The 66-kDa form of the EPO-R reportedly represents the Golgi form of the EPO-R and is the predominant one found in other tissues, including the placenta (current study). In immunoblots of placental homogenates, a less intense band of —76-78 kDa was also observed. This protein is presumably the plasma membrane form of the receptor, which has a reported size range of 70-78 kDa. The TF-1 cells, an erythroleukemic cell line known to express high levels of EPO-R, also demonstrated a prominent band of corresponding size, thus serving as our positive control.
A recent investigation has suggested that TF-1 cells also express an abnormal transcript due to a translocation breakpoint in exon VIII, resulting in a truncated form of EPO-R (—46 kDa). Human bone marrow cells express another truncated receptor that lacks most of the cytoplasmic domain coded in exon VIII and that derives from the inclusion of the 95-bp intron VII, resulting in a premature stop codon. Further, the soluble form of EPO-R of —34 kDa lacks both the transmembrane and cytoplasmic domains and derives from the insertion of the terminal 104-bp sequence of intron IV, which contains a stop codon. In this regard, lower molecular mass bands were observed for the TF-1 and Jar trophoblast-derived choriocarcinoma cells on Western blot analysis (Fig. 6). Because the expression of these different forms of EPO-R, as well as of the full-length EPO-R, may vary according to the stage of erythroid differentiation, and potentially regulate the cellular action of EPO, it may be important to determine whether the truncated and soluble forms of EPO-R are expressed by trophoblast cells.