Transcriptional down-regulation of the retinoblastoma protein is associated with differentiation and apoptosis in human colorectal epithelial cells

Abstract

SummaryThe aim of this study was to investigate the regulation of Rb protein expression in relation to increased differentiation and induction of apoptosis in colonic epithelial cells. In vivo, Rb protein expression was found to be down-regulated towards the top of the normal colonic crypt, coincident with the region of differentiation and apoptosis, but highly expressed in colonic carcinoma tissue. Using in vitro models to study the regulation of Rb expression in pre-malignant colonic epithelial cells, we have been able to show for the first time that Rb protein expression is transcriptionally down-regulated in differentiated pre-malignant cells (in post-confluent cultures) but not in malignant colorectal epithelial cells. Furthermore, suppression of rb protein function by the HPV-E7 viral oncoprotein increased both spontaneous and DNA damage-induced apoptosis. These results suggest that Rb is able to act as a survival factor in colonic epithelial cells by suppressing apoptosis, and that over-expression of pRb in colorectal tumour cells can cause a loss of sensitivity to apoptotic signalling, resulting in aberrant cell survival and resistance to therapy. targeted inactivation of the RB-1 gene in mouse embryonic fibroblasts induces apoptosis. Therefore, pRb appears to suppress apoptosis in some cell systems, perhaps through the sequestration of the E2F-1 protein, which can induce apoptosis if over-expressed (Qin et al, 1994;Field et al, 1996). In this case, pRb needs to be removed or inactivated before apoptosis can occur. To support this, a 30 kD protein has been detected in apoptotic cells harvested from colon tumour cell lines, CMSV40 fibroblasts and BJA-B lymphocyte cells, suggesting that the Rb protein is cleaved during the apoptotic process (Browne et al, 1994;An and Dou, 1996). Furthermore, cleavage at the C-terminus of pRb results in a p100 and ~5 kDa polypeptide (Chen et al, 1997). The p100 Rb protein can still bind to E2F-1 and inhibit E2F-mediated transcriptional activity, but its anti-apoptotic function is reduced, perhaps through its inability to bind to the oncogene MDM2. Thus the growthsuppressive and anti-apoptotic functions of pRb appear to be distinct from one another. The notion that pRb functions can be separated from one another has been explored previously, and experimental evidence indicates that the multiple functions of pRb can be genetically uncoupled, providing distinct functions in cellcycle control or in tissue-specific gene expression during differentiation (reviewed in Yee et al, 1998). Mice with mutations of the N-terminal region of Rb protein exhibit defects in muscle differentiation, whilst retaining the ability to bind to E2F (Riley et al, 1997). Conversely, cells with pRb mutations in the pocket domain are able to differentiate normally even when E2F binding is abrogated (Sellers et al, 1998).The functional loss of the Rb protein, by deletion or mutation, has been implicated not only in retinoblastoma, but in many types of tumour, including bladder, breast, lung and ovarian ca...

Adipose tissue-specific inactivation of the retinoblastoma protein protects against diabesity because of increased energy expenditure

“…The early decrease in body fat content is probably not due to a differentiation defect of adipose tissue precursors, because such cells require a longer period to achieve full terminal differentiation. In agreement with previous studies in which low levels of pRb coincide with an enhanced rate of apoptosis (30)(31)(32)(33), a similar increase in the rate of apoptosis was also found in pRb-deficient MEFs as compared with control MEFs.…”

Molecular prognostic factors in rectal cancer treated by preoperative chemoradiotherapy

“…prb is able to act as a survival factor in colonic epithelial cells by suppressing apoptosis.the overexpression of prb in colorectal tumor cells can cause a loss of sensitivity to apoptotic signaling, resulting in aberrant cell survival and resistance to therapy (31). our study showed that a trend for a higher pathological cr rate was observed in tumors with a negative prb expression (table I).…”

PPARγ controls cell proliferation and apoptosis in an RB-dependent manner

“…Most interesting is our observation that activation of PPARg in differentiated colon cells, where RB is almost absent, induces apoptosis. This mimics the in vivo situation, since PPARg is strongly expressed (Lefebvre et al, 1999) and RB is almost absent (Guy et al, 2001;Yamamoto et al, 1999) in differentiated epithelial cells found at the top of the villi in the colon. One of the consequences of the high ratio of PPARg to RB expression in the villi is the stimulation of apoptosis, which contributes to cell turn-over in normal colon.…”