ASMA = α-smooth muscle actin; CDK = cyclin-dependent kinase; COX = cyclo-oxygenase; ER = estrogen receptor; ESA = epithelial-specific
antigen; HMEC = human mammary epithelial cell; HPV = human papillomavirus; hTERT = catalytic subunit of human telomerase; PD = population
doubling; Ral-GEF = Ral guanine nucleotide exchange factor; TDLU = terminal ductal–lobular unit.
Available online http://breast-cancer-research.com/contents/7/4/171
Normal human mammary epithelial cells (HMECs) have a finite life
span and do not undergo spontaneous immortalization in culture.
Critical to oncogenic transformation is the ability of cells to overcome
the senescence checkpoints that define their replicative life span and
to multiply indefinitely – a phenomenon referred to as immortalization.
HMECs can be immortalized by exposing them to chemicals or
radiation, or by causing them to overexpress certain cellular genes or
viral oncogenes. However, the most efficient and reproducible model
of HMEC immortalization remains expression of high-risk human
papillomavirus (HPV) oncogenes E6 and E7. Cell culture models
have defined the role of tumor suppressor proteins (pRb and p53),
inhibitors of cyclin-dependent kinases (p16INK4a, p21, p27 and p57),
p14ARF, telomerase, and small G proteins Rap, Rho and Ras in
immortalization and transformation of HMECs. These cell culture
models have also provided evidence that multiple epithelial cell
subtypes with distinct patterns of susceptibility to oncogenesis exist
in the normal mammary tissue. Coupled with information from distinct
molecular portraits of primary breast cancers, these findings suggest
that various subtypes of mammary cells may be precursors of
different subtypes of breast cancers. Full oncogenic transformation
of HMECs in culture requires the expression of multiple gene
products, such as SV40 large T and small t, hTERT (catalytic subunit
of human telomerase), Raf, phosphatidylinositol 3-kinase, and Ral-
GEFs (Ral guanine nucleotide exchange factors). However, when
implanted into nude mice these transformed cells typically produce
poorly differentiated carcinomas and not adenocarcinomas. On the
other hand, transgenic mouse models using ErbB2/neu, Ras, Myc,
SV40 T or polyomavirus T develop adenocarcinomas, raising the
possibility that the parental normal cell subtype may determine the
pathological type of breast tumors. Availability of three-dimensional
and mammosphere models has led to the identification of putative
stem cells, but more studies are needed to define their biologic role
and potential as precursor cells for distinct breast cancers. The
combined use of transformation strategies in cell culture and mouse
models together with molecular definition of human breast cancer
subtypes should help to elucidate the nature of breast cancer
diversity and to develop individualized therapies.
More than 80% of adult human cancers are carcinomas,
tumors originating from malignant transformation of epithelial
cells. However, much of our understanding of oncogenic
transformation comes from fibroblast transformation systems.
Breast cancer is the second leading cause of cancer-related
deaths among women in the USA . The vast majority of
breast cancers are carcinomas that originate from cells lining
the milk-forming ducts of the mammary gland (for review ).
Deliberate transformation of these cells provides a practical
window into human epithelial oncogenesis. Malignant
transformation represents a complex multistep process in
which genetic, environmental, and dietary factors together are
thought to alter critical cell growth regulatory pathways
resulting in uncontrolled proliferation, which is a hallmark of
tumorigenesis [3,4]. Understanding the nature of these
cellular pathways is of central importance in cancer biology.
The growth of normal human mammary epithelial cells
(HMECs), which include luminal, myoepithelial and/or basal
cells (described below), is tightly controlled. These cells grow
for a finite life span and eventually senesce (for review [5-7]).
Both cell culture and mouse models have provided evidence
that essential initial steps in tumorigenesis involve the loss of
senescence checkpoints and immortalization, which allow a
cell to grow indefinitely and to go through further oncogenic
steps, resulting in fully malignant behavior. In addition, cell
culture model systems have identified a number of genes
whose alterations are involved in HMEC immortalization and
thereby have provided significant insights into the biology of
early breast cancer [5,7,8]. Use of oncogene combinations
has allowed researchers to create cell culture models of full
HMEC transformation, thereby illuminating the process of
Mammary epithelial cell transformation: insights from cell
culture and mouse models
Goberdhan Dimri1, Hamid Band2and Vimla Band1
1Division of Cancer Biology, Department of Medicine, ENH Research Institute, and Robert H Lurie Comprehensive Cancer Center, Feinberg School of
Medicine, Northwestern University, Evanston, Illinois, USA
2Division of Molecular Oncology, Department of Medicine, ENH Research Institute, and Robert H Lurie Comprehensive Cancer Center, Feinberg
School of Medicine, Northwestern University, Evanston, Illinois, USA
Corresponding author. Vimla Band, email@example.com
Published: 3 June 2005
This article is online at http://breast-cancer-research.com/content/7/4/171
© 2005 BioMed Central Ltd
Breast Cancer Research 2005, 7:171-179 (DOI 10.1186/bcr1275)
Breast Cancer Research July 2005 Vol 7 No 4Dimri et al.
breast cancer progression [9-11]. Additional insights have
come from mouse models, using transgenic overexpression of
oncogenesis-promoting genes and deletion of tumor
suppressor genes, which often produce breast adeno-
carcinomas that closely resemble human breast cancers.
