The Promoter of the Long Variant of Collagen XVIII, the Precursor of Endostatin, Contains Liver-specific Regulatory Elements

The endostatin precursor collagen XVIII is expressed at high levels in human livers, the main source being hepatocytes. We have studied the regulatory elements in the promoter 2 of the Col18a1 gene that directs the transcription of the NC1-517 variant of collagen alpha1(XVIII), which is the main form expressed in the liver. The 5'-flanking region of Col18a1 gene was cloned, and a series of 5'-deletions from -3286 bp to +285 bp were linked to the luciferase reporter gene. Transfection experiments in HepG2 cells allowed to identify a silencer-like element containing putative HNF1 and HNF3 sites and activator elements containing stretches of GC-rich sequences. Another putative HNF3 site in close apposition to a NF1/CTF site was localized upstream of the silencer-like element. Cotransfection experiments showed that the Col18a1 promoter 2 was transactivated by Sp1 and HNF3alpha. Gel-shift analyses showed that HNF3, NF1/CTF, and Sp1-like sites specifically recognized nuclear factors. Super-shift experiments indicated that HNF3beta was the major form of HNF3 interacting with the HNF3/NF1 site. The well-differentiated hepatoma cell line mhATFS315 transfected with a truncated form of HNF3beta, which competitively blocks HNF3 transactivating activity, expressed the Col18a1gene at a very low level. Taken together, these data strongly suggest that Col18a1 is a liver-specific gene. Furthermore, gel-shift analyses performed with nuclear factors prepared from well-differentiated hepatocellular carcinomas showed increased HNF3/NF1 binding activity compared with normal livers. Consequently, the precursor of endostatin might be differently expressed according to the differentiated and/or transformed state of hepatocytes.