Page 49 - Read Online

Zhang et al. Vessel Plus 2021;5:48 https://dx.doi.org/10.20517/2574-1209.2021.64 Page 3 of 14

Table 1. Expected normal-range PRG values from serum/plasma samples in various age groups of both women and men
Physiological stages (women) Values for ELISA kit detection
Follicular phase 0.2-1.4 ng/mL
Luteal phase 4.0-25 ng/mL
Menopause 0.1-1.0 ng/mL
Normal men 0.1-1.0 ng/mL
The dynamic range of PRG assay with human serum/plasma samples is 0-40 ng/mL. PRG: Progesterone.

vastly fluctuate between 4.0 and 25 ng/mL for premenopausal women during the luteal phase of their
menstrual cycle [Table 1] . Only 2% of total blood PRG is in its free, active form, which has a very short
[72]
half-life (5-10 min) . Over 98% of PRG in the blood is believed to be physiologically inactive and passively
[73]
transported by blood proteins , mainly by two major PRG-binding proteins: serpin A6 (binds ~18% of
[74]
PRG) and albumin (binds ~80% of PRG) [75-77] .
CSC COUPLES BOTH CLASSIC AND NON-CLASSIC PRG RECEPTOR SIGNALING
As a sex steroid hormone, PRG elicits its cellular responses through two major signaling pathways. PRG
binds to either nuclear progesterone receptors (nPRs) to enact classic PRG effects or to membrane
[78]
progesterone receptors (mPRs/PAQRs) [79,80] and PRG receptor membrane components [81,82] to enact non-
classic PRG effects. Currently, the intricate balance and switch mechanisms between these two signaling
cascades remain unknown. Recently, we found that CSC can modulate PRG receptor-mediated signaling,
coupling both classic and non-classic signaling by establishing crosstalk between them in nPR positive (+)
breast cancer T47D cells. Based on our findings, under PRG actions, CSC stability is regulated by two major
signaling cascades: (1) by the negative effects of PRG or its antagonist (nPRs only), mifepristone, via both
classes of PRG receptors; and (2) by the positive effects of nPRs signaling . This discovery reveals that the
[51]
balance between classic and non-classic PRG signaling impacts CSC function and identifies CSC as an
important mediator of nPR and mPR crosstalk in nPR(+) cells. Our observation is further supported by a
previous finding that PRG can act simultaneously on both nPRs and mPRs, and the activation of mPR
signaling can potentiate the hormone-activated nPR-2 isoform . The intricate feedback regulation among
[78]
the PRG-activated CSC-mPRs-PRG-nPRs (CmPn) signaling network in nPR(+) T47D cells can be
summarized as a common mechanism that exists among the CmPn signaling network under steroid
actions . In this CmPn signaling network, PRG and its nPR-specific antagonist, MIF, work independently
[51]
or synergistically to disrupt CSC through their common targets, mPRs, in a backward fashion (CSC←mPRs
←PRG) [Figure 1] .
[51]
A COMMON REGULATORY MECHANISM UNDERLYING THE PROMOTIVE EFFECTS OF
CSC ON MPRS IN BREAST CANCER CELLS
PRG can activate downstream signaling in both nPR(+) and nPR(-) cells by binding to mPRs [51,83-85] . Distinct
from nPRs, mPRs represent a unique class of membrane steroid receptors that mediate non-classic PRG
actions in nPR(+) and nPR(-) cells [78,86] . Numerous studies have implicated mPRs in breast cancer [87-95] ,
especially nPR(-) breast cancers [84,88,93] . After defining the CmPn signaling network in nPR(+) breast cancer
T47D cells , we shifted our focus to two nPR(-) breast cancer cells (MDA-MB231 and MDA-MB468), both
[51]
of which are triple-negative breast cancer (TNBC) cells. Using these two nPR(-) cell models, we confirmed
[96]
the presence of the CSC-mPRs-PRG (CmP) signaling network in nPR(-) breast cancer cells . We also
demonstrated that a common core mechanism exists among nPR(-) breast cancer cells, termed the CmP
signaling network. In the CmP signaling network, CSC can stabilize mPRs under steroid actions in a
forward fashion (CSC→mPRs), which overlaps with the CmPn signaling network in nPR(+) breast cancer