Studies using cell culture transformation models have pointed
to the existence of HMEC subtypes with distinct suscepti-
bilities to oncogenesis by different oncogenes [5,8].
Remarkably, direct cDNA microarray profiling of human
breast cancers has led to similar insights, identifying multiple
subtypes of human breast cancer with distinct outcomes;
phenotypic and genotypic characteristics of these breast
cancer subtypes point to their possible origin from specific
subtypes of HMECs, such as basal or luminal cells .
Finally, cell culture and mouse model systems have begun to
identify mammary stem cells that may provide progenitors for
oncogenic transformation  and have led to an appreciation
of the microenvironment for oncogenesis [14,15].
Thus, studies conducted over the past several years have
established the importance of HMEC transformation models
to our understanding of the pathways that control normal
mammary cell growth, development, and oncogenesis.
However, many challenges remain, including the identification
of mammary cell subtypes or oncogenic strategies that result
in cancers that resemble naturally occurring human breast
cancers, and translation of new research to devise more
specific diagnostic and treatment strategies for different
subtypes of breast cancer.
Mammary gland and various epithelial cell
The mammary gland consists of a branching ductal system
that ends in terminal ducts with their associated acinar
structures, termed the terminal ductal–lobular units (TDLUs),
together with interlobular fat and fibrous tissue [16,17]. Most
breast cancers arise in the TDLU (Fig. 1). Unlike other
epithelial cancers, such as that of colon, different stages of
breast cancer are not clearly defined. However, it is clear that
benign stages (such as typical and atypical hyperplasia),
noninvasive cancers (such as carcinoma in situ – ductal or
lobular), and invasive cancers (such as invasive ductal or
lobular carcinomas) do exist. Additionally, multiple types of in
situ carcinomas, such as solid, cribiform, papillary and
comedo types, have been reported and it is possible that
these represent tumors originating from different epithelial
Histological examination of TDLU reveals two major types of
cells: inner secretory luminal cells and outer contractile
myoepithelial cells (Fig. 1). In addition to luminal and
myoepithelial cells, there is emerging evidence that basal
cells (presumed to be the progenitor for myoepithelial cells)
and stem cells exist in the TDLU [17,18]. Until recently it was
believed that the vast majority of breast carcinomas arise from
luminal epithelial cells . This was based on the keratin
expression and other phenotypic markers of cultured tumor
cell lines, mostly derived from metastatic lesions .
Unfortunately, the great majority of primary breast tumors
have proved difficult to establish in cultures, either on plastic
or as three-dimensional cultures [5-7,19-21]. However,
recent molecular profiling studies clearly show the existence
of multiple subtypes of breast cancers probably originating
from luminal, basal, and possibly stem cell compartments
 (described below in detail).
Culturing of various epithelial cell subtypes
For more than two decades, various investigators have
attempted to develop cell culture models that lead to isolation
of breast cancer cells resembling those found in human
breast cancers. In order to establish such models, it was
essential to culture normal HMECs. In 1980s, work from
several laboratories showed that normal HMECs could be
cultured in cell culture [22,23] (for review [2,5,7]).
In our laboratory we defined a medium, termed DFCI-1, that
helped us to establish and culture normal and some primary
breast cancers under identical conditions . However, in
general the difficulty in establishing primary tumor cells in cell
culture has persisted. Notably, early cultures derived from
reduction mammoplasty or mastectomy specimens exhibit
considerable heterogeneity (with multiple cell types – luminal,
stem cells, basal and myoepithelial cells) and grow for three
to four passages or about 15–20 population doublings
(PDs), and then senesce (Figs 2 and 3) [5-7]. The senes-
cence in these cells is also termed as M0 stage .
Structure of the mammary gland. Terminal ductal–lobular unit (TDLU),
composed of ductal cells, is the unit thought to be the origin of most
breast cancer. The stroma is composed of fatty tissue (adipocytes) and
fibroblasts. Also shown are the two primary types of cells in normal
ducts: outer contractile myoepithelial and inner columnar luminal cells.
A putative progenitor/stem cell is also indicated.
Terminal ductal-lobular unit
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Available online http://breast-cancer-research.com/contents/7/4/171