Discordant mRNA and protein expression of CXCR4 under in vitro CoCl₂-induced hypoxic conditions

Cobalt chloride (CoCl₂) has long been accepted as a suitable in vitro hypoxia-mimetic agent. The gene CXCR4, which encodes a chemokine receptor, plays a key role in hypoxic retinal disease. Here, we investigated the mRNA and protein expression of CXCR4 in WERI-Rb1 retinoblastoma cells and human umbilical vein endothelial cells (HUVECs) under CoCl₂-induced hypoxic conditions, by means of real-time PCR and western blot. We found that CoCl₂-induced hypoxia profoundly increased CXCR4 expression at the mRNA level, but not at the protein level, at 12, 24, 48 and 72 h in these cells. Interestingly, this result differed from observations of 1% O₂ hypoxic conditions. Additionally, luciferase assays demonstrated that CoCl₂-induced hypoxia significantly increased transcription at the CXCR4 promoter. In order to compare our in vitro findings with the effects of hypoxia in vivo, an OIR (Oxygen-induced retinopathy) rat model was constructed. However, both CXCR4 mRNA and protein levels in OIR rats were significantly increased compared to controls. Thus taken together, our findings suggest that the relationship between CXCR4 mRNA and protein expression is not strictly linear under in vitro CoCl₂-induced hypoxic conditions. through comparative in vitro and in vivo experiments, this study implies that CoCl₂ is an imperfect simulation of hypoxia in retinal disease.