Nasopharyngeal carcinoma and retinoblastoma gene expression

The pathogenesis of nasopharyngeal carcinoma (NPC) is not well defined yet. To evaluate oncosuppressor genes in NPC, two NPC cell lines were investigated for expression of the retinoblastoma (RB) gene.

We used Western blotting to identify RB protein species, and checked the RB gene conformation by Southern blotting. We also used immunohistochemistry, in situ nucleic acid hybridization, and in situ extraction of RB protein to observe RB protein in NPC culture cells.

Both cell lines as well as sublines could synthesize normal RB proteins of 110, 113 and 114 kilodaltons. These cells showed no RB DNA rearrangement. Most interphase cells showed variable amounts of anti-RB reaction product in their nuclei when immunostained by 13 monoclonal antibodies. However, all mitotic cells contained RB protein either in the cytosol or the chromosomes, depending upon the mitotic phase. The level of RB mRNA increased slightly in mitotic cells as compared with interphase cells. Double localization of bromodeoxyuridine and RB protein in NPC cells and localization of RB protein in synchronized NPC cells in different phases of the cell cycle revealed random RB protein expression in each individual tumor cell. Co-localization of RB mRNA and RB protein in interphase cells showed a different degree of RB message expression in each cell. Low-salt hypotonic buffer could remove a fraction of RB protein from stained interphase nuclei. A similar finding with slight variations was observed in 16 other cancer cell lines.

These data indicate that our NPC cell lines contain no obvious RB gene rearrangement, but each cancer cell may have an abnormal expression of RB mRNA and protein. This phenomenon may also be true in other cancer cell lines with a normal RB gene.