Specific expression of haptoglobin mRNA in implantation-stage rabbit uterine epithelium

A glycoprotein, termed GP42, was previously identified in uterine fluid obtained from peri-implantation-stage rabbits. N-terminus amino acid sequencing of purified GP42 demonstrated identity through the first 13 amino acids with the beta subunit of liver haptoglobin. The present study was undertaken to determine if GP42 is indeed identical to haptoglobin and, if so, to determine whether it is expressed in the uterus as opposed to being present as a transudate from plasma. Reverse transcription-PCR amplification of poly(A)+ RNA prepared from implantation-stage rabbit endometrium with GP42- and haptoglobin-specific primers yielded a predicted 667 bp cDNA product. Sequence analysis of the cloned cDNA confirmed the identity of GP42 with beta-haptoglobin. Northern blot analysis demonstrated the specific expression of haptoglobin mRNA in the peri-implantation-stage endometrium and the absence of its expression in the estrous or day 4 pseudopregnant endometrium. Non-isotopic in situ hybridization revealed that the haptoglobin mRNA was restricted to the epithelium lining the luminal surface and mucosal folds of day 6(3/4) pregnant or pseudo-pregnant uteri and that no haptoglobin mRNA was detectable in the epithelium of the deep glands or cells of the stroma or myometrium. Similarly, in situ hybridization revealed no expression of haptoglobin mRNA in any cell types of the estrous uterus. These data establish the identity of GP42 with beta-haptoglobin and demonstrate that it is expressed in a stage-specific manner just prior to implantation, correlating with uterine receptivity to blastocyst implantation. Endometrial GP42 mRNA expression is not dependent on the presence of blastocysts.