Loss of the retinoblastoma tumor suppressor: differential action on transcriptional programs related to cell cycle control and immune function

Abstract

Functional inactivation of the retinoblastoma tumor suppressor gene product (RB) is a common event in human cancers. Classically, RB functions to constrain cellular proliferation, and loss of RB is proposed to facilitate the hyperplastic proliferation associated with tumorigenesis. To understand the repertoire of regulatory processes governed by RB, two models of RB loss were utilized to perform microarray analysis. In murine embryonic fibroblasts harboring germline loss of RB, there was a striking deregulation of gene expression, wherein distinct biological pathways were altered. Specifically, genes involved in cell cycle control and classically associated with E2F-dependent gene regulation were upregulated via RB loss. In contrast, a program of gene expression associated with immune function and response to pathogens was significantly downregulated with the loss of RB. To determine the specific influence of RB loss during a defined period and without the possibility of developmental compensation as occurs in embryonic fibroblasts, a second system was employed wherein Rb was acutely knocked out in adult fibroblasts. This model confirmed the distinct regulation of cell cycle and immune modulatory genes through RB loss. Analyses of ciselements supported the hypothesis that the majority of those genes upregulated with RB loss are regulated via the E2F family of transcription factors. In contrast, those genes whose expression was reduced with the loss of RB harbored different promoter elements. Consistent with these analyses, we found that disruption of E2F-binding function of RB was associated with the upregulation of gene expression. In contrast, cells harboring an RB mutant protein (RB-750F) that retains E2F-binding activity, but is specifically deficient in the association with LXCXE-containing proteins, failed to upregulate these same target genes. However, downregulation of genes involved in immune function was readily observed with disruption of the LXCXE-binding function of RB.Thus, these studies demonstrate that RB plays a significant role in both the positive and negative regulations of transcriptional programs and indicate that loss of RB has distinct biological effects related to both cell cycle control and immune function.

Retinoblastoma Protein (pRb), but Not p107 or p130, Is Required for Maintenance of Enterocyte Quiescence and Differentiation in Small Intestine

“…Cyclin A was exclusively expressed in crypts in normal mice. Its expression has been shown to be regulated by pRb (13,20,21). However, in small intestine, its expression seemed unaffected by any pocket protein studied.…”

Regulation of miR106b cluster through the RB pathway

“…To demonstrate that altered regulation general, the loss of RB function is associated with the deregulation of the transcription of coding genes that contribute to diverse biological processes. 9,31 In particular, RB modulates the expression of genes involved in DNA replication and can correspondingly deregulate multiple facets of replication control. While MCM7 has a key role in replication control, 32,33 it also harbors the mir106b cluster that is disease-relevant and encodes three miRNA species that target multiple disease-relevant coding genes (e.g., PTEN and p21…”

Modeling the effect of the RB tumor suppressor on disease progression: dependence on oncogene network and cellular context

“…Proximally, the RB protein functions as a transcriptional repressor of the E2F family of transcription factors (Nevins, 2001;Dynlacht, 2004, 2007;Cobrinik, 2005). In doing so, RB restricts the expression of a transcriptional program that includes key regulators of cell cycle progression (for example, cyclin A and cyclin-dependent kinase (CDK) 2), DNA replication (for example, MCM7 and PCNA) and mitosis (for example, Plk1 and MAD2) (Nevins, 2001;Markey et al, 2002Markey et al, , 2007Dynlacht, 2004, 2007;Cobrinik, 2005;Iaquinta and Lees, 2007). Mitogenic signals lead to the activation of CDK/cyclin complexes that phosphorylate RB, alleviating the latent transcriptional repression and allowing for progression through the cell cycle (Weinberg, 1995;Peeper et al, 1997;Dyson, 1998).…”