Objective: To identify specific tyrosine kinases that are involved in endometrial signaling and to study their in vivo expression in normal and abnormal endometrium. We hypothesized that proteins that are differentially expressed would be mor likely to be important in regulated cellular events.

Methods: Complementary DNA libraries, constructed from human secretory (n = 5) and proliferative (n = 5) endomentrial specimens, were screened with a polyclonal anti-phosphotyrosine antibody. Positive clones were sequenced and screened for differential expression using immunoblotting and Northern analysis of samples from proliferative and secretory endometrium. The expression of one identified clone, lyn, a Src family member, was characterized further with Western and Northern blot analyses and immunolocalization.

Results: One protein identified by the above method was lyn, a member of the src family of protein tyrosine kinases, never described in the human endometrium. Western blot analysis revealed two forms of lyn protein p53^lyn and p56^lyn, that were most abundant in the late secretory phase. Immunohistochemistry demonstrated uniform protein expression by all cells in normal glandular epithelium and suggesed a correlation between lyn protein expression and cell differentiation for human endometrial adenocarcinomas, with markedly elevated levels noted in poorly differentiated adenocarcinomas compared with well-differentiated tumors (n = 3). Northern hybridization confirmed the presence of the expected 3.5-kb lyn transcript in normal and abnormal endometrium.

Conclusions: Our data demonstrate that human cDNA libraries created from different phases of the menstrual cycle can be screened successfully using anti-phosphotyrosine antibodies to identify differentially expressed protein tyrosine kinases. Although p53^lyn and p56^lyn expression has been thought of as a predominantly lymphoid-specific tyrosine kinase, we show prominent expression of lyn protein and mRNA by normal and malignant epithelium of the human endometrium, suggesting a role in endometrial signaling and human reproduction.