Milk yield is closely related to the quantity and
physiological activity of mammary epithelial cells (MECs). Among mammals in
general, the mammary gland undergoes gradual degeneration during the late
lactation period (Akhtar et al. 2016) and the state of balance between the proliferation and
apoptosis of MECs has an important impact on changes in milk yield (Knight 2000; Capuco et al. 2003). There are many factors that influence the growth of
MECs, including various hormones and cytokines, which are involved in
regulating and controlling various biological processes such as gene
transcription and key signaling pathways (McCormick et al. 2014). IGFBP5 is a major
member of the insulin-like growth factor-binding protein family, able to bind
to insulin-like growth factors and the extracellular matrix (ECM) as well as mucopolysaccharides (Mohan and Baylink 2002). Studies have shown that changes in the expression
changes of IGFBP5 are associated with mammary gland development. For instance,
during mammary gland degeneration in rodents, the expression of the IGFBP5 gene
increased in MECs (Tonner et al. 1997). Analysis of the expression profile of IGFBP5 protein in
Bos indicus and Bubalus bubalis at
various lactation stages has revealed that IGFBP5 protein is highly expressed
in the late stage of lactation. Additionally, compared with normal mammals,
animals with a history of short lactation length (short-lactating animals)
express increased higher levels of IGFBP5 protein (Mohapatra et al. 2014).
IGFBP5, with its unique structure, is an important
regulator of activity of IGFs in the mammary gland. This is because the
N-terminal domain of IGFBP5 contains special sites that bind to IGF-Ⅰ (Ravid et al. 2008). Additionally, although the
C-terminal domain of IGFBP5 does not bind directly to IGF-I, it potentially
affects the IGF-Ⅰ binding affinity of the protein. Studies have shown that the IGFBP5 gene participates in
the regulation of MECs apoptotic processes and may influence cell survival (Dupont et al. 2002; Allan et al. 2004). Subcutaneous injection of mice with a recombinant IGFBP5
vector late in pregnancy led to mammary gland injuries, as evidenced by a
decrease in mammary fat pad infiltration (Allan et al. 2002). Additionally, a study in IGFBP5 transgenic mice
revealed that the DNA content was significantly reduced in the mammary glands
of transgenic mice on the tenth day of gestation. Moreover, the number of MECs
and the level of milk yield were decreased by approximately 50% within the
first 10 d of lactation (Allan et al. 2004). However, the above studies merely
explored the effect of IGFBP5 on MECs growth; little has been reported
concerning further details or clear regulatory mechanisms. Recently, the PI3K/Akt signaling pathway has become a research hotspot. PI3K/Akt signal transduction plays as a crucial role
in cell metabolism, survival, proliferation and migration (Kawiak and Lojkowska
2016). Activated PI3K and Akt activate or inhibit a series of downstream substrates
such as Bcl-2-associated agonist of cell death (BAD), caspase
3 and Bcl-2 through phosphorylation, thereby regulating cell proliferation,
differentiation, apoptosis and migration (Manning and Cantley 2007). For example, adolescent mammary gland development is
affected by hormones and growth factors that activate the PI3K/Akt signaling pathway to induce epithelial cell
proliferation and stimulate terminal end bud (TEB) formation and ductal
branching (Meng et al. 2017).
The objectives of this study were to search for any
changes in the expression of IGFBP5 gene in the mammary gland tissues of dairy
goats at three different lactation stages, to explore the effect of the IGFBP5
gene on the growth of MECs, and to reveal the molecular mechanism by which
IGFBP5 promotes the apoptosis of MECs through the PI3K/Akt
signaling pathway. This study provides a new theoretical reference for the
mechanism of MECs apoptosis in late lactation.
Materials
and Methods
Animals and ethical statement
The five
4-year-old, third-parity Laoshan dairy goats used in the present study were obtained
from the Aote
goat breeding farm (a Laoshan dairy goat
stock farm) in the Shandong Province, China. The goats were healthy and
free of disease and were kept under the
identical feeding and housing conditions. Mammary gland tissues were
collected surgically under general anesthesia from goats at the early lactation
stage (20 d postpartum) and peak lactation stage (90 d postpartum). The goats
were sacrificed in the late lactation stage (210 d postpartum). Goat heart,
liver, spleen, lung, colon, muscle, brain and mammary tissues were collected, frozen rapidly in liquid nitrogen and
cryopreserved. All animal experiments were carried out under the guidance of
the Shandong Agricultural University Animal Care and Use Committee
(SDAUA-2017-40), and best efforts were made to reduce animal suffering during
the operation.
