Vera S. Donnenberg

Multiple Drug Resistance in Cancer Revisited: The Cancer Stem Cell Hypothesis

The failure to eradicate cancer may be as fundamental as a misidentification of the target. Current therapies succeed at eliminating bulky disease but often miss a tumor reservoir that is the source of disease recurrence and metastasis. Recent advances in the understanding of tissue development and repair cause us to revisit the process of drug resistance as it applies to oncogenesis and tumor heterogeneity. The cancer stem cell hypothesis states that the cancer‐initiating cell is a transformed tissue stem cell, which retains the essential property of self‐protection through the activity of multiple drug resistance (MDR) transporters. This resting constitutively drug‐resistant cell remains at low frequency among a heterogeneous tumor mass. In the context of this hypothesis, the authors review the discovery of MDR transporters in cancer and normal stem cells and the failure of MDR reversal agents to increase the therapeutic index of substrate antineoplastic agents.

Stromal vascular progenitors in adult human adipose tissue

The in vivo progenitor of culture‐expanded mesenchymal‐like adipose‐derived stem cells (ADSC) remains elusive, owing in part to the complex organization of stromal cells surrounding the small vessels, and the rapidity with which adipose stromal vascular cells adopt a mesenchymal phenotype in vitro. Immunohistostaining of intact adipose tissue was used to identify three markers (CD31, CD34, and CD146), which together unambiguously discriminate histologically distinct inner and outer rings of vessel‐associated stromal cells, as well as capillary and small vessel endothelial cells. These markers were used in multiparameter flow cytometry in conjunction with stem/progenitor markers (CD90 and CD117) to further characterize stromal vascular fraction (SVF) subpopulations. Two mesenchymal and two endothelial populations were isolated by high speed flow cytometric sorting, expanded in short term culture, and tested for adipogenesis. The inner layer of stromal cells in contact with small vessel endothelium (pericytes) was CD146+/α‐SMA+/CD90±/CD34−/CD31−; the outer adventitial stromal ring (designated supra adventitial‐adipose stromal cells, SA‐ASC) was CD146−/α‐SMA−/CD90+/CD34+/CD31−. Capillary endothelial cells were CD31+/CD34+/CD90+ (endothelial progenitor), whereas small vessel endothelium was CD31+/CD34−/CD90− (endothelial mature). Flow cytometry confirmed these expression patterns and revealed a CD146+/CD90+/CD34+/CD31− pericyte subset that may be transitional between pericytes and SA‐ASC. Pericytes had the most potent adipogenic potential, followed by the more numerous SA‐ASC. Endothelial populations had significantly reduced adipogenic potential compared with unsorted expanded SVF cells. In adipose tissue, perivascular stromal cells are organized in two discrete layers, the innermost consisting of CD146+/CD34− pericytes, and the outermost of CD146−/CD34+ SA‐ASC, both of which have adipogenic potential in culture. A CD146+/CD34+ subset detected by flow cytometry at low frequency suggests a population transitional between pericytes and SA‐ASC.

