Gone without blebs
Cell to matrix adhesion is required for survival of most human cells and activates anoikis (a form of programmed cell death) when disrupted. Melanoma cells have high metastatic potential, increased cell motility, and resist anoikis. Recent work from Andrew Weems et al., published in Nature, reports pro-survival mechanisms initiated by continuous blebbing after cell detachment – a unique morphological feature. The authors show the presence of signaling hubs in the blebs that activate pro-survival pathways via NRAS (neuroblastoma ras viral oncogene homolog) and effectors. RAS oncogenes are mutated in a variety of human cancers leading to constitutive activation of downstream cell signaling and consequent uncontrolled cell growth & survival.
Conversion of raw light-sheet microscope data to 3D surfaces using u-Shape3D. Poorly adhered MV3 cell embedded in soft bovine collagen. Septins visualized with mouse SEPT6–GFP probe.
When blebbing was inhibited, detached melanoma cells from three mutational backgrounds (MV3 (NRAS(Q61R)-driven), A375 (BRAF(V600E)-driven) and M498 (BRAF(V600E)-driven primary cell line)) were more susceptible to cell death. Importantly, A375 BRAF-driven mutants continued to resist anoikis unless treated with frontline cancer therapies, suggesting bleb formation confers greater survival of metastatic cancer cells as well as resistance to drug treatments. Cross validation with mass spectrometry and immunofluorescence revealed that the specific geometry of blebs recruits septins, which are GTP-binding cytoskeletal proteins that self-assemble and are implicated in protein scaffolding and receptor signaling. High-speed time-lapse light-sheet microscopy showed ‘pulses’ of septin accumulation and stabilization of higher order assemblies at dynamic blebbing sites that were maintained beyond blebbing timescales. Bleb morphology was unaffected by septin inhibition, but localization of septin assemblies at sites of blebbing was required for anoikis resistance. This indicated that the morphology adapted by the cell surface preceded functional downstream signaling pathways. Perturbation of NRAS signaling increased cell death in detached cells and stabilization of downstream ERK signaling in these detached cells required blebbing and septins. To validate that cell surface morphology promotes survival of non-malignant detached cells, anoikis-resistant blebbing was induced in MEF cell lines. Prolonged blebbing led to reduced activation of cell death. When blebbing or septins were inhibited, cell death increased.
The work presented here offers insight into molecular mechanisms at the blebs that involve septins and NRAS -mediated downstream signaling pathways for survival of cancer cells. These findings open the door to the investigation of new therapeutic options for treatment-resistance RAS mutation-driven cancers. Most importantly, these results show that the mutational state of a potent oncogene per se is not sufficient to maintain oncogenic functions like anoikis. Morphological conditions, and probably several other non-genetic factors play an equal if not even more decisive role in cancer development. Our lab is very excited by this discovery and driven to pursue this unique line of research further.
Several journals and news outlets have picked up this story in excellent commentaries, some of which are listed here:
