We have previously shown that heregulin-betal (HRG-beta 1) was involved in the development and survival of cardiomyocytes derived from embryonic stem (ES) cells. This study was conducted to investigate the intracellular signal mechanisms by which HRG-beta 1 stimulates cardiogenesis in ES cells. The treatment with ErbB receptor inhibitor decreased the population of cardiomyocytes and transcripts levels of cardiac genes (Nkx2.5, beta-MHC, cTnI, and MLC2a). The phosphorylation of ERK and development of cardiomyocytes by treatment with HRG-beta 1 was suppressed upon treatment with MEK1 inhibitor. Furthermore, cardiomyocytes and level of MHC protein were significantly increased by overexpression of wild type MEK1 or constitutive active MEK1, but not dominant negative MEK1. These results suggest that HRG-beta 1 promotes the development of cardiomyocytes predominantly by activation of MEK-ERK. (c) 2007 Elsevier Inc. All rights reserved.