The differential localization of HO-1 in renal cells under conditions of injury, and the demonstration that exaggerated HO-1 expression can have detrimental rather than beneficial effects, raises the question of whether HO-1 expression in these cells is subject to control. The present study identifies a unique HO-1 expression pattern in the renal glomerulus indicative of presence of HO-1 expression control following prolonged HO-1 induction. HO-1 and HO-2 expression in response to the natural HO substrate/inducer Fe++ protoporphyrin (PP) IX (hemin) was assessed in normal rat glomeruli. Following 18 h incubations with hemin (0-200 mu M), HO-1 expression increased in a concentration-dependent manner and via a hemopexin (HPX) independent mechanism with no effect on HO-2. In incubations with higher hemin concentrations (400 mu M), likely to be encountered in hemolytic disorders, HO-1 expression, decreased. This was preceded by a prolonged and sustained increase in HO-1 protein and was independent of the Fe++ moiety as incubations with Cobalt protoporphyrin (CoPP) resulted in an identical expression pattern. The decrease of HO-1 protein could not be accounted for by proteasomal degradation since it was not reversed in co-incubations with hemin and the proteasome inhibitor, MG132, at concentrations sufficient to increase HO-1 glomerular content when used alone. Moreover, in the presence of MG132, a decrease of HO-1 expression also occurred at 100 and 200 I.EM hemin. The effect of MG132 was mimicked by two additional mechanistically different approaches which also raised HO-1 content: a) coincubations of hemin with ZnPP which increased HO-1 protein when used alone, and b) glomerular j-10-1 over expression achieved by SB transposon mediated transgenesis. In contrast, the decrease in HO1 levels observed at high hemin concentrations was reversed in co-incubations with hemin and SnPP, which reduced HO-1 content when used alone. Expression of NF-E2 related factor 2 (Nrf2) protein, which mediates HO-1 induction in response to hemin, had a similar expression pattern with that of HO-1 protein indicating involvement of Nr12 in the response of HO-1 to hemin. The above observations indicate presence of a HO-1 expression control mechanism in the glomerulus that may serve to protect it against potentially detrimental effects of exaggerated HO-1 expression. (C) 2015 Elsevier Inc. All rights reserved.