Loss of CSL Unlocks a Hypoxic Response and Enhanced Tumor Growth Potential in Breast Cancer Cells

Abstract

SummaryNotch signaling is an important regulator of stem cell differentiation. All canonical Notch signaling is transmitted through the DNA-binding protein CSL, and hyperactivated Notch signaling is associated with tumor development; thus it may be anticipated that CSL deficiency should reduce tumor growth. In contrast, we report that genetic removal of CSL in breast tumor cells caused accelerated growth of xenografted tumors. Loss of CSL unleashed a hypoxic response during normoxic conditions, manifested by stabilization of the HIF1α protein and acquisition of a polyploid giant-cell, cancer stem cell-like, phenotype. At the transcriptome level, loss of CSL upregulated more than 1,750 genes and less than 3% of those genes were part of the Notch transcriptional signature. Collectively, this suggests that CSL exerts functions beyond serving as the central node in the Notch signaling cascade and reveals a role for CSL in tumorigenesis and regulation of the cellular hypoxic response.

Notch signalling in context

“…Although further studies are needed to determine the extent to which enhancers are 'marked' by CSL prior to Notch activation, CSL-binding co-repressors are nevertheless likely to contribute to the regulatory landscapes. First, loss or down-modulation of CSL leads to derepression of tumour-promoting genes in several contexts 115,116 . Second, as several co-repressors bind directly to CSL in a manner that would preclude concomitant NICD binding, they will titrate the availability of CSL and hence set a threshold that NICD would need to exceed in order to activate transcription 110,111,[117][118][119] .…”

Notch Signaling in Development, Tissue Homeostasis, and Disease

“…At the molecular level, hypoxia induces stabilization of Notch ICD (45,251) and also increases the activity of the ␥-secretase complex (745), which further enhances Notch signaling. A recent unexpected finding is that ablation of CSL elicits a normoxic stabilization of HIF1␣ (68). Interactions between Notch and the Sonic Hedgehog signaling mechanism are less well explored, but it has recently been shown that Notch signaling plays a critical role in regulating intracellular transport to the primary cilia, which is a critical cellular structure for Sonic Hedgehog signaling (357,681).…”

“…Small molecule inhibition of the transcriptional activation complex, using a compound called IMR-1, has also been attempted (22). Direct interference with CSL might be expected to raise problems beyond those seen with GSIs, given recent data suggesting that CSL loss promotes tumor growth (68,373). As a means of competitively inhibiting ligand binding to receptors, numerous laboratories have tested receptor or ligand fragments as decoys.…”

Polyploid giant cancer cells: Unrecognized actuators of tumorigenesis, metastasis, and resistance

“…There is a small body of literature specifically related to PGCCs (74 PubMed‐listed entries with query: [PGCCs] vs 3,857,567 with query [cancer]; accessed 10 May 2019; Figure ), but this literature, taken holistically, makes a compelling case for defining them as the essential keystone cancer species and actuators of tumorigenesis, therapeutic resistance, and recurrence in metastatic disease. Keystone species are relatively few in number, but have a significant impact on the health and composition of the ecosystem.…”