Cell culture and
transfection
Mammary tissue
specimens were collected from Laoshan dairy goats,
and the MECs were isolated and cultured using the tissue-block method. The MECs
culture medium was prepared according to the following formula: 100 mg/mL streptomycin, 10 ng/mL
epidermal growth factor (Invitrogen, Carlsbad, CA, USA), 100 U/mL penicillin,
12% fetal bovine serum (Gibco, Grand Island, NY, USA),
and 88% Dulbecco's modified Eagle's medium (DMEM)/F12, 5 mg/L insulin (Sigma,
St. Louis, MO, USA). A culture environment was provided for cells at 37°C and
5% CO2 concentration in a temperature-controlled incubator. The in
vitro-purified dairy goat
MECs were plated in 24-well plates. The cells were subjected to transfections
when grown to approximately 65% confluence. Transfection with the
overexpression vector was performed using LipofectamineTM
2000 Transfection Reagent (Invitrogen). The IGFBP5 interference vector was
synthesized by RiboBio (Guangzhou, China).
Transfection with the interference vector was performed using a small
interfering RNA kit.
RNA sample preparation and gene expression profile analysis
In this study, the
tissues or cells were disrupted using a Bionoon-48 homogenizer (BIONOON, Shanghai, China), and RNA was extracted by adding TRIzol
reagent (Transgen, Biotech, Beijing, China). The quality and purity of RNA were examined using agarose
gel electrophoresis (5% gel) and a NanoDrop ND-2000 ultramicrospectrophotometer (NanoDrop
Technologies, Wilmington, DE, USA). The PrimeScriptTM
1st Strand cDNA Synthesis Kit (Takara, Dalian, China)
was applied to perform a reverse transcription
reaction. The PCR system (20 µL)
was set up according to the instructions provided in the TB Green Premix Ex Taq kit (Takara) as follows: 10 µL of TB Green, 0.4 µL
of forward primer, 0.4 µL of reverse primer, 2 µL of DNA template
and 7.2 µL of sterile water (primers
of IGFBP5: forward 5′-GGGTTTGCCTGAACGA-3′, reverse 5′-TCTCCTCTGCCATCTCG-3′, product size: 102 bp; primers of GAPDH: forward 5′-AGATAGCCGTAACTTCTGTG-3′,
reverse 5′-GGGTGGAATCATACTGGA-3′，product size: 198 bp). PCR amplification was carried
out using the LightCycler 96 PCR system (Roche,
Basel, Switzerland). The PCR conditions included a pre-denaturation step at 95°C for 30 s, followed by 35 cycles of denaturation
at 95°C for 5 sec, and extension at 60°C for 20 s. The melting curves were then
analyzed. Each sample was subjected to three independent amplifications. Glyceraldehyde-3-phosphate dehydrogenase
(GAPDH) was selected as the internal reference. The relative gene expression
was calculated using the 2-ΔΔCt method (Livak and Schmittgen
2001).
Detection and analysis of protein expression
RIPA lysis buffer (Beyotime, Shanghai, China) was applied to lyse cells; the liquid lysate was then centrifuged
and total protein was obtained from the supernatant. The Enhanced BCA Protein
Assay Kit (Tiangen Biotech, Beijing, China) was used
to measure the protein concentration. A separation gel
with a concentration of 10% and an upper gel with a concentration of 5% were
prepared and then the target proteins were separated by sodium dodecyl
sulfate-polyacrylamide gel electrophoresis. The electrophoresis time was 150
min. Next, the proteins in the gel were transferred
to polyvinylidene fluoride (PVDF) membranes by
electrophoresis. Subsequently, the PVDF membranes were blocked for 1 h using a
blocking solution (5% fat-free milk in 1× Tris-buffered saline/TWEEN 20 (TBST)) and
then were incubated with specific antibodies overnight. The antibodies used in
this experiment were as follows: rabbit anti-goat IGFBP5
polyclonal antibody (Abcam, Shanghai, China), rabbit anti-goat PI3K polyclonal
antibody (CST, Shanghai, China), rabbit anti-goat Akt
polyclonal antibody (Abcam), rabbit anti-goat Bcl-2
monoclonal antibody (Abcam), and rabbit anti-goat
Bcl-2-associated X (Bax) polyclonal antibody (Abcam), rabbit anti-goat casepase3 polyclonal antibody (Abcam), mouse anti-goat β-actin monoclonal antibody (Abcam), rabbit anti-goat GAPDH monoclonal antibody (CST),
goat anti-mouse IgG HRP conjugated antibody (CWBIO, Beijing,
China), and goat
anti-rabbit IgG HRP conjugated antibody (CWBIO).
