Figure 4A Immunoblot analysis of the serine/threonine phosphorylation (pS/T) status of HIF2α in GSCs in response to 5% oxygen. GSCs were cultured under 20% or 5% oxygen for 24 hours, and then harvested for immunoprecipitation with anti-HIF2α antibodies or IgG control. Immunoprecipitated endogenous HIF2α were immunoblotted with anti-pan-pS/T antibodies. An increased pS/T of HIF2α was detected in GSCs cultured under 5% oxygen relative to 20% oxygen. B Immunoblot analysis of the serine/threonine phosphorylation (pS/T) status of HIF2α in GSCs after inhibition of ERK1/2 activity. GSCs were cultured under 20% or 5% oxygen for 24 hours, then treated with the MEK inhibitor U0126 (10µM) for 30 minutes to inhibit ERK1/2 activity, and harvested for immunoprecipitation with anti-HIF2α antibodies. Immunoprecipitated endogenous HIF2α were immunoblotted with anti-pan-pS/T antibodies. In comparison with 20% oxygen treatment, the treatment with 5% oxygen elevated pS/T of HIF2α in GSCs, but the increased pS/T of HIF2α was abolished by U0126 treatment. C, Co-immunoprecipitation to detect the interaction between endogenous HIF2α and USP33 in GSCs after inhibition of ERK1/2 activity. GSCs were cultured under 20% or 5% oxygen for 24 hours, then treated with the MEK inhibitor U0126 (10µM) for 30 minutes to inhibit ERK1/2 activity, and harvested for immunoprecipitation with anti-HIF2α antibodies. Co-immunoprecipitated products were immunoblotted with the indicated antibodies. Relative to 20% oxygen treatment, 5% oxygen treatment promoted the interaction between HIF2α and USP33 in GSCs. However, inhibition of ERK1/2 activity by U0126 treatment suppressed the interaction between HIF2α and USP33. D Co-immunoprecipitation to detect the interaction between endogenous HIF2α and USP33 in GSCs after inhibition of ERK1/2 activity. GSCs were cultured under 20% or 5% oxygen for 24 hours, then treated with the MEK inhibitor U0126 (10µM) for 30 minutes to inhibit ERK1/2 activity, and harvested for immunoprecipitation with anti-USP33 antibodies. Co-immunoprecipitated products were immunoblotted with the indicated antibodies. Relative to 20% oxygen treatment, 5% oxygen treatment promoted the interaction between HIF2α and USP33 in GSCs. However, inhibition of ERK1/2 activity by U0126 treatment suppressed the interaction between HIF2α and USP33. E Immunoblot analysis of the poly-ubiquitination of endogenous HIF2α proteins in GSCs after inhibition of ERK1/2 activity. GSCs were cultured under 20% or 5% oxygen for 24 hours before harvest. MG132 (20µM) was added to cell culture 6 hours before harvest, and U0126 (10µM) was added to cell culture 30 minutes before harvest. Cell lysates were subjected to immunoprecipitation with anti-HIF2α antibodies and immunoblot with anti-Ub antibodies. Whereas 5% oxygen suppressed the poly-ubiquitination of HIF2α, inhibition of ERK1/2 activity by U0126 restored the poly-ubiquitination of HIF2α in GSCs. F, Co-immunoprecipitation to determine the serine phosphorylation sites in human HIF2α responsible for its binding to USP33. Ectopic GFP-tagged USP33 along with HA-tagged wild type HIF2α or HIF2α mutants with indicated serine to alanine point mutations were expressed in 293T cells. Cell lysates were subjected to immunoprecipitation with anti-USP33 antibodies Co-immunoprecipitated products were immunoblotted with the indicated antibodies. The S484A mutation largely attenuated the interaction of HIF2α with USP33, indicating that the serine 484 site in HIF2α was the most important phosphorylation site for the interaction between HIF2α and USP33. G Co-immunoprecipitation to determine the serine phosphorylation sites in human HIF2α responsible for its binding to USP33. Ectopic GFP-tagged USP33 along with HA-tagged wild type HIF2α or HIF2α mutants with indicated serine to alanine point mutations were expressed in 293T cells. Cell lysates were subjected to immunoprecipitation with anti-HA agaroses Co-immunoprecipitated products were immunoblotted with the indicated antibodies. The S484A mutation largely attenuated the interaction of HIF2α with USP33, indicating that the serine 484 site in HIF2α was the most important phosphorylation site for the interaction between HIF2α and USP33.