A haptoglobin-like glycoprotein is produced by implantation-stage rabbit endometrium

A polypeptide of 42 kDa was previously identified in rabbit uterine epithelium during the peri-implantation period as a progesterone-dependent, stage-specific protein. Binding of the lectin RCA-I to the 42-kDa band on Western blots demonstrated that it was a glycoprotein, here designated GP42. With use of a polyclonal antiserum to this glycoprotein, strong immunostaining was present on the surface of epithelial cells in implantation-stage uteri (6-7 days pregnant). Uteri of 4-day pseudopregnant females had only trace reactivity, and estrous uteri were devoid of immunostaining. Comparison of GP42 staining on immunoblots of uterine luminal samples, obtained using buffer with or without the detergent Triton X-100, demonstrated that GP42 is either loosely associated with the epithelial surface or is a secretory product. The N-terminal sequence of GP42 was identical through 13 amino acids with the beta subunit of haptoglobin, an acute-phase protein secreted by the liver. Additional immunoblot analyses were carried out after one- or two-dimensional PAGE separation of polypeptides of rabbit uterine samples and human haptoglobin. These employed anti-GP42 as well as antibodies directed against haptoglobin, and results confirmed the similarity of GP42 with beta-haptoglobin. In nonreducing gels, reactivity with anti-GP42 was present in a band of 110 kDa. This is comparable to the molecular size of serum haptoglobin, which occurs as a tetramer of two alpha (15-kDa) and two beta (38-42-kDa) chains. Cultured epithelial cells, derived from 4-day pseudopregnant uteri, released GP42 into the medium, but stromal cells appeared not to produce the glycoprotein. We conclude that GP42 is a uterine glycoprotein, related or identical to haptoglobin, and produced by rabbit uterine epithelial cells during the peri-implantation period. Possible roles for GP42 in relation to ovo-implantation are discussed in light of known functions for haptoglobin and haptoglobin-related protein.