Abstract 726: Expression of an Active LKB1 Complex in Cardiac Myocytes Prevents Protein Synthesis Associated with Phenylephrine-induced Hypertrophy
AMP-activated protein kinase (AMPK) is a major metabolic regulator in the cardiac myocyte. Recently, LKB1 was identified as a kinase that regulates AMPK. Using immunoblot analysis we confirmed high expression of LKB1 in isolated rat cardiac myocytes, but show that under basal conditions LKB1 is primarily localized to the nucleus, where it is inactive. We examined the role of LKB1 in cardiac myocytes, using adenoviruses that express LKB1, and its binding partners, STRADα, and MO25α. Infection of neonatal rat cardiac myocytes with all 3 adenoviruses substantially increased LKB1/STRADα/MO25α expression, LKB1 activity (LKB1/STRAD/MO25 11.5 ± 2.9 vs. control 0.99 ± 0.05, pmol/min/mg p < 0.01) and AMPKα phosphorylation at its activating phosphorylation site (threonine 172). Since activation of AMPK can inhibit hypertrophic growth and since LKB1 is upstream of AMPK, we hypothesized that expression of an active LKB1 complex would also inhibit protein synthesis associated with hypertrophic growth. Expression of the LKB1/STRADα /MO25α complex in neonatal cardiac myocytes significantly inhibited the increase in protein synthesis (measured via 3[H]-phenylalanine incorporation) observed in cells treated with phenylephrine (PE) (control 100, control/PE 167 ± 16 vs. LKB1/PE 98 ± 12 arbitrary units (AU), p < 0.05). This was associated with a significant decrease of phosphorylation of p70S6 kinase (control 100 vs. LKB1 35 ± 8.5 AU, p < 0.05) and its substrate S6 ribosomal protein (control 100 vs. LKB1 53 ± 16 AU, p < 0.063), key regulators of protein synthesis. In addition, we show that the pathological cardiac hypertrophy in transgenic mice with cardiac-specific expression of activated calcineurin (calcineurin-TG) is associated with a significant decrease in LKB1 expression (wildtype 101 ± 11 vs. calcineurin-TG 42 ± 4.9 (AU), p < 0.001). Together, our data show that increased LKB1 activity in the cardiac myocyte can decrease hypertrophy-induced protein synthesis, and suggest that LKB1 activation may be a method for the prevention of pathological cardiac hypertrophy.