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, STRAD, and MO25. Infection of neonatal rat cardiac myocytes with all 3 adenoviruses substantially increased LKB1/STRAD/MO25 expression, LKB1 activity, and AMPK 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/STRAD/MO25 complex in neonatal rat cardiac myocytes inhibited the increase in protein synthesis observed in cells treated with phenylephrine (measured via ³[H]-phenylalanine incorporation). This was associated with a decreased phosphorylation of p70S6 kinase and its substrate S6 ribosomal protein, key regulators of protein synthesis. In addition, we show that the pathological cardiac hypertrophy in transgenic mice with cardiac-specific expression of activated calcineurin is associated with a significant decrease in LKB1 expression. 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.