Background: After intracerebral hemorrhage (ICH), the red blood cells (RBC) and their hemolytic products within brain hematoma trigger adverse biochemical events, leading to secondary brain injury and neurological deficits. Thus, efficient removal of hematoma components is essential for achieving inflammation resolution and functional recovery. The inducible heme-oxygenase (HO-1) is a key rate-limiting enzyme that catabolizes heme into iron, CO, and biliverdin. The present study investigated the role of HO-1 in microglia/macrophages (MΦ)-mediated phagocytosis of RBC; and also assessed the spatial and temporal expression of HO-1 in ICH-affected brain, as well as its possible role in the clearance of hematoma components following ICH modeled in rodents.
Methods and Results: First, we employed the rat brain MΦ. Upon exposing to RBC, MΦ phagocytize RBC; and HO-1 was induced during this process. Co-incubating tin-protoporphyrin IX (SnPP, a competitive HO-1 inhibitor) with RBC significantly delayed RBC internalization by MΦ. Removal of SnPP from the culture medium led to a rapid recovery of MΦ’s phagocytic function, suggesting that SnPP-induced inhibition is a reversible process. Subjecting neuron-microglia co-cultures to RBC plus sublethal dose of oxygen-deprivation (an ICH-like insult) triggered neuronal injury, as assessed using neurofilament degradation assay and loss of NeuN-positive cells; and addition of SnPP further aggravated the neuronal injury. Additional studies showed that after ICH, HO-1 is up-regulated in hematoma-affected rat brain tissues starting from 6h, reaching the maximum level at 3-7days, and persisting for at least 10 days after ICH. Double immunohistochemistry of HO-1 and brain cell markers shows that the most HO-1-positive cells are Iba1-positive MΦ. Administration of SnPP for 7 days, (7.5 mg/kg, ip, twice a day) delayed hematoma clearance by 27.8% and significantly impaired the functional recovery, as measured 7 days after ICH. Histological analyses showed that there are more TUNEL-positive neurons in the hematoma-affected brain tissue in SnPP-treated mouse brains.
Conclusion: Our study suggests that HO-1 is essential for phagocytosis of RBC by MΦ, which is critical for endogenous clearance of hematoma after ICH.