Mesenchymal markers on human adipose stem/progenitor cells

The stromal‐vascular fraction (SVF) of adipose tissue is a rich source of multipotent stem cells. We and others have described three major populations of stem/progenitor cells in this fraction, all closely associated with small blood vessels: endothelial progenitor cells (EPC, CD45−/CD31+/CD34+), pericytes (CD45−/CD31−/CD146+), and supra‐adventitial adipose stromal cells (SA‐ASC, CD45−/CD31−/CD146−/CD34+). EPC are luminal, pericytes are adventitial, and SA‐ASC surround the vessel like a sheath. The multipotency of the pericytes and SA‐ASC compartments is strikingly similar to that of CD45−/CD34−/CD73+/CD105+/CD90+ bone marrow‐derived mesenchymal stem cells (BM‐MSC). Here, we determine the extent to which this mesenchymal pattern is expressed on the three adipose stem/progenitor populations. Eight independent adipose tissue samples were analyzed in a single tube (CD105‐FITC/CD73‐PE/CD146‐PETXR/CD14‐PECY5/CD33‐PECY5/CD235A‐PECY5/CD31‐PECY7/CD90‐APC/CD34‐A700/CD45‐APCCY7/DAPI). Adipose EPC were highly proliferative with (14.3 ± 2.8)% (mean ± SEM) having >2N DNA. About half (53.1 ± 7.6)% coexpressed CD73 and CD105, and (71.9 ± 7.4)% expressed CD90. Pericytes were less proliferative [(8.2 ± 3.4)% >2N DNA)] with a smaller proportion [(29.6 ± 6.9)% CD73+/CD105+, (60.5 ± 10.2)% CD90+] expressing mesenchymal associated markers. However, the CD34+ subset of CD146+ pericytes were both highly proliferative [(15.1 ± 3.6)% with >2N DNA] and of uniform mesenchymal phenotype [(93.3 ± 3.7)% CD73+/CD105+, (97.8 ± 0.7)% CD90+], suggesting transit amplifying progenitor cells. SA‐ASC were the least proliferative [(3.7 ± 0.8)%>2N DNA] but were also highly mesenchymal in phenotype [(94.4 ± 3.2)% CD73+/CD105+, (95.5 ± 1.2)% CD90+]. These data imply a progenitor/progeny relationship between pericytes and SA‐ASC, the most mesenchymal of SVF cells. Despite phenotypic and functional similarities to BM‐MSC, SA‐ASC are distinguished by CD34 expression.

Identification of Human Aldehyde Dehydrogenase 1 Family Member A1 as a Novel CD8+ T-Cell–Defined Tumor Antigen in Squamous Cell Carcinoma of the Head and Neck

Few epitopes are available for vaccination therapy of patients with squamous cell carcinoma of the head and neck (SCCHN). Using a tumor-specific CTL, aldehyde dehydrogenase 1 family member A1 (ALDH1A1) was identified as a novel tumor antigen in SCCHN. Mass spectral analysis of peptides in tumor-derived lysates was used to determine that the CTL line recognized the HLA-A*0201 (HLA-A2) binding ALDH1A1(88-96) peptide. Expression of ALDH1A1 in established SCCHN cell lines, normal mucosa, and primary keratinocytes was studied by quantitative reverse transcription-PCR and immunostaining. Protein expression was further defined by immunoblot analysis, whereas ALDH1A1 activity was measured using ALDEFLUOR. ALDH1A1(88-96) peptide was identified as an HLA-A2-restricted, naturally presented, CD8(+) T-cell-defined tumor peptide. ALDH1A1(88-96) peptide-specific CD8(+) T cells recognized only HLA-A2(+) SCCHN cell lines, which overexpressed ALDH1A1, as well as targets transfected with ALDH1A1 cDNA. Target recognition was blocked by anti-HLA class I and anti-HLA-A2 antibodies. SCCHN cell lines overexpressing ALDH1 had high enzymatic activity. ALDH1A1 protein was expressed in 12 of 17 SCCHN, and 30 of 40 dysplastic mucosa samples, but not in normal mucosa. ALDH1A1 expression levels in target cells correlated with their recognition by ALDH1A1(88-96) peptide-specific CD8(+) T cells. Our findings identify ALDH1A1, a metabolic antigen, as a potential target for vaccination therapy in the cohort of SCCHN subjects with tumors overexpressing this protein. A smaller cohort of subjects with SCCHN, whose tumors express little to no ALDH1A1, and thus are deficient in conversion of retinal to retinoic acid, could benefit from chemoprevention therapy.