After being washed three times with 1× TBST, the protein-loaded PVDF membranes
were incubated with horseradish peroxidase-labeled secondary antibodies for 1 h
at room temperature. The proteins were then visualized using a BeyoECL Plus kit (Beyotime) on an
Azure Biosystem C300 apparatus (Azure Biosystem, Dublin, CA, USA). The protein image was processed and quantified by ImageJ 1.48 software (National Institutes of Health,
Bethesda, MD, USA). The
relative levels of the proteins of interest were calculated based on GAPDH or
β-actin as an internal control protein.
Cell viability
evaluation
MECs were recovered
and cultured in 96-well plates. The number of cells seeded into each well was
approximately 3,000. Then, the cells were cultured overnight at a temperature of 37°C under an
atmosphere of 5% CO2.
A Cell Counting Kit-8 (CCK8; Sigma-Aldrich, Beijing, China) was used to examine
cell viability. Specifically, cells were collected
after 0, 24, 48 and 72 h of cell culture, and 10 ul
of CCK8 reagent was added to each well. Then the 96-well plate was incubated at
37°C for 40 min. After incubation, the absorbance of the samples was measured
at 450 nm using a SpectraMax iD3 microplate
reader (Molecular Devices, San Jose, CA, USA). An overexpression group, an
interference group, a blank control group and a negative control group were set
up for the experiment. Each group contained 5 replicate wells, and every
experiment was repeated three times independently.
Apoptotic assay
After 48 h of cell
transfection, the MECs were resuspended in PBS buffer, and the number of cells was counted. The cells were mixed with binding buffer at a concentration of 1×106
cells/mL. According to the instructions provided in
the Dead Cell Apoptosis Kit with Annexin V-FITC and
PI (Invitrogen, Carlsbad, CA, USA), the cells were mixed thoroughly with 5 µL of Annexin V-fluorescein isothiocyanate (FITC) and 5 µL of propidium iodide (PI) staining solution and incubated at
4°C for 10 min in the dark. The cells were examined immediately after the incubation period using a
flow cytometry sorter (BD, Franklin Lake, NJ, USA). FlowJo 7.6.1 software (Ashland, OR, USA) was applied to
analyze and visualize the original flow cytometry
data.
Data statistics and processing
Data collation and statistical
analysis were performed using R software (Version 3.5.1). Statistical
significance was analyzed by repeated-measures ANOVA and Tukey’s
honestly significant difference (HSD) test. When the P value was less
than 0.05, the difference between different groups was considered significant.
Unless otherwise stated, all the data shown in the present study were obtained
from at least three biological replicates. The data are expressed as the mean ±
standard error of the mean (SEM).
Results
The expression profile of the IGFBP5 gene in the mammary
tissue of dairy goats at various lactation stages
The qRT-PCR was employed to examine the expression level of the
IGFBP5 gene in the mammary tissue of dairy goats during the early (20 d), peak
(90 d) and late (210 d) stages of lactation. The expression level of the IGFBP5
gene was significantly higher at the early and late lactation stages than at
the peak lactation stage (Fig. 1A). Compared with the peak lactation stage, the
expression level of the IGFBP5 gene was increased by 6.78- and 5.92-fold at the
early and late lactation stages, respectively (P < 0.01). Additionally, examination of the IGFBP5 gene
expression levels in multiple tissues of
dairy goats revealed that the IGFBP5 gene was widely expressed in many tissues (Fig. 1B). Among the multiple tissues of the dairy goat, the highest
expression of IGFBP5 was observed in ovarian tissue, followed by kidney and
mammary tissues. In contrast, IGFBP5 gene expression was significantly reduced
in the colon, heart, spleen, muscle, brain, liver and adipose tissues (P < 0.05).
