Cancer cells acquire resistance to DNA-damaging therapeutic agents, such as cisplatin, but the genetic mechanisms through which this occurs remain unclear. We show that the c-MYC oncoprotein increases cisplatin resistance by decreasing production of the c-MYC inhibitor BIN1 (bridging integrator 1). The sensitivity of cancer cells to cisplatin depended on BIN1 abundance, regardless of the p53 gene status. BIN1 bound to the automodification domain of and suppressed the catalytic activity of poly(ADP-ribose) polymerase 1 (PARP1, EC 2.4.2.30), an enzyme essential for DNA repair, thereby reducing the stability of the genome. The inhibition of PARP1 activity was sufficient for BIN1 to suppress c-MYC-mediated transactivation, the G(2)-M transition, and cisplatin resistance. Conversely, overexpressed c-MYC repressed BIN1 expression by blocking its activation by the MYC-interacting zinc finger transcription factor 1 (MIZ1) and thereby released PARP1 activity. Thus, a c-MYC-mediated positive feedback loop may contribute to cancer cell resistance to cisplatin.

ETOC: Ribosome synthesis is a multistep process initiated in the nucleolus with the transcription of a precursor rRNA that is subjected to a series of modification and processing steps to generate the mature rRNA. In this paper, we describe a novel 60S ribosome biogenesis complex associating with LAS1L that controls rRNA processing and synthesis of the 28S rRNA.

Ribosome biogenesis is a highly regulated process ensuring that cell growth (increase in biomass) is coordinated with cell proliferation. The formation of eukaryotic ribosomes is a multistep process initiated by the transcription and processing of rRNA in the nucleolus. Concomitant with this, several preribosomal particles, which transiently associate with numerous nonribosomal factors before mature 60S and 40S subunits are formed and exported in the cytoplasm, are generated. Here we identify Las1L as a previously uncharacterized nucleolar protein required for ribosome biogenesis. Depletion of Las1L causes inhibition of cell proliferation characterized by a G 1 arrest dependent on the tumor suppressor p53. Moreover, we demonstrate that Las1L is crucial for ribosome biogenesis and that depletion of Las1L leads to inhibition of rRNA processing and failure to synthesize the mature 28S rRNA. Taken together, our data demonstrate that Las1L is essential for cell proliferation and biogenesis of the 60S ribosomal subunit.Cell growth (increase in cell mass) and cell division must be coordinated during proliferation for cells to remain at a constant size (13). Biogenesis of eukaryotic ribosomes is a finely tuned process ensuring that the protein synthesis capacity of a cell meets the demands of growth associated with proliferation (31). Ribosome biogenesis is initiated by the transcription of rRNA by RNA polymerase I (Pol I) (28S, 18S, and 5.8S rRNA) in the nucleolus and by RNA polymerase III (5S rRNA) in the nucleoplasm. After undergoing cotranscriptional site-specific modification, the 47S pre-rRNA transcript is submitted to a series of endonucleolytic cleavages and exonucleolytic digestion steps to remove internal and external transcribed spacer (ITS and ETS) regions and generate the mature 18S, 28S, and 5.8S rRNAs (21,26,27,36). Mature rRNAs are then assembled with ribosomal proteins to form the 40S (18S rRNA) and 60S (28S, 5.8S, and 5S rRNA) ribosomal particles. These steps involve a large assortment of factors in addition to the RNA and protein components of the ribosome itself. More than 200 additional accessory proteins and noncoding small RNAs engage in complex reactions of processing, assembly, and nuclear export to generate functional cytoplasmic 40S and 60S subunits (36). Although a large collection of evolutionarily conserved proteins has been implicated in processing and assembly of ribonucleoproteins, many details of these pathways and the precise function of several of these factors remain unresolved (28, 54).Perturbation of rRNA transcription or rRNA processing or deletion of ribosomal proteins has been shown to induce nucleolar stress and stabilization of the tumor suppressor p53 (4, 9-11, 16, 23, 25, 32, 58, 61, 62). It was previously demonstrated that, following nucleolar stress, several ribosomal proteins can bind to HDM2 (an E3 ubiquitin ligase) and inhibit its ability to ubiquitinate and target p53 for degradation, leading to accumulation of p53 and subsequent G 1 cell cycle arrest. This suggests t...

The E2F1 transcription factor, which was originally identified as a cell-cycle initiator, mediates apoptosis in response to DNA damage. As E2F1-induced apoptosis is an attractive mechanism for cancer therapy, it is critical to fully elucidate its effector pathways. Here, we show that the c-MYC-interacting proapoptotic tumor suppressor, BIN1, is transcriptionally activated by E2F1 and mediates E2F1-induced apoptosis in response to DNA damage. Acting through the DNA-binding and transactivation domains, ectopically expressed E2F1 activated the human BIN1 promoter, which contains canonical E2F-recognition sites. Conversely, depletion of E2F1 by small interfering RNA or germline deletion led to BIN1 deficiency. DNA-damaging agents (which included etoposide) increased BIN1 levels, unless E2F1 was deficient. Moreover, endogenous E2F1 protein interacted directly with the BIN1 gene promoter in chromatin, particularly after etoposide treatment. Notably, suppression of BIN1 expression using an antisense (AS) technique attenuated the cell death mediated by E2F1 and etoposide. Although the p53 tumor suppressor, its sibling protein p73, and caspases are well-known E2F1 effectors for DNA damage-induced apoptosis, AS-BIN1 did not compromise their apoptotic functions. Our results collectively suggest that BIN1 is a novel transcriptional target of E2F1 that triggers a unique mode of cell death in response to DNA damage.