Lack of nuclear RB protein staining in G0/middle G1 cells: correlation to changes in total RB protein level

Functional loss of the retinoblastoma (RB) gene has been implicated in a diverse group of human malignancies. The potential clinical importance of such loss demands a reproducible RB cell staining method to distinguish RB+ from RB- tumors. We report that normal and known RB+ tumor cells either in culture or in vivo display a mosaic RB staining pattern with variable proportions of cells having unstained nuclei which are indistinguishable from truly RB- tumor cells. We also show that the apparent lack of nuclear staining corresponds to RB+ cells which are in the G0 or middle G1 phase, where as much as 10-fold decrease in total cellular RB protein occurs compared to their G2/M phase counterparts. These findings not only provide new clues to RB protein function in cell growth and differentiation, but also need to be considered when evaluating the RB status in primary malignancies for its role in tumor initiation/progression or as a prognostic parameter.