MECs proliferation
was regulated by IGFBP5
To discover and
clarify the effect of the IGFBP5 gene on the growth activity of MECs in dairy
goats, we constructed IGFBP5 overexpression and interference vectors and then
transfected them into the in vitro-cultured
%20434-440_files/image002.png)
Fig. 2: Western blot analysis of the expression levels of the IGFBP5 gene in vitro in cultured MECs after
overexpression or inhibition of the IGFBP5 gene
(A) Western blot results. (B) Gray value analysis of Western blot
bands using ImageJ. Control refers to the blank
control group, IGFBP5-OE refers to the IGFBP5 overexpression group, and
IGFBP5-Si refers to the IGFPB5 interference group. ** indicates highly
significant differences (P < 0.01)
%20434-440_files/image004.png)
Fig. 3: Examination of the effect of IGFBP5 on the proliferative activity of in vitro-cultured MECs by the CCK8 assay
(A) Effect of IGFBP5
overexpression on the proliferative activity of MECs cultured in vitro. (B) Effect of the
inhibition of IGFBP5 expression on the proliferative activity of MECs cultured in vitro. Blank refers to the blank
control group, IGFBP5-OE refers to the IGFBP5 overexpression group, and
negative control refers to the negative control group for the IGFBP5 overexpression vector. The cells were examined at 0,
24, 48 and 72 h after transfection using the SpectraMax
iD3 microplate reader. * indicates significant
differences (P < 0.05) and **
indicates highly significant differences (P
< 0.01)
%20434-440_files/image006.png)
Fig. 1: Analysis of the expression profile of the IGFBP5 gene in various tissues
of dairy goats
(A) The results of
real-time fluorescence-based quantitative PCR analysis of IGFBP5 gene
expression at different lactation stages. The abbreviation “20 d” refers to 20
days postpartum (early lactation stage), “90 d” refers to 90 days postpartum
(peak lactation stage), and “210 d” refers to 210 days postpartum (late
lactation stage). (B) Analysis of the expression abundance of the IGFBP5
gene in various tissues of dairy goats at the late stage of lactation. **
indicates highly significant differences (P
< 0.01). Different lowercase letters above the columns indicate that the
differences between the groups reached the significance level (P < 0.05)
dairy goat MECs. In the
protein expression assay (Fig. 2), the group transfected with the IGFBP5
overexpression
%20434-440_files/image008.png)
Fig. 4: Flow cytometric analysis of the effect of IGFBP5
on the apoptosis of in vitro-cultured
MECs
(A-D) Effects of the in vitro
overexpression and inhibition of IGFBP5 expression on apoptosis. (E-F)
Western blot analysis of caspase 3
expression after in vitro
overexpression and inhibition of IGFBP5 expression. The data are
expressed as the mean ± SEM. Control refers to the blank control group,
IGFBP5-OE refers to the IGFBP5 overexpression group, and IGFBP5-Si refers to
the IGFPB5 interference group. * indicates significant differences (P < 0.05), and ** indicates highly
significant differences (P < 0.01)
%20434-440_files/image010.png)
Fig. 5: Effects of in vitro
overexpression and inhibition of IGFBP5 expression on the relative expression
of the proteins in the PI3K/Akt signaling pathway
(A-B) Western blot analysis of the expression of
PI3K and Akt proteins. (C-D) Western blot analysis of the expression of Bcl-2 and Bax protein. All the data are expressed as the mean
± SEM. Control refers to the blank control group, IGFBP5-OE refers to the
IGFBP5 overexpression group, and IGFBP5-Si refers to the IGFPB5 interference
group. * indicates significant differences (P
< 0.05), and ** indicates highly significant differences (P < 0.01)
vector had a
significantly increased level of IGFBP5 (P
< 0.01) compared with the control group, while in the group transfected
with the interference vector, the expression level markedly decreased (P < 0.01) after 48 h. Additionally,
in order to examined the proliferative capacity of the cells, a CCK8 test kit was specifically used for detection at four different
time points—0, 24, 48 and 72 h after cell transfection. The proliferation of
the group overexpressing the IGFBP5 gene began to slow down at 24 h after
transfection (Fig. 3A). Compared to negative group, the overexpression group
exhibited significantly reduced viability at 48 h (P < 0.05). As shown in Fig. 3B, the group with interfered IGFBP5
gene expression exhibited proliferative activity similar to that of the
negative control group until the 24-h time point. However, at 48 h, the
proliferative activity was significantly higher in the IGFBP5 interference
group than in the negative control group (P
< 0.05). The above data indicate that the IGFBP5 gene exerts a certain
inhibitory effect on the proliferation of MECs.
The IGFBP5 gene
promotes the apoptosis of MECs
After 48 h of
transfection, the cells were
harvested into a sterile centrifuge
tube with a capacity of 1.5 mL and the
fluorescent dyes Annexin V-FITC and PI were used to
stain the cells. The effect of the IGFBP5 gene on apoptosis was examined by
flow cytometry and the results are shown in Fig.
4A-D. Based on comparison with the results of the
control group, the number of apoptotic cells
(Q2+Q3) was significantly increased in the group overexpressing the IGFBP5 gene
(P < 0.01) but was markedly
reduced in the interference group (P <
0.05). Flow cytometric analysis also found that the
number of early apoptotic cells was increased by 8.08%, while the number of
late apoptotic cells was increased by 4.9%, in the group overexpressing the
IGFBP5 gene. By contrast, the percentages of early and late apoptotic cells
were reduced in the group with interfered IGFBP5 gene expression compared with
those in the control group (3.24% vs.
