Regulation in vitro of an L-CAM enhancer by homeobox genes HoxD9 and HNF-1.

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Previous studies have shown that in vitro expression of the neural cell adhesion molecule (N-CAM) can be regulated by the products of homeobox genes HoxB9, -B8, and -C6. N-CAM is a Ca(2+)-independent immunoglobulin-related CAM that plays an important role in neural development. In the present study, we investigated whether the liver cell adhesion molecule (L-CAM) a member of the Ca(2+)-dependent CAM family (cadherins) is also regulated by homeobox-containing genes. In transient cotransfection experiments of NIH 3T3 cells, we observed that both HoxD9 and liver-enriched POU-homeodomain transcription factor, HNF-1, activated chloramphenicol acetyltransferase gene reporter constructs containing the L-CAM promoter and an enhancer present in the second intron of the chicken L-CAM gene. Using electrophoretic mobility-shift assays, we found that components of cell extracts from NIH 3T3 cells transfected with HoxD9 bound to a small region of the L-CAM enhancer having a consensus sequence that is a putative binding site for HNF-1. Components of extracts from the chicken hepatoma cell line LMH that had been transfected with an HNF-1 expression vector also bound to this same site. In nuclear run-on experiments with nuclei from LMH cells that were transfected with expression vectors for HoxD9 or HNF-1, L-CAM RNA levels were increased 33-fold and 4-fold respectively. Using the same run-on procedure, it was confirmed that nuclei prepared from normal embryonic chicken liver cells expressed the RNAs for HoxD9, HNF-1, and L-CAM. Taken together with previous observations, these data raise the possibility that homeobox-containing genes will have a widespread role in the place-dependent expression of CAMs belonging both to immunoglobulin-related and to cadherin families.

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