MOLECULAR-CLONING OF THE ONCOFETAL ISOFORM OF THE HUMAN PANCREATIC BILE SALT-DEPENDENT LIPASE

Specific transcripts for bile salt-dependent lipase (BSDL), a 100-kDa glycoprotein secreted by the human pancreas, were immunodetected in Bx PC-3 and SOJ-6 pancreatic tumoral cell lines. Sequencing of fragments, obtained by mRNA reverse transcription and amplification, confirmed t he presence of BSDL transcripts in these cancer cells. The protein was detected in lysates of pancreatic tumoral cells, where it was mainly associated with membranes. Only a minute amount of the enzyme was dete cted in the culture media. Immunofluorescence studies demonstrated tha t in SOJ-6 cells, BSDL colocates with the p58 Golgi protein and sugges ted that the protein may be sequestrated within the Golgi compartment, These results demonstrated that BSDL is expressed in human pancreatic tumoral cells and cannot be secreted (or for the least very poorly). Subsequently, a cDNA covering the entire sequence of BSDL was obtained by reverse transcription-polymerase chain reaction. The sequence of t his cDNA indicated that the N-terminal domain encoded by exons 1-10 wa s identical to that of BSDL expressed by the human normal pancreas, Ho wever, the sequence corresponding to exon 11, which should code for th e 16 tandem-repeated identical mucin-like sequences of BSDL, was delet ed by 330 base pairs (bp) and encoded only 6 of these repeated sequenc es. We conclude that this truncated variant of BSDL would be its oncof etal form, referred to as feto-acinar pancreatic protein. We then inve stigated whether the deletion of 330 bp affected the secretion of the protein. For this purpose, the cDNA corresponding to the mature form o f the BSDL variant expressed in SOJ-6 cells was cloned into an express ion/secretion vector and transfected into CHO-K1 cells. Results indica ted that the variant of BSDL isolated from SOJ-6 cells was expressed a nd secreted by transfected cells. However, the level of BSDL secreted by these transfected CHO-K1 cells was significantly higher than that o bserved for SOJ-6 cells. Consequently, the retention of the oncofetal variant of BSDL observed in human pancreatic tumoral cells might not r esult from inherent properties of the protein.