In vivo imaging, tracking, and targeting of cancer stem cells.
BACKGROUND
There is increasing evidence that solid cancers contain cancer-initiating cells (CICs) that are capable of regenerating a tumor that has been surgically removed and/or treated with chemotherapy and/or radiation therapy. Currently, cell surface markers, like CD133 or CD44, are used to identify CICs in vitro; however, these markers cannot be used to identify and track CICs in vivo. The 26S proteasome is the main regulator of many processes within a proliferating cell, and its activity may be altered depending on the phenotype of a cell.


METHODS
Human glioma and breast cancer cells were engineered to stably express ZsGreen fused to the carboxyl-terminal degron of ornithine decarboxylase, resulting in a fluorescent fusion protein that accumulates in cells in the absence of 26S proteasome activity; activities of individual proteases were monitored in a plate reader by detecting the cleavage of fluorogenic peptide substrates. Proteasome subunit expression in cells expressing the fusion protein was assessed by quantitative reverse transcription-polymerase chain reaction, and the stem cell phenotype of CICs was assessed by a sphere formation assay, by immunohistochemical staining for known stem cell markers in vitro, and by analyzing their tumorigenicity in vivo. CICs were tracked by in vivo fluorescence imaging after radiation treatment of tumor-bearing mice and targeted specifically via a thymidine kinase-degron fusion construct. All P values were derived from two-sided tests.


RESULTS
Cancer cells grown as sphere cultures in conditions, which enrich for cancer stem cells (CSCs), had decreased proteasome activity relative to the respective monolayers (percent decrease in chymotryptic-like activity of sphere cultures relative to monolayers--U87MG: 26.64%, 95% confidence interval [CI] = 10.19 to 43.10, GL261, 52.91%, 95% CI = 28.38 to 77.43). The cancer cells with low proteasome activity can thus be monitored in vitro and in vivo by the accumulation of a fluorescent protein (ZsGreen) fused to a degron that targets it for 26S proteasome degradation. In vitro, ZsGreen-positive cells had increased sphere-forming capacity, expressed CSC markers, and lacked differentiation markers compared with ZsGreen-negative cells. In vivo, ZsGreen-positive cells were approximately 100-fold more tumorigenic than ZsGreen-negative cells when injected into nude mice (ZsGreen positive, 30 mice per group; ZsGreen negative, 31 mice per group), and the number of CICs in tumors increased after 72 hours post radiation treatment. CICs were selectively targeted via a proteasome-dependent suicide gene, and their elimination in vivo led to tumor regression.


CONCLUSIONS
Our results demonstrate that reduced 26S proteasome activity is a general feature of CICs that can easily be exploited to identify, track, and target them in vitro and in vivo.
Vlashi
Erina
E
Division of Molecular and Cellular Oncology, Department of Radiation Oncology, David Geffen School of Medicine at University of California, Los Angeles, Los Angeles, CA 90095-1714, USA.
Kim
Kwanghee
K
Lagadec
Chann
C
Donna
Lorenza Della
LD
McDonald
John Tyson
JT
Eghbali
Mansoureh
M
Sayre
James W
JW
Stefani
Encrico
E
McBride
William
W
Pajonk
Frank
F
D000818:Animals; D001943:Breast Neoplasms / Q000378:metabolism* / Q000532:radiotherapy; D045744:Cell Line, Tumor; D019583:Dose Fractionation, Radiation; D005260:Female; D005455:Fluorescent Antibody Technique / Q000379:methods; D015972:Gene Expression Regulation, Neoplastic; D005910:Glioma / Q000378:metabolism* / Q000532:radiotherapy; D049452:Green Fluorescent Proteins / Q000378:metabolism*; D006801:Humans; D007150:Immunohistochemistry; D008297:Male; D051379:Mice; D008819:Mice, Nude; D014411:Neoplastic Stem Cells / Q000378:metabolism*; D009955:Ornithine Decarboxylase / Q000378:metabolism*; D010641:Phenotype; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D012107:Research Design; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013937:Thymidine Kinase / Q000235:genetics / Q000378:metabolism*; D014183:Transplantation, Heterologous
D049452:Green Fluorescent Proteins; D013937:Thymidine Kinase; D046988:Proteasome Endopeptidase Complex; C061553:ATP dependent 26S protease; D009955:Ornithine Decarboxylase