MULTIPLE TRANSCRIPTS ENCODING HEME OXYGENASE-2 IN RAT TESTIS - DEVELOPMENTAL AND CELL-SPECIFIC REGULATION OF TRANSCRIPTS AND PROTEIN

We report for the first time that heme oxygenase-2 (HO-2) expression i s regulated by developmental and cell type-specific factors in the tes tis, and we describe the presence of three unique sizes of HO-2 transc ripts in the testis. HO-2, together with HO-1 (HSP32), catalyzes oxida tive cleavage of the heme molecule to biliverdin, carbon monoxide, and iron; HO-2 is the major isozyme of the testis. Northern blot analysis was used to demonstrate the presence of five transcripts for HO-2 in rat testis mRNA; they range from similar to 1.3 to similar to 2.1 kb i n length with a predominant 1.45-kb message; three of the transcripts, similar to 1.45 kb, similar to 1.7 kb, and similar to 2.1 kb, are uni que to testis. The two other transcripts of similar to 1.3 and similar to 1.9 kb are common to every tissue examined, including the testis. Analysis of three distinct cDNAs isolated from rat libraries in phage lambda indicates that all are identical from similar to 37, relative t o translation initiation through the coding region to the first of two poly(A) signals previously identified in the HO-2 gene (McCoubrey and Maines, 1994), Upstream of similar to 37, the 5' untranslated sequenc es of the isolates differ in both length and sequence. Comparison with the genomic sequence suggests that the multiple transcripts arise by splicing of alternative first exons as well as use of alternate poly(A ) signals. Northern hybridization with probes specific for the unique portion of each cDNA are consistent with this interpretation. Further, unlike HO-1, HO-2 messages are developmentally regulated; only simila r to 1.3 and similar to 1.9-kb transcripts were detected, at minute le vels, in the testis RNA of 7-day-old rats. A pronounced increase in to tal message level was observed by Day 28 postparium, although the leve l had not reached the marked amplification seen in the adult testis. F urther the transcript patterns differed when Day 28 and adult testis w ere compared to Day 7 testis. The very predominant similar to 1.45-kb band acid the similar to 1.7 and similar to 2.1-kb bands were absent f rom Day 7 testis, Heme oxygenase activity and HO-2 protein levels, as assessed by Western blot, reflect the increases at the RNA level. Inte restingly, although abundant HO-2 mRNA can be detected by in situ hybr idization in spermatogonia, spermatocytes, and spermatids, HO-2 protei n was detected, by immunocytochemistry, only in spermatids. These obse rvations demonstrate tissue and cell specificity of HO-2 gene expressi on and suggest that in the testis, HO-2 expression is regulated at the transcriptional and translational levels.