Breast cancer occurs at rate of 1 out of 7. After so many years of research, even
though treatment has been dramatically improved and survival rate and length significantly
increased, there is still not cure for the disease. The possible existence of mammary
gland stem cell and breast cancer stem cell 1, 2 lights the hope for finding a cure
to this deadly disease. The rational behind is that once the stem cell can be isolated,
the signalling pathways that regulate its proliferation, self-renewal, survival and
differentiation will be illustrated, and this might shed light on the mechanisms of
breast cancer formation, therefore leading to a better therapeutic treatment.
Existence of stem cell in normal mammary gland has been convincingly demonstrated
by Kordon and Smith 1. In their report, an entire mammary gland could be regenerated
with the progeny of a single cell following transplantation into cleared mammary fat
pads. By definition, mammary gland stem cells are those that rarely divide and persist
throughout reproductive life. Classical markers 3 for identifying and purifying mammary
stem cells are label retention (tritiated thymidine or BrdU), stem cell antigen Sca-1
expression, ability to exclude dyes such as Hoechst 33342 or rhodamine 123 (side population
-- SP, e.g. Hoechst 33342 negative) because of elevated expression of membrane transporter
proteins, such as P-glycoproteins, and “small light” cell by electronic microscopy.
However, these profiling methods are controversial and confusing. Sometimes, for a
layman, it is very difficult to handle.
But this situation is about to change with two recent publications in Nature. In January,
Shackleton et al published “Generation of a functional mammary gland from a single
stem cell” 4. In this report, the authors cleared mammary gland mixture with CD31
(endothelial marker), CD45 and TER119 (haematopoietic antigens) by FACS sorting (Lin-
population). Using repopulating cleared mammary fat pad (Mammary repopulating 'units'--MRUs)
as criteria, they were able to increase the MRUs from 1/4900 to 1/64 just by applying
two more markers on Lin- population -- CD29 (beta1-integrin) and CD24 (heat-stable
antigen). Lin-CD29hiCd24+ cells have expended differentiation ability and colony-formation
ability. A single Lin-CD29hiCD24+ cell can repopulate cleared mammary fat pad and
develop into a fully functioning mammary gland, demonstrating its high proliferating
and multi-lineage differentiation capacity in vivo. Lin-CD29hiCd24+ cells can self-renew.
In mammary gland of MMTV-Wnt-1 transgenic mice, Lin-CD29hiCd24+ population are increased
and mammary gland outgrowths of Lin-CD29hiCd24+ MMTV-Wnt-1 cells are profoundly hyperplastic.
Lin-CD29hiCd24+ cells were enriched for long-term label-retaining cells, CD49f+ cells.
However, neither high Sca-1 expression nor Hoechst33342 dye exclusion was enriched
in this population.
In the February publication “Purification and unique properties of mammary epithelial
stem cells” 5, Stingl et al purified CD45-Ter119-CD31-CD49fhiCD24med cells and demonstrated
that they were the mammary gland stem cells. In consistency, CD45-Ter119-CD31-CD49fhiCD24med
cells were Sca-1 negative and only minority of these cells can efflux Hoechst 33342
and Rhodamine-123. Interestingly, the authors took one step further to illustrate
that CD45-Ter119-CD31-CD49fhiCD24med cells are in G1 or S/G2/M fractions, indicating
the stem cell population is a cycling population.
Most notably, these two publications completely changed the old mammary gland stem
cell picture -- Hoechst 33342 negative, slowly dividing and Sca-1 positive. They demonstrated
that CD45-Ter119-CD31-CD49fhiCD24med and Lin-CD29hiCd24+ are the mammary stem cell
populations, whereas previous SP and Sca-1+ cells only take very few percentage of
these two populations if not at all. Since label retention coincides very well with
Lin-CD29highCD24+ or CD45-Ter119-CD31-CD49fhiCD24med, it joins CD29, CD49f and CD24
as one of the most efficient 4 mammary gland stem cell markers. These new markers
make it easier to isolate mammary gland stem cells, therefore open a door for further
characterizing these cells. Importantly, with the same markers, cancer stem cells
can be purified as well. This provides a new opportunity to develop new targeted therapies
to killing cancer stem cells. Finally, the report proved that mammary gland stem cells
were actually cycling within cell cycle. This observation lays an important foundation
for testing new methods of chemoprevention and chemotherapy.