Adipogenic Potential of Adipose Stem Cell Subpopulations

Background: Adipose stem cells represent a heterogenous population. Understanding the functional characteristics of subpopulations will be useful in developing adipose stem cell–based therapies for regenerative medicine applications. The aim of this study was to define distinct populations within the stromal vascular fraction based on surface marker expression, and to evaluate the ability of each cell type to differentiate to mature adipocytes. Methods: Subcutaneous whole adipose tissue was obtained by abdominoplasty from human patients. The stromal vascular fraction was separated and four cell populations were isolated by flow cytometry and studied. Candidate perivascular cells (pericytes) were defined as CD146+/CD31–/CD34–. Two CD31+ endothelial populations were detected and differentiated by CD34 expression. These were tentatively designated as mature endothelial (CD 31+/CD34–), and immature endothelial (CD31+/CD34+). Both endothelial populations were heterogeneous with respect to CD146. The CD31–/CD34+ fraction (preadipocyte candidate) was also CD90+ but lacked CD146 expression. Results: Proliferation was greatest in the CD31–/CD34+ group and slowest in the CD146+ group. Expression of adipogenic genes, peroxisome proliferator-activated receptor-&ggr;, and fatty acid binding protein 4, were significantly higher in the CD31–/CD34+ group compared with all other populations after in vitro adipogenic differentiation. This group also demonstrated the highest proportion of AdipoRed lipid staining. Conclusions: The authors have isolated four distinct stromal populations from human adult adipose tissue and characterized their adipogenic potential. Of these four populations, the CD31/CD34+ group is the most prevalent and has the greatest potential for adipogenic differentiation. This cell type appears to hold the most promise for adipose tissue engineering.

Mesenchymal stem cell secretome and regenerative therapy after cancer

Cancer treatment generally relies on tumor ablative techniques that can lead to major functional or disfiguring defects. These post-therapy impairments require the development of safe regenerative therapy strategies during cancer remission. Many current tissue repair approaches exploit paracrine (immunomodulatory, pro-angiogenic, anti-apoptotic and pro-survival effects) or restoring (functional or structural tissue repair) properties of mesenchymal stem/stromal cells (MSC). Yet, a major concern in the application of regenerative therapies during cancer remission remains the possible triggering of cancer recurrence. Tumor relapse implies the persistence of rare subsets of tumor-initiating cancer cells which can escape anti-cancer therapies and lie dormant in specific niches awaiting reactivation via unknown stimuli. Many of the components required for successful regenerative therapy (revascularization, immunosuppression, cellular homing, tissue growth promotion) are also critical for tumor progression and metastasis. While bi-directional crosstalk between tumorigenic cells (especially aggressive cancer cell lines) and MSC (including tumor stroma-resident populations) has been demonstrated in a variety of cancers, the effects of local or systemic MSC delivery for regenerative purposes on persisting cancer cells during remission remain controversial. Both pro- and anti-tumorigenic effects of MSC have been reported in the literature. Our own data using breast cancer clinical isolates have suggested that dormant-like tumor-initiating cells do not respond to MSC signals, unlike actively dividing cancer cells which benefited from the presence of supportive MSC. The secretome of MSC isolated from various tissues may partially diverge, but it includes a core of cytokines (i.e. CCL2, CCL5, IL-6, TGFβ, VEGF), which have been implicated in tumor growth and/or metastasis. This article reviews published models for studying interactions between MSC and cancer cells with a focus on the impact of MSC secretome on cancer cell activity, and discusses the implications for regenerative therapy after cancer.

Rare Event Detection and Analysis in Flow Cytometry: Bone Marrow Mesenchymal Stem Cells, Breast Cancer Stem/Progenitor Cells in Malignant Effusions, and Pericytes in Disaggregated Adipose Tissue

