Immunoblotting Detection of GLUL and EPO

To detect GLUL by immunoblotting, cell pellets were first homogenized in phosphatase inhibitor lysis buffer. Lysed cell pellets were then diluted in LDS sample buffer and heated in reducing agent prior to loading onto the gel for Western blotting as previously described^{29 (link)}. To detect EPO, culture supernatant were similarly diluted and reduced prior to loading and immunoblotting. Blots were incubated with antibodies: anti-EPO (1:1000; MAB2871, R&D systems, MN, USA), anti-GLUL (1:5000; MAB302, Merck Millipore) and anti-actin (Fig. 1b: sc-47778, Santa Cruz, RX, USA; Figs 1f and 4c: 1:6000; A2066, Sigma-Aldrich, MO, USA). Figure 1b was imaged using ChemiDoc (Bio-Rad), while Figs 1f and 4c were imaged on a chemiluminescent CCD camera, ImageQuant LAS 500 (GE, MA, USA). Full length gel images are shown in Supplementary Table and Figures File.