6.32% and 5.01% vs. 11.4%,
respectively). Additionally, Western blot analysis was also applied to examine
the expression of the apoptosis-related factor caspase
3 in this study; experimental results are shown in Fig. 4E-F. Based on comparison with the results of the control group, the expression level of caspase
3 protein was significantly increased in the IGFBP5 overexpression group (P < 0.01). The opposite results were
found in the group with interfered IGFBP5 gene expression. The above results
demonstrate that the IGFBP5 gene is related to the apoptosis of MECs.
Overexpression of the IGFBP5 gene promoted the early apoptosis of MECs.
Regulatory effect of the IGFBP5 gene on the components of
the PI3K/Akt signaling pathway
To elucidate whether the effect of the IGFBP5 gene on MECs growth is related to the PI3K/Akt signaling pathway, we applied Western blotting to
examine the changes in the expression levels of PI3K and Akt
proteins in the MECs of the dairy goats from the normal control, IGFBP5
overexpression and interference groups. The experimental results showed that
the expression of PI3K and Akt proteins was markedly
reduced in MECs after overexpression of the IGFBP5 gene for 48 h (P < 0.01). By contrast, the
interference group expressed significantly increased levels of PI3K (P < 0.01) and Akt
proteins (P < 0.05) (Fig. 5A–B). Additionally, we examined the changes in the expression of
the apoptotic factor Bax and the antiapoptotic
factor Bcl-2 in MECs after overexpression and inhibition of the IGFBP5 gene. In
the group overexpressing the IGFBP5 gene, the Bcl-2 expression level was
significantly decreased, while the Bax expression
level was considerably elevated. Interference with IGFBP5 gene expression
yielded completely opposite results (P <
0.05) (Fig. 5C–D). Thus, from the above experiments, regulation of IGFBP5 gene
expression altered the expression of PI3K and Akt,
which are two key proteins in the PI3K/Akt signal
transduction pathway. Moreover, the expression levels of apoptosis-related
factors also changed significantly. Therefore, IGFBP5 could promote the apoptosis of MECs through the PI3K/Akt signaling pathway.
Discussion
The mammary glands of dairy goats undergo structural and
physiological functional changes at different
stages of lactation, often accompanied by changes in various physiological
activities of the MECs, which may include gene replication, transcription,
translation, material transport, energy metabolism and other activities (Tong and Hotamisligil 2007; Ji et al.
2013). As an important gene that can influence the growth of
mammary gland epithelial cells, IGFBP5 is a very highly conserved member of IGF-binding protein family (James et al. 1993). The expression pattern of this protein in breast
tissues is closely related to mammary gland development, milk secretion and
breast cancer diseases (Liu et al. 2012). The results from our study indicated that the IGFBP5
gene was highly expressed in the mammary tissue of dairy goats at the early and
late stages of lactation. In contrast, IGFBP5 gene expression was low in the
peak lactation period. This expression pattern has also been found in previous
lactation-related studies conducted in mice (Affolter et al. 2003) and cattle (Plath-Gabler et al. 2001). Interestingly, most studies have focused on the high
expression of IGFBP5 in the mammary gland in late lactation, ignoring the role
of IGFBP5 in mammary gland development and lactation during early lactation. On
the one hand, the reasons for the high expression level of IGFBP5 during early
lactation may be related to the higher expression of IGFBP5 in puberty and
pregnancy (Allar and Wood 2004); on the other hand, it may be that IGFBP5 expression
level is affected by related hormones, such as prolactin (PRL), which inhibits
the production of IGFBP5 during lactation (Colitti and Farinacci 2009). In addition, IGFBP5 can be produced either by local
cells in the mammary gland or from the liver to the mammary gland region (Phillips et al. 1993). To further understand the expression pattern of the
IGFBP5 gene, we examined the
expression status of IGFBP5 in various tissues of dairy goats at the late
lactation stage (Fig. 1B). The IGFBP5 gene was particularly highly expressed in
mammary tissue. A study has reported that a high level of IGFBP5 expression
damages the mouse mammary gland and promotes the apoptosis of mouse MECs (Phillips et al. 1993). Therefore, we speculated that the IGFBP5 gene might
have a similar effect on the growth of dairy goat MECs. In the present study,
the IGFBP5 gene was overexpressed or inhibited in in vitro-cultured
dairy goat MECs. The results
showed that IGFBP5 overexpression inhibited the proliferation of MECs and
promoted the apoptosis of MECs in dairy goats. Combined with the expression
characteristics of IGFBP5 in different lactation stages, the potential function of IGFBP5 in mammary gland-related
physiological activities was further revealed.
