Rb2/p130 tumor suppressor protein regulates cell cycle progression primarily through interactions with members of the E2F family of transcription factors and repression of the transactivation of E2F target genes. In ATRA sensitive ovarian carcinoma CA-OV3 cells, a dramatic increase in Rb2/p130 protein mediates growth arrest at G0/G1. However, although Rb2/p130 is expressed at high levels in SK-OV3 cells, they fail to growth arrest in response to ATRA treatment. We show that the functional activity of Rb2/p130 in SK-OV3 cells is reduced when compared to CA-OV3 cells. To determine the basis for the reduced functional activity, we characterized the Rb2/p130 protein in SK-OV3 cells and investigated the possible role of alterations to this molecule in mediating resistance to ATRA growth suppression. Direct sequencing of Rb2/p130 cDNA cloned from SK-OV3 cells revealed the presence of two homozygous missense mutations (T178C and C259G) which result in amino acid changes Ser60 to Pro60 and Pro87 to Ala87 respectively. Unfortunately the same missense mutations were observed in Rb2/p130 cDNA cloned from ATRA sensitive CA-OV3 cells. We next investigated differences in Rb2/p130 protein subcellular localization. While Rb2/p130 was localized in the nucleus in both cell lines, we observed regions of intense staining within the nucleus of SK-OV3 cells. This is suggestive of aggregation and/or subnuclear sequestration of the Rb2/p130 protein. Finally, the PAGE migration pattern of Rb2/p130 suggested that a hyperphosphorylated form of Rb2/p130 accumulated in SK-OV3 cells but not in CA-OV3 cells. It is possible that this hyperphosphorylated form can be responsible for the decreased Rb2/p130 functional activity observed in SK-OV3 cells and may contribute to the resistance of these cells to ATRA mediated growth suppression.
(c) 2008 Wiley-Liss, Inc.