Molecular cloning and characterization of LAPTM4B, a novel gene upregulated in hepatocellular carcinoma

Lysosomal-associated protein transmembrane-4 beta (LAPTM4B), a novel gene upregulated in hepatocellular carcinoma (HCC), was cloned using fluorescence differential display, RACE, and RT-PCR. It contains seven exons and encodes a 35-kDa protein with four putative transmembrane regions. Both the N- and C-termini of the protein are proline-rich, and may serve as potential ligands for the SH3 domain. Immunohistochemical analysis localized the protein predominantly to intracellular membranes. Northern blot showed that the LAPTM4B mRNAs were remarkably upregulated in HCC (87.3%) and correlated inversely with differentiation status. LAPTM4B was also overexpressed in many HCC-derived cell lines. It was also highly expressed in fetal livers and certain adult normal tissues including the heart, skeletal muscle, testis, and ovary. Promoter function assays showed a distinct difference in the gene's activities between BEL7402 and HLE cell lines, suggesting that the transcription factors responsible for regulation of the gene in the two cell lines are different, and that possible negative regulatory cis-elements may exist upstream of the promoter region. It was demonstrated that the N-terminus of LAPTM4B was essential for survival of the cells. Cells harboring the full-length LAPTM4B cDNA expression clone displayed a slightly increased efficiency in colony formation. These results suggest that LAPTM4B is a potential protooncogene, whose overexpression is involved in carcinogenesis and progression of HCC. In normal cells, it may also play important roles such as regulation of cell proliferation and survival.