Existing evidence suggests that oxidative insults and
antioxidant defense mechanisms play a critical role in the host cell response during
infection of endothelial cells by Rickettsia rickettsii, the causative agent of
Rocky Mountain spotted fever.
Heme oxygenase (HO), a rate-limiting
enzyme in the pathway for
heme catabolism, protects against
oxidant damage in a variety of stress situations. Here, we report on the expression of the inducible and constitutive HO
isozymes, HO-1 and HO-2, during R. rickettsii
infection of endothelial cells. Steady-state levels for HO-1
mRNA were increased two- to threefold, as early as 4 h postinfection, whereas HO-2
mRNA was not affected. Induction of HO-1
mRNA was dependent on the dose of
infection and occurred in a time-dependent manner, reaching maximal levels at 4 to 7 h. The increase in HO-1
mRNA occurred at the level of trancription as it was blocked by the transcriptional inhibitors,
actinomycin D and
alpha-amanitin. The eukaryotic
protein synthesis inhibitor,
cycloheximide, caused a >50% reduction in the
infection-induced increase in HO-1
mRNA level, suggesting its dependence on de novo
protein synthesis of host cell. The uptake of viable organisms appeared to be necessary, since inactivation of R. rickettsii by heat or
formalin fixation, or incubation of cells with
cytochalasin B to prevent entry resulted in marked inhibition of HO-1 response.
N-Acetyl-L-cysteine, a known
oxidant scavenger, inhibited the HO-1 induction by R. rickettsii. Finally, Western analysis with a specific
monoclonal antibody revealed higher levels of HO-1
protein ( approximately 32 kDa), confirming that changes in HO-1
mRNA levels were followed by increases in the levels of
protein. The findings indicate that R. rickettsii
infection induces HO-1 expression in host endothelial cells and suggest an important role for this
enzyme in cellular response to
infection, possibly by serving a protective function against oxidative injury.