IGF-I is a well-known
survival-promoting antiapoptic factor. The PI3K/Akt pathway is related to IGF-Ⅰ-induced cell survival
(Kim and Park 2018). IGFBP5 binds to IGF-I, thereby affecting cell
proliferation and apoptosis, and several studies have confirmed this
conclusion. For example, intact IGFBP5 promotes chondrocyte proliferation
through binding to IGF-I. The C-terminal domain of IGFBP5 binds firmly to the
cell membrane, improving the presentation of IGF-I to its receptor (Kiepe et al. 2005). Additionally, IGFBP5 is a potential tumor suppressor
that inhibits the signal transduction and functional output of IGF-Ⅰ and
blocks the proliferation and migration of cancer cells (Ding et al. 2016). Therefore, we
speculated whether IGFBP5 participates in the regulation of the PI3K/Akt signaling pathway by affecting IGF-I. However, IGFBP5
was also reported to exert its regulatory effects independently. For example, a
study has shown that when exerting its effect independently, IGFBP5 acts as a
survival factor to promote the proliferation of muscle cells (Cobb et al. 2004). IGFBP5 not only exerts its effect through different
modes of action but also plays distinct roles in various cell types and
environments. As mentioned earlier, IGFBP5 promotes apoptosis in nerve cells
and mouse MECs. However, in an inflammatory environment, recombinant human
IGFBP5 stimulated the proliferation and migration of mesenchymal
stem cells and induced the differentiation of mesenchymal
stem cells toward bone/dentine (Schmidt et al. 2014). Therefore, we speculate that the diversity of IGFBP5
function is related to cell type, environment, and the dependence status on its
ligands (such as IGF-I).
The activated PI3K/Akt signaling
pathway participates in the regulation of proliferation of various cells,
including muscle cells (Fang et al. 2016), fibroblasts (Jung et al. 2007), cancer cells ( Vara et al. 2004), stem cells (Ling et al. 2013) and MECs (Zhu and Nelson 2013). Additionally, mammary gland development is under the
potential regulation of the PI3K/Akt signaling
pathway (Wickenden and Watson 2010;
Schmidt et al. 2014). Many functions of PI3K in the regulation of cellular
physiological activity are related to its ability to activate the
serine/threonine kinase Akt. Therefore, Akt, located downstream of the signaling pathway can be
regulated by PI3K. Activated Akt can affect the
morphology of MECs and the branches of mammary ducts (Engelman 2009). IGF-I inhibited the expression of connective tissue
growth factor through the PI3K-Akt signaling pathway, thereby promoting the
proliferation of MECs in dairy cows (Zhou et al. 2008). Recently, a study reported that lauric
acid stimulated the development of the mammary gland in adolescent mice by
activating the PI3K/Akt signaling pathway (Meng et al. 2017). The present study showed that overexpression of the
IGFBP5 gene in the MECs of dairy goats inhibited the PI3K/Akt
signaling pathway, resulting in the downregulation of
PI3K and Akt expression levels. Interference with
IGFBP5 gene expression yielded completely opposite results. Bcl-2 and Bax are significant factors involved in apoptotic
regulation, and they are part of the PI3K/Akt signal transduction pathway. Their
interaction and balance affect cell survival. Therefore, changes in Bcl-2 and Bax were also detected after upregulation or interference of IGFBP5 in MECs of dairy
goats. When the
IGFBP5 gene was overexpressed in MECs, the expression of Bcl-2 was inhibited,
while the expression of Bax was increased. The above
experimental data demonstrated that IGFBP5 is a key factor in the regulation of
MECs survival in dairy goats. Changes in the expression of IGFBP5 can induce
changes in apoptosis-related proteins and then cause a transition of cell fate.
IGFBP5 could inhibit the proliferation and promote the apoptosis
of dairy goat MECs by participating in the regulation of the PI3K/Akt signaling pathway. However, the process of apoptosis is
complex and changeable. This study did not verify whether IGFBP5 relies on
IGF-I to participate in the regulation of the PI3K/Akt
pathway or exerts a regulatory effect independently. In summary, our
results demonstrate that IGFBP5 is an important regulatory factor capable of
affecting mammary epithelial growth. IGFBP5 can promote the apoptosis of MECs
through the PI3K/Akt signaling pathway.
Conclusion
By constructing
IGFBP5 overexpression and interference vectors and employing in vitro-cultured MECs as the
experimental model, the present study demonstrated that the IGFBP5 gene
exhibits distinct differential expression patterns in various tissues of dairy
goats at different lactation stages. The IGFBP5 gene is highly expressed in the
early and late lactation periods. Upregulation of the IGFBP5 gene can prevent cell growth and accelerate apoptosis of MECs. The
changes in the PI3K/Akt signaling pathway in MECs were also verified, indicating that the
IGFBP5 gene can affect the survival of mammary cells via this pathway. The present study
explored the molecular mechanisms
with regard to the effects of the IGFBP5 gene on the growth of dairy goat MECs,
providing a theoretical basis to understand the role of IGFBP5 in mammary gland
development and lactation in dairy goats.
