Post-transcriptional regulation of the human liver/bone/kidney alkaline phosphatase gene

Osteoblasts express high levels of liver/bone/kidney alkaline phosphatase (LBK AP), an enzyme critical for bone formation. Other tissues and cell types generally express much lower levels of LBK AP and correspondingly lower levels of mRNA. In light of our early observations that the human LBK AP promoter is expressed equally when transfected into a variety of different cells, we have carried out a detailed study of LBK AP gene expression in Saos-2 cells which are osteoblast-derived and express high levels of LBK AP mRNA, and in HepG2 hepatoblastoma cells which express LBK AP mRNA at levels which are approximately 1000-fold lower. Our results indicate that both of these cells utilize the same promoter sequences to initiate transcription of their LBK AP genes at roughly the same rates. Moreover, the stability of cytoplasmic LBK AP mRNA is equal in both cell types. The lack of any apparent buildup of unspliced precursor mRNA in the nucleus of HepG2 cells leads us to the conclusion that splicing (and nuclear export) is equivalent. It is therefore likely that differential expression is controlled at a very early step post-transcription, possibly by sequences that destabilize the nascent RNA in HepG2 cells. We reason that these destabilizing sequences are located in the gene's introns because a transfected LBK AP minigene, comprised of the full length cDNA and flanking sequences, is expressed efficiently in both cell types.