passages.id: split_4_test_1170_passage
passages.type: progene_text
passages.text: Transient transfection of a constitutively active Mek1 , a specific upstream activator of ERKs , maintained myoblast viability in the absence of growth factors , while inhibition of Mek1 by the drug UO126 blocked PDGF - mediated but not IGF - stimulated survival .