tokens: TPO -induced proliferation also was reduced by a specific inhibitor of the mitogen-activated protein kinase ( MAPK ) pathway ( PD098059 ) , which inhibits activation of the MAPK extracellular signal-regulated kinases ( ERK ) ERK1 and ERK2 , and AG490 , an inhibitor of Janus kinase-2 , which completely blocked TPO -induced proliferation .