Acknowledgements
This work was
carried out and completed with the supports of National Natural Science
Foundation of China (31672401), National Key R & D Program of China (2018YFD0501906), Funds
of Shandong “Double Tops” Program (SYL2017YSTD12).
References
Affolter M, S Bellusci, N Itoh,
B Shilo, JP Thiery, Z Werb (2003). Tube or not tube: Remodeling epithelial
tissues by branching morphogenesis. Dev Cell 4:11‒18
Akhtar N, W Li, A Mironov, CH Streuli
(2016). Rac1 Controls Both the Secretory Function of the Mammary Gland and Its
Remodeling for Successive Gestations. Dev Cell 38:522‒535
Allan GJ, J
Beattie, DJ Flint (2004). The role of IGFBP-5 in mammary
gland development and involution. Domest Anim Endocrinol
27:257‒266
Allan GJ, E
Tonner, MC Barber, MT Travers, JH Shand, RG Vernon,
PA Kelly, N Binart, DJ Flint (2002). Growth
hormone, acting in part through the insulin-like growth factor axis, rescues
developmental, but not metabolic, activity in the mammary gland of mice
expressing a single allele of the prolactin receptor. Endocrinology 143:4310‒4319
Allar MA, TL Wood (2004).
Expression of the insulin-like growth factor binding proteins
during postnatal development of the murine mammary gland. Endocrinology 145:2467‒2477
Capuco AV, SE Ellis,
SA Hale, E Long, RA Erdman, X Zhao, MJ Paape (2003).
Lactation persistency: Insights from mammary cell proliferation studies. J Anim Sci 81:18‒31
Cobb LJ, DAM Salih, I Gonzalez, G Tripathi, EJ
Carter, F Lovett, C Holding, JM Pell (2004). Partitioning of
IGFBP-5 actions in myogenesis: IGF-independent
anti-apoptotic function. J Cell Sci 117:1737‒1746
Colitti M, M Farinacci (2009). Expression of a
putative stem cell marker, Musashi 1, in mammary
glands of ewes. J Mol Histol 40:139‒149
Ding M, RK Bruick, Y Yu (2016). Secreted IGFBP5 mediates
mTORC1-dependent feedback inhibition of IGF-1 signalling.
Nat Cell Biol
18:319‒327
Dupont J, JP Renou, M Shani, L Hennighausen, D LeRoith (2002).
PTEN overexpression suppresses proliferation and differentiation and enhances
apoptosis of the mouse mammary epithelium. J
Clin Invest 110:815‒825
Engelman JA (2009). Targeting PI3K signalling in cancer: Opportunities,
challenges and limitations. Nat
Rev Cancer 9:550‒562
Fang X, X Chen,
G Zhong, Q Chen, C Hu (2016). Mitofusin
2 Downregulation Triggers Pulmonary Artery Smooth
Muscle Cell Proliferation and Apoptosis Imbalance in Rats with Hypoxic
Pulmonary Hypertension Via
the PI3K/Akt and Mitochondrial Apoptosis Pathways. J Cardiovasc Pharmacol 67:164‒174
James PL, SB Jones, WH Busby, DR Clemmons, P Rotwein
(1993). A highly
conserved insulin-like growth factor-binding protein (IGFBP-5) is expressed
during myoblast differentiation. J Biol Chem 268:22305‒22312
Ji Z, G Wang, C
Zhang, Z Xie, Z Liu, J Wang (2013). Identification and Function Prediction of Novel MicroRNAs in Laoshan Dairy Goats. Asian-Aust
J Anim Sci
26:309‒315
Jung SA, HK Lee,
JS Yoon, SJ Kim, CY Kim, H Song, KC Hwang, JB Lee, JH Lee (2007). Upregulation of
TGF-beta-induced tissue transglutaminase expression
by PI3K-Akt pathway activation in human subconjunctival
fibroblasts. Invest Ophthalmol Vis Sci 48:1952‒1958
Kawiak A, E Lojkowska (2016). Ramentaceone, a
naphthoquinone derived from Drosera spp., induces apoptosis
by suppressing PI3K/Akt signaling in breast cancer
cells. PLoS One 11; Article e0147718
Kiepe D, S Ciarmatori, A Hoeflich,
E Wolf, B Tönshoff (2005). Insulin-like growth factor
(IGF)-I stimulates cell proliferation and induces IGF binding protein (IGFBP)-3
and IGFBP-5 gene expression in cultured growth plate chondrocytes via distinct
signaling pathways. Endocrinology
146:3096‒3104
Kim C, S Park (2018).
