Ablation of the Retinoblastoma gene family deregulates G(1) control causing immortalization and increased cell turnover under growth-restricting conditions

The retinoblastoma suppressor pRB belongs to the family of so-called pocket proteins, which also includes p107 and p130. These proteins may functional ly overlap in cell cycle control and tumor suppression. We have generated a n isogenic set of embryonic stem (ES) cell lines carrying single or compoun d loss-of-function mutations in the Rb gene family, including a cell line c ompletely devoid of all three pocket proteins. None of the knockout combina tions affected the,growth characteristics of ES cells; however, concomitant ablation of all three pocket proteins strongly impaired their differentiat ion capacity. For the generated genotypes, primary mouse embryonic fibrobla sts (MEFs) also were obtained. While inactivation of Rb alone did not allev iate the senescence response of MEFs, pRB/p107-deficient MEFs, after having adapted to in vitro culturing, continued to proliferate at modest rate. Ad ditional ablation of p130 rendered MEFs completely insensitive to senescenc e-inducing signals and strongly increased their proliferation rate. Althoug h triple-knockout MEFs retained anchorage dependence, they lacked proper G( 1) control and showed increased cell turnover under growth-inhibiting condi tions.