Distribution of constitutive (HO-2) and heat-inducible (HO-1) heme oxygenase isozymes in rat testes: HO-2 displays stage-specific expression in germ cells

The heme oxygenase isozymes, HO-1 and HO-2, oxidatively cleave the heme molecule to produce antioxidants, the bile pigments, the gaseous cellular messenger, CO, and iron, a regulator of transferrin, ferritin, and nitric oxide synthase gene expression. HO-1 (hsp32) is a stress-inducible enzyme, whereas HO-2 is constitutively expressed at high levels in the testes and brain. In the present study, using immunohistochemical and in situ hybridization techniques, we report for the first time the cellular distribution of HO-1 and HO-2 in the testes of normal and heat-shocked rats and define a cell-specific expression of the isozymes and a stage-specific expression of HO-2 in the organ. In normal tissue, HO-1 was present at low levels in the Sertoli cells and could not be detected in germ or Leydig cells. HO-2, on the other hand, was most prominently expressed in residual bodies and was not detected in spermatogonia. Modest levels of HO-2 were observed in spermatocytes, spermatids, and select Leydig cells. In contrast, prominent expression of HO-2 messenger RNAs (mRNAs) was detected by in situ hybridization in spermatogonia, as well as spermatocytes, spermatids, and residual bodies of the seminiferous epithelium. The expression pattern of HO-2 protein and transcript in testes of heat-stressed (42 C; 20 min) rats did not differ from that in the control animals, whereas the expression pattern of HO-1 differed from that in the controls, in which distinct populations of Leydig and Sertoli cells displayed intense immunoreactivity. Thermal stress also resulted in an increase (2.8-fold) in the testicular HO-1 mRNA level within 1 h after treatment, followed by a significant increase (32%) in total microsomal heme oxygenase activity 6 h after treatment. Notably, this increase followed a significant depression (36%) in enzyme activity, which was detected 1 h after hyperthermia. The disparity between HO-2 mRNA and protein distribution clearly indicates cell-specific differences in the translational efficiency of HO-2 transcripts. It appears that HO-2 mRNA translation is linked to the maturation and expression of a factor(s) that regulates this process. This, in turn, appears to coincide with sperm development. HO-1 activity, on the other hand, which has a transcriptional component to its regulation, may have a role in maintenance of the conditions required for spermatogenesis.