IGF-1 protects SH-SY5Y cells against MPP(+)-induced
apoptosis via PI3K/PDK-1/Akt pathway. Endocr Connect
7:443‒455
Knight CH (2000).
The importance of cell division in udder development and
lactation. Livest Prod Sci 66:169‒176
Ling LS, D Voskas, JR Woodgett
(2013). Activation of
PDK-1 maintains mouse embryonic stem cell self-renewal in a PKB-dependent
manner. Oncogene 32:5397‒5408
Liu BY, I Soloviev, X Huang, P Chang, JA
Ernst, P Polakis, C Sakanaka
(2012). Mammary tumor
regression elicited by Wnt signaling inhibitor
requires IGFBP5. Cancer Res 72:1568‒1578
Livak KJ, TD Schmittgen (2001). Analysis of Relative
Gene Expression Data Using Real-Time Quantitative PCR and the
2−ΔΔCT Method. Methods
25:402‒408
Manning BD, LC Cantley (2007). AKT/PKB signaling: Navigating downstream. Cell 129:1261‒1274
McCormick NH, SR Hennigar, K Kiselyov, SL Kelleher (2014). The biology of zinc transport in mammary
epithelial cells: Implications for mammary gland development, lactation, and
involution. J Mamm Gland Biol Neopl 19:59‒71
Meng Y, C Yuan, J
Zhang, F Zhang, Q Fu, X Zhu, G Shu, L Wang, P Gao, Q Xi, J Sun, Y Zhang, Q Jiang, S Wang (2017). Stearic
acid suppresses mammary gland development by inhibiting PI3K/Akt signaling pathway through GPR120 in pubertal mice. Biochem Biophys Res Commun 491:192‒197
Mohan S, DJ Baylink (2002). IGF-binding proteins are multifunctional
and act via IGF-dependent and -independent mechanisms. J Endocrinol 175:19‒31
Mohapatra SK, S Singh, S
Kumar, AK Dang, TK Datta, SK Das, TK Mohanty, JK Kaushik, AK Mohanty (2014). Comparative expression profiling of
insulin-like growth factor binding protein-5 in milk of Bos indicus and Bubalus bubalis during lactation. Animal 9:643‒649
Phillips ID, E Arany, AJ Strain, VK Han, DJ
Hill (1993). Rapid clearance of insulin-like growth factor (IGF)-binding protein
species from blood and an associated fall in circulating IGF-I following
partial hepatectomy in the rat. J
Endocrinol 137:271‒280
Plath-Gabler A, C Gabler,
F Sinowatz, B Berisha, D Schams (2001). The expression of the IGF family and GH receptor in
the bovine mammary gland. J Endocrinol 168:39‒48
Ravid D, D Chuderland, L Landsman, Y Lavie,
R Reich, M Liscovitch (2008). Filamin
A is a novel caveolin-1-dependent target in IGF-I-stimulated cancer cell
migration. Exp Cell Res 314:2762‒2773
Schmidt JW, BL Wehde, K Sakamoto, AA Triplett, SM Anderson, PN Tsichlis, G Leone, KU Wagner (2014). Stat5 regulates the
phosphatidylinositol 3-kinase/Akt1 pathway during mammary gland development and
tumorigenesis. Mol Cell Biol 34:1363‒1377
Tong Q, GS Hotamisligil (2007). Cell fate in the
mammary gland. Nature 445:724‒726
Tonner E, MC Barber, MT Travers, A Logan, DJ Flint (1997). Hormonal control of insulin-like growth factor-binding protein-5
production in the involuting mammary gland of the rat. Endocrinology
138:5101‒5107
Vara JAF, E Casado, JD Castro, P Cejas, C Belda-Iniesta, M González-Barón
(2004). PI3K/Akt signalling
pathway and cancer. Cancer Treat Rev 30:193‒204
Wickenden JA, CJ Watson (2010).
Key signalling nodes in mammary gland development and
cancer. Signalling downstream of PI3 kinase in
mammary epithelium: A play in 3 Akts. Breast Cancer Res 12:202‒211
Zhou Y, AV Capuco, H Jiang (2008). Involvement of connective tissue
growth factor (CTGF) in insulin-like growth factor-I (IGF1) stimulation of
proliferation of a bovine mammary epithelial cell line. Domest Anim Endocrinol
35:180‒189
Zhu W, CM Nelson
(2013). PI3K regulates branch initiation and extension of cultured mammary
epithelia via Akt
and Rac1 respectively. Dev Biol 379:235‒245