Figure 1. Design of injectable BRET probe regulated by HIF-1α-specific UPS.

(a) A schematic diagram indicating the regulation of PTD-ODD-Luciferase-NIRF dye (POL-N) imaging probe in cells with (+) or without (−) HIF activity. CTZ, luciferase substrate coelenterazine. (b) Regulation of POL protein in cancer cells. SUIT-2 cells treated with POL were incubated for 30 min in normoxia (21% O₂) or hypoxia (1% O₂), or normoxia with proteasome-inhibitor MG132 (MG). Then protein levels of HIF-1α and POL were examined by western blotting with anti-HIF-1α and anti-Renilla luciferase antibodies, respectively. (c) SUIT-2 cells treated with POL (WT) or POmL (MT) are incubated for 30 min in normoxia (21% O₂) or hypoxia (1% O₂). Then POL or POmL was detected by western blotting. The ODD domain of POmL does not contain the proline residue, whose hydroxylation is required for VHL-recognition.