycle regulators such as cyclin D2 have been shown to be required for pathologic cardiac hypertrophy; therefore we studied cyclin D2 null mice. Mice were divided into sedentary and 21 days of voluntary exercise on a cage wheel. Across the seven different mouse models, exercise capacity was similar with regards to running duration, distance, and speed. Importantly, we analyzed the impact of exercise on cardiac hypertrophy by measuring heart weight-to-body weight (HW/BW) ratios of sedentary and exercised mice. While exercise had no impact on body weight, heart weight increased significantly in all mouse models except the cyclin D2-/- mice. Overall there was a 3.5-fold range of percent increase in HW/BW ratios from the highest (caGSK-3β) to the lowest (cyclin D2-/-). In conclusion, genetic manipulation of these hypertrophic signaling pathways has little impact on exercise performance and only the loss of cyclin D2 attenuates exercise-induced cardiac growth. These data establish cyclin D2 as an important regulator of physiological hypertrophy and underscore the differences in pathologic and physiologic cardiac hypertrophy.
- © 2012 by American Heart Association, Inc.