of the hallmarks of malignant cell populations is the ability to undergo continuous proliferation. to induce malignancy cell senescence could provide increased patient survival with fewer and less severe side effects than standard cytotoxic regimens. This positive aspect is usually countered by important caveats regarding senescence reversibility genomic instability and paracrine effects that may Docetaxel (Taxotere) increase heterogeneity and adaptive resistance of surviving malignancy cells. Nevertheless brokers that effectively disrupt replicative immortality will likely be useful components of new combinatorial approaches to malignancy therapy. and (examined in ). Although this senescence response has been shown to involve many of the same DNA damage response mediators (family functions that distinguish reversible cell cycle arrest from Docetaxel (Taxotere) irreversible senescence-associated changes. Despite the similarities among family proteins defects in pRB but not in p107 or p130 have been associated with human cancers. This suggests that pRB has unique tumor suppressor properties not attributable to p107 or p130. In support of this concept pRB has been shown to be preferentially associated with E2F targets involved in DNA replication during OIS and suppression of pRB but not p107 or p130 allowed continued DNA synthesis after induction of oncogenic RAS . The pRB protein contains multiple phosphorylation sites and interacts with multiple protein complexes. It remains to be determined whether the spectrum of pRB dependent changes in a given cell type under specific conditions is simply determined by the period of pRB activation or by qualitative differences in pRB modifications/binding interactions. Changes initiated by p16 expression are qualitatively and quantitatively unique from those in cells undergoing transient pRB-dependent growth arrest. For example in U2OS cells exposed Docetaxel (Taxotere) to p16 pRB augments p130 at E2F-regulated promoters. Dean and co-workers  used chromatin immunoprecipitation (ChIP) assays to assess protein association with the E2F responsive cyclin E and A promoters. A COL18A1 6-day induction of p16 resulted in a dramatic increase in pRB and E2F-4 associated with these promoters. Additional promoter-specific changes in the extent of binding to histone deacetylase HDAC1 SWI/SNF chromatin remodeling complex components BRG1 and Brm and polycomb group protein HPC2 were noted. Distinctions in pRB-associated phenotypes may be due to differences in the functionality of different phosphorylated forms of pRB (Fig. 1). Although growth factors are required for cyclin D1 synthesis transiently growth-arrested cells often contain significant amounts of cyclin D3 associated with CDK4 and the level of CDK4 activity is sufficient for cell cycle progression if CDK Docetaxel (Taxotere) inhibitors are removed . Thus in transiently growth-arrested cells pRB may be held preferentially in a hypophosphorylated rather than an unphosphorylated state. While many past studies have relied on the effect of hyperphosphorylation around the electrophoretic mobility of pRB to distinguish the hyperphosphorylated from your hypophosphorylated form few have distinguished the unphosphorylated from your hypophosphorylated form . E2Fs are more very easily co-immunoprecipitated with the hypophosphorylated form of pRB than the unphosphorylated form of pRB in peripheral blood lymphocytes (PBLs) during early G1 . Interestingly transduction of p16 protein into PBLs leads to loss of pRB hypophosphorylation and loss of detectable pRB association with E2F-4. The lack of detectable association might be due to reduced affinity of the unphosphorylated form of pRB for E2F-4 or alternatively to relative insolubility of larger chromatin complexes Docetaxel (Taxotere) made up of both pRB and E2F-4. Regardless of the interpretation the results suggest that pRB managed in a minimally Docetaxel (Taxotere) or completely unphosphorylated state in the presence of p16 is likely to have properties that differ from those of the hypophosphorylated form. Confirmation of this concept is apparent in the results of an expression profiling study of rat fibroblast cell lines . In this study the effects on..