Mammosphere Formation Assay from Human Breast Cancer Tissues and Cell Lines

Summary

Floating mammosphere assays can investigate the subset of stem-like breast cancer cells that survive in suspension conditions and show enhanced tumorigenesis when implanted into mice. This protocol provides a convenient in vitro measure of sphere-forming ability, a proxy for in vivo tumorigenesis, while facilitating analysis of the stem-associated transcriptional landscape.

Abstract

Similar to healthy tissues, many blood and solid malignancies are now thought to be organised hierarchically, with a subset of stem-like cancer cells that self-renew while giving rise to more differentiated progeny. Understanding and targeting these cancer stem cells in breast cancer, which may possess enhanced chemo- and radio-resistance compared to the non-stem tumor bulk, has become an important research area. Markers including CD44, CD24, and ALDH activity can be assessed using fluorescence activated cell sorting (FACS) to prospectively isolate cells that display enhanced tumorigenicity when implanted into immunocompromised mice: the mammosphere assay has also become widely used for its ability to retrospectively identify sphere-forming cells that develop from single stem cell-like clones. Here we outline approaches for the appropriate culturing of mammospheres from cell lines or primary patient samples, their passaging, and calculations to estimate sphere forming efficiency (SFE). First we discuss key considerations and pitfalls in the appropriate planning and interpretation of mammosphere experiments.

Introduction

The existence of tumor cell lineages headed by stem-like cancer stem cells has greatly added to our understanding of tumor heterogeneity. While some phenotypic diversity in tumors does arise from the clonal outgrowth of genetically distinct clones, a substantial component appears to result from epigenetic differences: cancer cells can transition (sometimes reversibly) between stem, progenitor, and differentiated states via activation or repression of specific gene expression programs^1–3. This may reflect cell intrinsic or extrinsic factors, reflecting the gene expression program currently being expressed in a cell with its result....

Protocol

The procedures below have been ethically approved by Imperial College, London.

1. Generation of Primary Mammospheres from Human Breast Cancer Cell Lines

NOTE: Perform the following steps under a sterile culture hood.

1. Prepare Mammosphere Media containing DMEM/F12 supplemented with 2 mM L-glutamine, 100 U/ml penicillin, 100 U/ml streptomycin. Prepare complete media immediately before use by adding 20 ng/ml recombinant human epidermal growth factor (EGF; Sigma), 10 ng/ml recombinant human basic fibroblast growth factor (bFGF; R&D Systems) and 1x B27 supplement.
2. Aspirate media from flask con....

Results

Different samples or those subjected to different treatments may vary in the number of mammospheres >40 µm that form after normalizing for initial cells seeded. Calculate mammosphere forming efficiency (MFE) for each treatment grown in triplicate. This enables experiments with different seeding densities to be compared. Data is best displayed on a bar graph, ideally with positive and negative controls, and displaying the standard deviation across the triplicate wells. Consistently transfer adherent cells into non-adhe.......

Discussion

Successful assessment of primary and secondary mammospheres relies on cells being plated at sufficiently low densities that mammospheres form from single clones, with minimal sphere aggregation. However at densities that are too low, too few mammospheres may form to distinguish the effects of treatments statistically. Seeding density should be optimised for each cell line used since they can vary considerably in their sphere forming efficiencies (those expressing low E-cadherin may form less stable and shorter term mammo.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
DMEM/F12	Lonza	CC-3151
2 mM L-glutamine	Sigma Aldrich	G8540
100 U/ml Penicillin & Streptomycin	Sigma Aldrich	P4083
20 ng/ml Recombinant human epidermal growth factor (EGF)	Sigma Aldrich	E9644
20 ng/ml Recombinant human basic fibroblast growth factor (bFGF)	R&D systems	233-FB-025
1x B27 supplement	Invitrogen	17504-044
Phosphate buffered saline (PBS)	Thermo Scientific	12399902
0.5% trypsin/0.2% EDTA	Sigma Aldrich	59418C
Fetal calf serum	First Link UK	02-00-850
Trypan blue	Sigma Aldrich	93595
Low attachment 6-well plates	Corning	CLS3814
Collagenase type 1A	Sigma Aldrich	C9891
Hyaluronidase	Sigma Aldrich	H3506
Sterile razor blades	Fisher Scientific	12443170
Sterile scalpel	Fisher Scientific	11758353
Sterile micro-dissecting scissors	Sigma Aldrich	S3146