Discovery of Power-Law Growth in the Self-Renewal of Heterogeneous Glioma Stem Cell Populations.
BACKGROUND
Accumulating evidence indicates that cancer stem cells (CSCs) drive tumorigenesis. This suggests that CSCs should make ideal therapeutic targets. However, because CSC populations in tumors appear heterogeneous, it remains unclear how CSCs might be effectively targeted. To investigate the mechanisms by which CSC populations maintain heterogeneity during self-renewal, we established a glioma sphere (GS) forming model, to generate a population in which glioma stem cells (GSCs) become enriched. We hypothesized, based on the clonal evolution concept, that with each passage in culture, heterogeneous clonal sublines of GSs are generated that progressively show increased proliferative ability.


RESULTS
To test this hypothesis, we determined whether, with each passage, glioma neurosphere culture generated from four different glioma cell lines become progressively proliferative (i.e., enriched in large spheres). Rather than monitoring self-renewal, we measured heterogeneity based on neurosphere clone sizes (#cells/clone). Log-log plots of distributions of clone sizes yielded a good fit (r>0.90) to a straight line (log(% total clones) = k*log(#cells/clone)) indicating that the system follows a power-law (y = xk) with a specific degree exponent (k = -1.42). Repeated passaging of the total GS population showed that the same power-law was maintained over six passages (CV = -1.01 to -1.17). Surprisingly, passage of either isolated small or large subclones generated fully heterogeneous populations that retained the original power-law-dependent heterogeneity. The anti-GSC agent Temozolomide, which is well known as a standard therapy for glioblastoma multiforme (GBM), suppressed the self-renewal of clones, but it never disrupted the power-law behavior of a GS population.


CONCLUSIONS
Although the data above did not support the stated hypothesis, they did strongly suggest a novel mechanism that underlies CSC heterogeneity. They indicate that power-law growth governs the self-renewal of heterogeneous glioma stem cell populations. That the data always fit a power-law suggests that: (i) clone sizes follow continuous, non-random, and scale-free hierarchy; (ii) precise biologic rules that reflect self-organizing emergent behaviors govern the generation of neurospheres. That the power-law behavior and the original GS heterogeneity are maintained over multiple passages indicates that these rules are invariant. These self-organizing mechanisms very likely underlie tumor heterogeneity during tumor growth. Discovery of this power-law behavior provides a mechanism that could be targeted in the development of new, more effective, anti-cancer agents.
Department of Integrative Neuroscience, University of Toyama, 2630 Sugitani, Toyama, Toyama 930-0194, Japan.
Michiya
M
Sugimori
Department of Neurosurgery, University of Toyama, 2630 Sugitani, Toyama, Toyama 930-0194, Japan.
Yumiko
Y
Hayakawa
Center for Translational Cancer Research, Departments of Biology and Mathematics, University of Delaware, Helen F Graham Cancer Center and Research Institute, Newark, DE 19711 United States of America.
Bruce M
BM
Boman
Biotechnical Research, CATX, Inc., Gladwyne, PA 19035 United States of America.
Jeremy Z
JZ
Fields
Department of Integrative Neuroscience, University of Toyama, 2630 Sugitani, Toyama, Toyama 930-0194, Japan.
Miharu
M
Awaji
Department of Integrative Neuroscience, University of Toyama, 2630 Sugitani, Toyama, Toyama 930-0194, Japan.
Hiroko
H
Kozano
Department of Integrative Neuroscience, University of Toyama, 2630 Sugitani, Toyama, Toyama 930-0194, Japan.
Ryoi
R
Tamura
Department of Pathology, University of Toyama, 2630 Sugitani, Toyama, Toyama 930-0194, Japan.
Seiji
S
Yamamoto
The Center of Sports Science and Health Promotion in the NRCD Hospital, National Rehabilitation Center for Persons with Disabilities, 4-1 Namiki, Tokorozawa, Saitama 359-8555, Japan.
Toru
T
Ogata
Department of Neuropsycopharmacology, National Institute of Mental Health, National Center of Neurology and Psychiatry, 4-1-1 Ogawahigashi, Kodaira, Tokyo 187-8553, Japan.
Mitsuhiko
M
Yamada
Department of Neurosurgery, University of Toyama, 2630 Sugitani, Toyama, Toyama 930-0194, Japan.
Shunro
S
Endo
Department of Neurosurgery, University of Toyama, 2630 Sugitani, Toyama, Toyama 930-0194, Japan.
Masanori
M
Kurimoto
Department of Neurosurgery, University of Toyama, 2630 Sugitani, Toyama, Toyama 930-0194, Japan.
Satoshi
S
Kuroda
D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D000066673:Cell Self Renewal / Q000187:drug effects; D048429:Cell Size; D002999:Clone Cells / Q000187:drug effects / Q000473:pathology; D003606:Dacarbazine / Q000031:analogs & derivatives / Q000494:pharmacology / Q000627:therapeutic use; D005910:Glioma / Q000188:drug therapy / Q000473:pathology*; D006801:Humans; D008954:Models, Biological; D058990:Molecular Targeted Therapy; D014411:Neoplastic Stem Cells / Q000187:drug effects / Q000473:pathology*; D000077204:Temozolomide
D003606:Dacarbazine; D000077204:Temozolomide