Unregistered

Normal and neoplastic nonstem cells can
spontaneously convert to a stem-like state
Christine L. Chaffera,b, Ines Brueckmanna, Christina Scheela,b, Alicia J. Kaestlia, Paul A. Wigginsa,
Leonardo O. Rodriguesa,b, Mary Brooksa,b, Ferenc Reinhardta,b, Ying Suc, Kornelia Polyakc, Lisa M. Arendtd,e,
Charlotte Kuperwasserd,e, Brian Bieriea,b, and Robert A. Weinberga,b,f,1
aWhitehead Institute for Biomedical Research, Cambridge, MA 02142; bLudwig MIT Center for Molecular Oncology, Cambridge, MA 02139; cDepartment
of Medical Oncology, Dana-Farber Cancer Institute, Boston, MA 02115; dDepartment of Anatomy and Cellular Biology, Sackler School, Tufts University
School of Medicine, Boston, MA 02111; eMolecular Oncology Research Institute, Tufts Medical Center, Boston, MA 02111; and fDepartment of Biology,
Massachusetts Institute of Technology, Cambridge, MA 02139
Contributed by Robert A. Weinberg, March 2, 2011 (sent for review December 8, 2010)
Current models of stem cell biology assume that normal and
neoplastic stem cells reside at the apices of hierarchies and differentiate
into nonstem progeny in a unidirectionalmanner. Here we
identify a subpopulation of basal-like human mammary epithelial
cells that departs from that assumption, spontaneously dedifferentiating
into stem-like cells. Moreover, oncogenic transformation
enhances the spontaneous conversion, so that nonstem cancer cells
give rise to cancer stem cell (CSC)-like cells in vitro and in vivo. We
further show that the differentiation state of normal cells-of-origin is
a strong determinant of posttransformation behavior. These findings
demonstrate that normal and CSC-like cells can arise de novo from
more differentiated cell types and that hierarchical models of mammary
stem cell biology should encompass bidirectional interconversions
between stem and nonstem compartments. The observed
plasticity may allow derivation of patient-specific adult stem cells
without genetic manipulation and holds important implications for
therapeutic strategies to eradicate cancer.

Discussion:

The most unanticipated discovery that has emerged from this
study is the plasticity that we can now ascribe to human mammary
epithelial cells. We have shown that differentiated mammary
epithelial cells can convert to a stem-like state, doing so in
an apparent stochastic manner in vitro. This conversion occurs
in transformed and nontransformed HMECs isolated from cell
lines and primary tissue. In each case, the conversion proceeded
without genetic manipulation.
These findings represent a profound divergence from the
currently accepted unidirectional hierarchical model of mammary
epithelial cells and have widespread implications for the
use of cultured cells. In mammalian cells, the idea that nonstem
cells dedifferentiate to form functional stem cells has been restricted
to the notion that progenitor cells can reacquire stem
cell activity in mouse differentiating spermatogonia (26). As
such, our work demonstrates in mammalian cells that differentiated
epithelial cells can revert to a stem-like state.
Our findings also hold implications for the development of
anticancer therapeutics. As we previously reported, cells that
have been forced experimentally into a mesenchymal/stem-like
state can be used to screen for candidate therapeutic agents that
specifically target CSCs (27); the intent here was to eliminate
these cells and thereby deprive tumors of their ability to regenerate
and thrive following initial therapy. However, if non-
CSCs can spontaneously dedifferentiate into CSCs, then targeting
CSC populations will, on its own, be unlikely to yield durable
clinical responses, because the therapeutic eradication of existing
CSC populations might be followed by their regeneration from
non-CSCs within the tumor under treatment. Given the present findings, the known ability of microenvironmental
signals to provoke epithelial–mesenchymal transitions
(EMTs) and the close connection between passage through an
EMT and entrance into a stem-cell state, we suspect that the
presently observed spontaneous conversion in vitro may be
augmented in vivo by contextual signals in the tumor microenvironment,
such as those that drive the EMT (28, 29). Relevant
here are studies demonstrating that hypoxia-inducible factors
(HIFs) can induce the EMT phenotype and promote metastasis
and the CSC phenotype (30, 31). Spontaneous dedifferentiation
in vivo may involve the reactivation of one or more of the described
pluripotency factors (Oct4, Klf4, c-myc, and Sox-2) (32).
Hence, the representation of CSCs within tumor cell populations
is likely to be influenced both by contextual signals and by the
intrinsic phenotypic plasticity of these cells, as observed here.
The ability of non-CSCs to convert into CSCs in vivo might
resolve many of the current inconsistencies of the CSC model. In
particular, the observed plasticity that was once reserved for CSCs
alone can now be associated with nonstem cells. As such, CSC
populations may differ profoundly between various tumor types
according to the inherent plasticity of cells in their respective
nonstem fractions and their ability to spawn CSCs de novo.
The present observations lend further support to the emerging
view that the biological state of cells-of-origin is an important
determinant of the phenotype of their transformed derivatives
(8, 9), where experimental transformation of cells that have a
phenotype related to that of mammary stem cells generates cell
populations with a high frequency of tumor-initiating cells
(∼1:1,420–1:1,804 cells) and metastasis, which contrasts with the
low tumor-initiating ability and nonmetastatic nature of tumors
derived via transformation of more differentiated cell types.
The present findings hold the implication that patient- and
tissue-specific stem-like cells may one day be created in vitro via
spontaneous conversion of a patient’s own terminally differentiated
epithelial cells, a process that would not require any genetic
alteration of these cells. Such stem-like cells could be important
for regenerative therapies. Our results further emphasize the
pathological implications of cellular plasticity in cancer development,
progression, and recurrence. Further research needs to be
undertaken to determine the mechanism underlying the de novo
generation of CSCs from non-CSCs in vivo, with the promise
of potential novel targets for future cancer therapies aimed at
eradicating CSCs.