Transcriptional regulation of the KEL gene and Kell protein expression in erythroid and non-erythroid cells

The Kell blood-group antigen was originally reported to be a protein expressed in erythroid tissue only. Transcriptional analysis of the KEL promoter activity in human erythroleukaemia K562 and epithelial HeLa cells by electrophoretic mobility-shift and supershift assays, chloramphenicol acetyltransferase assays, co-transfection studies and site-directed mutagenesis provided the following results: (i) the KEL promoter exhibits a strong transcriptional activity in K562 cells and, unexpectedly, a basal non-erythroid activity in HeLa cells, (ii) up-regulation of the 5' distal promoter activity occurs only in the erythroid context, and (iii) two motifs localized in the exon 1 region, which bind the Sp1/Sp3 and the human GATA-1/Ku70/80 factors, were required for down-regulation of the promoter activity, but inhibition of the promoter activity by the repressing factors in HeLa cells was incomplete. KEL expression in HeLa cells was performed further by primer-extension analysis, which revealed the presence of a low amount of Kell transcript correlating with basal expression of the Kell protein in these cells, as shown by immunopurification and Western-blot analysis. DNA sequencing of the transcript revealed a sequence identical to that obtained from erythroid tissue. In human tissues, KEL expression was investigated by dot-blot analysis and revealed high levels of Kell mRNAs, particularly in brain tissues, testis and lymphoid tissues. Moreover, most tissues analysed exhibited low levels of Kell transcripts. The Kell protein was also detected by immunohistochemistry in the Sertoli cells of the testis and in lymphoid tissues like spleen and tonsil, specifically localized in the follicular dendritic cells. Altogether, the results indicated that KEL expression is not restricted to erythroid tissue.