Gravar-mail: Signal Integration and Coincidence Detection in the Mitogen-activated Protein Kinase/Extracellular Signal-regulated Kinase (ERK) Cascade: CONCOMITANT ACTIVATION OF RECEPTOR TYROSINE KINASES AND OF LRP-1 LEADS TO SUSTAINED ERK PHOSPHORYLATION VIA DOWN-REGULATION OF DUAL SPECIFICITY PHOSPHATASES (DUSP1 AND -6)