Differential expression of heme oxygenase-1 in cultured cortical neurons and astrocytes determined by the aid of a new heme oxygenase antibody. Response to oxidative stress

Permanent focal and transient global cerebral ischemia increase glial and neuronal expression of heme oxygenase-1, but not heme oxygenase-2, protein in rat brain

Two heme oxygenase (HO) proteins have been identified to date; HO-1, a stress-induced protein, and HO-2, a constitutively expressed isoform. Recently, it was demonstrated that HO-1 mRNA expression is increased following transient global ischemia. The present study examined the effects of global and focal ischemia on HO-1 and HO-2 protein, using immunocytochemistry. Following 20 min of ischemia (rat 4 vessel occlusion model with hypotension) and 6 h of recirculation, increased HO-1 immunoreactivity was evident in hippocampal neurons. After 24 h of recirculation, HO-1 was observed in both hippocampal neurons and astroglial cells. By 72 h, expression was primarily glial and restricted to CA1 and CA3c. In addition to hippocampus, HO-1 was also evident in both neurons and glia in cerebral cortex and thalamus, and in striatal glial cells. Twenty-four hours following permanent focal ischemia, HO-1 immunoreactivity was observed in astroglial cells in the penumbra region surrounding the infarct. In contrast to HO-1, the pattern of HO-2 immunoreactivity was not altered following transient global or permanent focal ischemia. The increased expression of HO-1 following ischemia may confer protection against oxidative stress, but might also contribute to the subsequent neuronal degeneration.

Transient induction of heme oxygenase after cortical stab wound injury

Heme oxygenase (HO) exists as two isoenzymes designated heme oxygenase-1 (HO-1) and heme oxygenase-2 (HO-2). HO-1 has been identified as a heat shock or stress protein and is inducible whereas HO-2 is largely refractory to induction. HO-2 is the predominant isoenzyme in normal brain and appears to have a predominantly neuronal distribution in cerebral cortex. Cortical stab wound injury resulted in HO-1 induction as determined by Western blot analysis. Immunohistochemical analysis suggested that induced HO-1 was largely restricted to reactive astrocytes and macrophage-like cells. Enhanced HO-1 immunoreactivity was observed in hypertrophied, GFAP+ reactive astrocytes near the wound margin as early as 12 h after injury. Very rarely were HO-1+ neurons observed and then only up to 6 h after stabbing. Maximal numbers of HO-1+ astrocytes were found 3 days after stabbing. Their numbers declined thereafter. By 5 days after stab injury few HO-1+ reactive astrocytes were observed although GFAP+ reactive astrocytes were still prominent near the wound margin. HO-1+ macrophage-like cells were initially observed between 1 and 3 days after injury and they persisted in the margin of the wound for at least 14 days. The proximity of HO-1+ cells to the wound margin suggests that factors associated with injury contribute to the regulation of HO-1 in injured cortex.

Evidence for different mechanisms of induction of HSP70i: a comparison of cultured rat cortical neurons with astrocytes

This study is a follow-up of previous work which demonstrated that cultured cortical neurons did not synthesize HSP70i immediately after heat stress when compared with cultured cortical astrocytes. We have extended the period of observation for HSP70i induction of cultured cortical neurons and astrocytes up to 24 h after heat stress. Cultured rat cortical neurons derived from 16-day-old fetal rats respond differently to heat stress than cultured rat astrocytes derived from newborn rats. They showed a delayed HSP70i induction in the majority of cultured neurons and the response was heterogeneous and was absent in most smaller neurons. The delayed neuronal induction was accompanied by a prolonged activation of heat-shock transcription factor 1 (HSF-1) and prolonged transcription of HSP70i mRNA. In comparison astrocytes showed a marked early induction of HSP70i mRNA and protein. In addition the induction of HSP70i in astrocytes was followed by translocation of the protein into the nucleus, a finding which we failed to demonstrate in neurons. Immunostaining for HSP70i was more uniform in astrocytes than neurons. Many neurons did not stain for up to 24 h after heat shock in this study. Immunocytochemical staining of HSF-1 and 2 showed major differences between neurons and astrocytes. Astrocytes showed localization of HSF-1 to the nucleus before and after heat stress, while neurons showed HSF-1 localization to the cytoplasm and nucleus before and after heat stress. Finally HSF-2 was undetectable in neurons when compared with astrocytes by Western immunoblot analysis. However, astrocytes and neurons revealed weak immunostaining of HSF-2 in the cytoplasm and nucleus. The staining in the neurons was likely secondary to cross-reactivity to an unidentified protein. We conclude that HSP70i expression after heat shock is delayed in rat cortical neurons when compared with rat cortical astrocytes. In addition most small neurons did not synthesize HSP70i after heat shock. This difference in induction of HSP70i may be secondary to localization and activation of HSF-1 but not HSF-2. Neuronal susceptibility to injury may be related to the delayed induction of HSP70i and also the possible failure of newly synthesized HSP70i to translocate into the nucleus.

Localization of heme oxygenase in rat retina: effect of light adaptation

Heme oxygenase-2 isozyme is the predominant form of heme oxygenase in rat brain by western blot analysis. Heme oxygenase-1 isozyme is not induced by light adaptation in rat retina by western blot analysis. Immunocytochemistry localizes heme oxygenase-2 in three areas of the retina: the retinal pigment epithelium, inner segment and external nuclear layers of the rat retina. Ganglion cells and cell bodies of the internal nuclear layer of the retina and Müller cells were largely unstained for heme oxygenase-2. The localization of heme oxygenase-2 in the retina implies that its function is not associated with phototransduction. Also, light adaptation does not appear to induce heme oxygenase-1, a measure of oxidative injury.

Pharmacological induction of heat shock protein 68 synthesis in cultured rat astrocytes

The induction of the highly inducible 70-kDa heat shock protein (HSP 70) is associated with thermotolerance and survival from many other types of stress. This investigation studied the pharmacological induction of HSP 68 (HSP 68 is the rat homolog of human HSP 70) by 1,10-phenanthroline in cultured rat astrocytes under conditions that activated heat shock transcription factor-1 without inducing HSP 68 synthesis. Two conditions that activate heat shock transcription factor-1 and promote its binding to the heat shock element without subsequent transcription of HSP 68 mRNA, intracellular acidosis and exposure to salicylate, showed synthesis of HSP 68 when 1,10-phenanthroline was added to culture medium after the activation of heat shock transcription factor-1. 1,10-phenanthroline mimicked heat shock by inducing HSP 68 mRNA and protein under both conditions. 1,10-phenanthroline added alone to culture medium did not induce the synthesis of HSP 68 or activate heat shock transcription factor-1. These findings strongly suggest a multistep activation for HSP 68 synthesis and also demonstrate that the synthesis of HSP 68 can be pharmacologically regulated.