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

Abstract

The retinoblastoma tumor suppressor, RB, is a key regulator of cellular proliferation that is functionally inactivated at high frequency in human cancer. Although RB has been extensively studied with regard to tumor etiology, loss of tumor-suppressor function often occurs relatively late in tumor progression. Therefore, inactivation of RB could have a profound impact on the behavior of tumors driven by discrete oncogenes. Here, collaboration between Ras or c-Myc deregulation and RB functional state was investigated in a model of conditional genetic deletion to decipher the effects related to disease progression. These studies showed that RB loss had a robust impact on mitogen dependence, anchorage dependence and overall survival, which was significantly modified by oncogene activation. Specifically, RB deficiency predisposed c-Myc-expressing cells to cell death and reduced overall tumorigenic proliferation. In contrast, RB deficiency exacerbated the tumorigenic behavior of Ras-transformed cells in both the model system and human tumor cell lines. As these tumors exhibited highly aggressive behavior, the possibility of exploiting the intrinsic sensitivity to cell death with RB loss was evaluated. Particularly, although Ras-transformed, RB-deficient cells bypassed the G1-checkpoint elicited by pharmacological activation of the p53 pathway, they were also highly sensitized to cell death. Altogether, these data suggest that the impact of RB deletion is dependent on the oncogene milieu, and can directly contribute to transformed phenotypes and response to therapeutic intervention.

Reduced Expression of the Retinoblastoma Protein Shows That the Related Signaling Pathway Is Essential for Mediating the Antineoplastic Activity of Erufosine

“…These findings correlate with the accelerated clonogenicity of our Rb-deficient cell panel. They are supported by earlier reports that loss or deficiency of Rb expression impairs the chemotherapy response [14], [20], [57], [58].…”

“…Cyclin E2 is over-expressed in tumor-derived cells and accelerates the G 1 phase [56]. Reduced Rb-levels were associated with diminished amounts of p27 Kip1 , p16 Ink4A and p53 in our cell panel, more pronounced in cells with 99% than in those with 83% Rb-knockdown, a status, which is known to cause loss in cell cycle control, deregulation of DNA damage repair and accelerated proliferation [14].…”

“…In contrast to its activity in NSO-cells, erufosine did not enhance in cells with complete Rb knockdown the protein levels of p16 Ink4A , p27 Kip1 , p53, or Cdk4, nor inhibited the expression of cyclin D3. Low levels of the cell cycle regulators p16 and p27 and the tumor suppressor protein p53 have been correlated with accelerated proliferation and loss of cell cycle control in other studies [14], [40], which explains the resistance of the treated cell population towards erufosine. In cells with 83% Rb-knockdown; however, exposure to erufosine caused a significant suppression of the investigated proteins, which points to their involvement in mediating the cytotoxic effect of the compound.…”

“…Cell survival and the stunned erufosine-induced G 2 arrest can be explained by the unchanged S-phase cell fraction due to inhibition of the cell cycle regulators and tumor suppressor proteins p16 Ink4A andp27 Kip1 under conditions of Rb deficiency and the increase in cyclin E2 after treatment of Rb-deficient cells with erufosine (Fig. 5) [14].…”

“…It also targets cell cycle regulators such as the retinoblastoma protein (Rb), p27 Kip1 , transcription factors from the E2F family and cyclin D1 [2], [9]–[13]. The Rb-pathway represents one of the most frequently inactivated signaling axes in human cancers [14]–[18]. The retinoblastoma tumor-suppressor gene RB1 controls, pending on its phosphorylation status, on one hand the cell cycle progression from G1-to S-phase and on the other hand the activation of apoptosis.…”