Science - USA (2020-05-01)

INSIGHTS | PERSPECTIVES

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cells but not basal cells, which are the stem
cells during embryonic development ( 4 ). A
critical question is whether there are special-
ized committed luminal progenitors or facul-
tative (that is, adaptive) cells that repopulate
the prostate on regeneration.
Karthaus et al. carried out scRNA-seq in
mouse prostate, which revealed several cell
types, including three AR-positive luminal
epithelial populations—L1, L2, and L3—and
AR-positive mesenchymal populations. L1
cells, the largest population, are secretory
cells located in the distal prostate of mice.
As shown by scRNA-seq, L1-like cells are
similarly enriched in the peripheral zone
of human prostates, the location of most
adenocarcinomas, which express many of
the phenotypic properties of human L1 cells
( 5 ). L2 cells represent a population that has
been identified and isolated on the basis of
the expression of a variety of markers ( 6 – 8 ).
Prior studies have shown that L2 cells are
located in the proximal prostate (adjacent
to the urethra) and have the highest ex vivo
and transplantable self-renewal activity,
lack secretory proteins, and survive andro-
gen withdrawal at a higher frequency than
L1 cells. These properties suggest that L2
cells act as luminal stem or progenitor cells.
In the human prostate, a population of
cells with transcriptomes approximating L2
cells has been identified in the proximal pros-
tate (called the transition zone in humans)
( 5 ). L2 gene expression profiles overlap with
those of pulmonary luminal epithelial club
cells, which line the trachea (windpipe) and
are self-renewing progenitors of differenti-
ated secretory cells ( 9 ). Karthaus et al. also
identified a rare L3 population in mouse
prostate on the basis of a segregating gene
expression profile similar to that of pulmo-
nary ionocytes, which regulate ion and fluid
balance in lung secretions ( 9 ).
To evaluate the numbers of individual re-

generating luminal progenitors in the mouse

prostate, Karthaus et al. used genetically en-

gineered (confetti) mice to demonstrate the

clonality of small clusters of repopulating lu-

minal cells distributed throughout the pros-

tate. Although prior work has suggested that

a relatively large proportion of cells in the

distal prostate that survive castration subse-

quently proliferate on androgen addition ( 10 ,

11 ), the clonal origins of proliferating cells

were not addressed. These findings establish

that most surviving luminal cells divide in

situ on average two to three times and that

regeneration is not due to a limited number

of highly migratory repopulating clones.

A question of great interest has been

whether the cells that survive castration

display distinct molecular characteristics.

Karthaus et al. found that there was no dis-

tinct signature that identified a preexisting

stem or progenitor cell population among

L1, L2, and L3 cells immediately surviving

castration. However, adaptive transcrip-

tional changes were observed among the re-

maining luminal cells. Under castrated con-

ditions, the gene expression profiles of the

surviving L1 secretory population approxi-

mated those of the less castration-sensitive

L2 population, although each population

also maintained distinct markers. On an-

drogen add-back, the L1 and L2 populations

expressed proliferative programs as well as

their characteristic transcription profiles.

Loss of AR-mediated gene regulation was

a major mechanism of the transcriptional

changes observed in L1 cells. Defining the

transcriptional networks that cross-regu-

late AR-mediated differentiated functions

with survival and self-renewal programs is

an important next step (see the figure).

What are the mechanisms that determine

castration resistance in normal luminal

cells? The prevailing evidence suggests that

AR activity in mesenchyme, rather than lu-

minal epithelium, may be responsible for

androgen-dependent regeneration in the

normal prostate ( 12 , 13 ). Perhaps luminal cell

survival and regrowth are determined by the

heterogeneity of microenvironmental niches,

which can vary in soluble ligand expression

( 14 ) and mechanical properties. Karthaus et

al. identified, in mesenchyme populations

within regenerating prostates, induced RNAs

encoding growth factors that functionally

enhanced the growth of ex vivo luminal epi-

thelial cells in organoid culture. Further work

to validate the in vivo role of specific mesen-

chymal growth factors will be important to

define signaling mechanisms associated with

regenerative growth.

In men, L1 populations are anatomically

associated with prostate cancer and L2 popu-

lations with a common hyperproliferative

disease, benign prostate hypertrophy. It is

possible that the intrinsic properties of L1

and L2 cells, as well as their niches, contrib-

ute to these distinct disease outcomes. Does

the potential of L1 cells to express a less dif-

ferentiated “stemlike” state in the context

of the distal microenvironment predispose

to malignant transformation? Conversely,

do factors enriched in the proximal micro-

environment, such as transforming growth

factor–b (TGF-b) ( 14 ), suppress transforma-

tion? It is unclear what physiological condi-

tions (e.g., inflammation) might lead to L1

cell dedifferentiation.

These findings have implications for pros-

tate cancer treatment. Prostate adenocar-

cinomas are autonomously dependent on

AR activity, and inhibiting AR signaling is a

mainstay of treating high-risk primary and

metastatic prostate cancer ( 1 ). Acute survival

mechanisms in response to AR signaling

inhibition of adenocarcinoma may include

signaling pathways normally initiated by

mesenchymal paracrine growth factors used

by normal luminal cells. Targeting such path-

ways in combination with AR inhibitors may

prove to be one approach to increasing treat-

ment efficacy for advanced prostate cancer. j

REFERENCES AND NOTES

1. P. A. Watson et al., Nat. Rev. Cancer 15 , 701 (2015).


2. W. R. Karthaus et al., Science 368 , 497 (2020).


3. J. J. Li et al., Cold Spring Harb. Perspect. Med. 9 ,
   a030395 (2019).


4. N. Choi et al., Cancer Cell 21 , 253 (2012).


5. G. H. Henry et al., Cell Rep. 25 , 3530 (2018).


6. A. Tsujimura et al., J. Cell Biol. 157 , 1257 (2002).


7. O. J. Kwon, L. Zhang, L. Xin, Stem Cells 34 , 191 (2016).


8. Y. A. Yoo et al., J. Natl. Cancer Inst. 111 , 311 (2019).


9. D. T. Montoro et al., Nature 560 , 319 (2018).


10. J. Liu et al., Mol. Endocrinol. 25 , 1849 (2011).


11. J. C. Pignon et al., PLOS ONE 10 , e0128489 (2015).


12. G. R. Cunha, B. Lung, J. Exp. Zool. 205 , 181 (1978).


13. Q. Xie et al., Nat. Commun. 8 , 14284 (2017).


14. X. Wei et al., Cell Stem Cell 24 , 753 (2019).
    ACKNOWLEDGMENTS
    K.K. is funded by the intramural program, Center for Cancer
    Research, National Cancer Institute, NIH.

10.1126/science.abb7052

Proximal L2 cells

(ex vivo stem cells)

Homeostasis

AR-driven

diferentiation

Quiescent survival

Loss of AR-driven

transcription

In situ self-renewal

Proliferative

transcription

Distal L1 cells (secretory)

Converging

transcriptomes

Stromal

growth

factors

Proliferation

Castration

90% epithelial

cell death

Androgen

add-back

Basal cell L1 cell L2 cell Neuroendocrine cell Stromal cell

468 1 MAY 2020 • VOL 368 ISSUE 6490

Prostate luminal cell responses to androgen
In the mouse prostate, secretory (L1) and nonsecretory (L2) luminal cell populations respond differently to the
presence and absence of androgen through androgen receptor (AR)–mediated transcription. After castration,
L1 and L2 populations demonstrate converging gene expression profiles. On androgen reintroduction, both L1
and L2 populations proliferate and regain their differentiated identities.