Role of the Rb-mediated cell cycle regulatory pathway in human ovarian cancer cell lines

Abstract

The p16^INK4A gene encoding a cell cycle regulatory protein, cyclin-dependent kinase 4 inhibitor, is a putative tumor suppressor gene. We examined p16 gene alterations in 11 ovarian cancer cell lines. Three cell lines had homozygous deletion of p16, 1 cell line had multiple intragenic mutations, and there was suppressed transcription of the p16 gene in 1 cell line. Some point mutations were also found in the conserved ankyrin consensus. These observations suggest that abnormalities of p16 are related to ovarian carcinogenesis. We then investigated whether gene products directly or indirectly linked to the Rb-mediated negative regulation of the cell cycle might have been targeted for mutation, resulting in a phenotype resembling loss of p16 function in ovarian cancer cells. p16 protein was undetectable in 5 cell lines due to homozygous deletion, missense mutations of both p16 alleles, or transcriptional silencing. These 5 cell lines expressed Rb protein at detectable levels. In contrast, Rb protein was undetectable in 4 of the 6 cell lines that expressed detectable p16 protein. Thus, tumor cell lines expressing Rb protein rarely expressed p16 protein, and conversely p16 expressing tumor cell lines rarely had detectable levels of Rb protein. The high level of aberrant expression of Rb or p16 in ovarian cancer cell lines supports the concept that these proteins interact to form a critical cell cycle checkpoint, and that the suppression of Rb or p16 is sufficient to disrupt the regulatory mechanism in a manner that favors proliferation.