Figure 6

Evidence for increased phosphatase activity in late passage ERK1-GFP cells. Panel A: Western blot analysis of P-ERK levels in serum starved (0.1% FBS) vs 10% FBS stimulated cells. Treatment of cells with 10% FBS for 5 min results in increased P-ERK levels in vector control and early passage ERK1-GFP cells, as expected, while late passage ERK1-GFP cells display diminished P-ERK, but not total ERK levels. Panel B: Treatment of late passage ERK1-GFP cells with a broad spectrum phosphatase inhibitor (1 mM Na₃VO₄) for 5 or 60 min resulted in reappearance of P-ERK levels, suggesting that the decrease in P-ERK levels is due to increased phosphatase activity. Similar results were observed in two independent experiments