One of the major strengths of Flow Cytometry is its ability to perform multiple measurements on single cells within a heterogeneous mixture. When the populations of interest are relatively rare, analytical methodology that is adequate for more prevalent populations is often overcome by sources of artifacts that become apparent only when large numbers of cells are acquired. This chapter presents three practical examples of rare event problems and gives detailed instructions for preparation of single cell suspensions from bone marrow, malignant effusions, and solid tissue. These examples include detection of mesenchymal stem cells in bone marrow, characterization of cycling/aneuploid cells in a breast cancer pleural effusion, and detection and subset analysis on adipose-derived pericytes. Standardization of the flow cytometer to decrease measurement variability and the use of integrally stained and immunoglobulin capture beads as spectral compensation standards are detailed. The chapter frames rare event detection as a signal-to-noise problem and provides practical methods to determine the lower limit of detection and the appropriate number of cells to acquire. Detailed staining protocols for implementation of the examples on a three-laser cytometer are provided, including methods for intracellular staining and the use of DAPI to quantify DNA content and identify events with ≥2N DNA. Finally, detailed data analysis is performed for all three examples with emphasis on a three step procedure: (1) Removal of sources of interference; (2) Identification of populations of interest using hierarchical classifier parameters; and (3) Measurement of outcomes on classifier populations.

Prevalence of endogenous CD34+ adipose stem cells predicts human fat graft retention in a xenograft model.

Background: Fat grafting is a promising technique for soft-tissue augmentation, although graft retention is highly unpredictable and factors that affect graft survival have not been well defined. Because of their capacity for differentiation and growth factor release, adipose-derived stem cells may have a key role in graft healing. The authors’ objective was to determine whether biological properties of adipose-derived stem cells present within human fat would correlate with in vivo outcomes of graft volume retention. Methods: Lipoaspirate from eight human subjects was processed using a standardized centrifugation technique and then injected subcutaneously into the flanks of 6-week-old athymic nude mice. Graft masses and volumes were measured, and histologic evaluation, including CD31+ staining for vessels, was performed 8 weeks after transplantation. Stromal vascular fraction isolated at the time of harvest from each subject was analyzed for surface markers by multiparameter flow cytometry, and also assessed for proliferation, differentiation capacity, and normoxic/hypoxic vascular endothelial growth factor secretion. Results: Wide variation in percentage of CD34+ progenitors within the stromal vascular fraction was noted among subjects and averaged 21.3 ± 15 percent (mean ± SD). Proliferation rates and adipogenic potential among stromal vascular fraction cells demonstrated moderate interpatient variability. In mouse xenograft studies, retention volumes ranged from approximately 36 to 68 percent after 8 weeks, with an overall average of 52 ± 11 percent. A strong correlation (r = 0.78, slope = 0.76, p < 0.05) existed between stromal vascular fraction percentage of CD34+ progenitors and high graft retention. Conclusion: Inherent biological differences in adipose tissue exist between patients. In particular, concentration of CD34+ progenitor cells within the stromal vascular fraction may be one of the factors used to predict human fat graft retention.

Localization of CD44 and CD90 Positive Cells to the Invasive Front of Breast Tumors

A variety of markers have been proposed to identify breast cancer stem cells. Here, we used immunohistostaining and flow cytometry to analyze their interrelationships and to sort cells for tumorigenicity studies.

Measurement of multiple drug resistance transporter activity in putative cancer stem/progenitor cells.

Multiple drug resistance, mediated by the expression and activity of ABC-transporters, is a major obstacle to antineoplastic therapy. Normal tissue stem cells and their malignant counterparts share MDR transporter activity as a major mechanism of self-protection. Although MDR activity is upregulated in response to substrate chemotherapeutic agents, it is also constitutively expressed on both normal tissue stem cells and a subset of tumor cells prior to the initiation of therapy, representing a built-in obstacle to therapeutic ratio. Constitutive and induced MDR activity can be detected in cellular subsets of disaggregated tissues, using the fluorescent substrates Rhodamine 123 and Hoechst 33342 for ABCB1 (also known as P-gp and MDR1) and ABCG2 (BCRP1). In this chapter, we will describe the complete procedure for the detection of MDR activity, including: (1) Preparing single-cell suspensions from tumor and normal tissue specimens; (2) An efficient method to perform cell surface marker staining on large numbers of cells; (3) Flow cytometer setup and controls; (4) Simultaneous measurement of Hoechst 33342 and Rhodamine123 transport; and (5) Data acquisition and analysis.