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Introduction

Postmenopausal osteoporosis (PMOP) is a common skeletal disorder in postmenopausal women that occurs due to the simultaneous interaction of independent predisposing factors, including aging and continuous calcium loss (1). In the developed world, depending on the method of diagnosis, 9–38% of females are affected by PMOP (2). It is estimated that one-third of adult women are affected by osteoporosis-associated fractures in their lifetime, which is a public health concern (3).

The mechanism underlying all cases of osteoporosis involves an imbalance between bone resorption and formation (4). In postmenopausal women, excessive bone resorption and an inadequate rate of formation of new bone tissue are associated with estrogen deficiency (5). Low estrogen levels have been hypothesized to cause osteoporosis in women since the 1940s (6). Based on the above mechanism, various medications, including alendronate, etidronate, risedronate and strontium ranelate have been employed to prevent osteoporotic fragility fractures in patients with PMOP (7). However, the efficacy of these medications have not been directly compared and it is not clear which of these medications are the most effective (8). Furthermore, the effect of medications is not ideal; patients with PMOP need continued treatment for up to 10 years (9). It has been previously demonstrated that the peak bone mass and early menopausal bone loss are associated with the level of expression of estrogen-associated genes (10). Previous study has demonstrated that mutations in osteoclast-associated genes, including parathyroid hormone 1 receptor, colony stimulating factor 1 and low-density lipoprotein receptor-related protein 5 are associated with PMOP (11). Furthermore, bioinformatics analysis of gene expression profiles in B cells from patients with PMOP demonstrated that filamin A a and transforming growth factor-β1 may be potential target genes associated with the pathogenesis of PMOP (12). In addition, certain transcription factors (TFs), such as catenin β1, and pathways, including the mitogen-activated protein kinase signaling pathway, have been demonstrated to be involved in primary osteoporosis by DNA microarray analysis (1). Therefore, a thorough understanding of the mechanisms underlying PMOP may be based on bioinformatics analysis and may contribute to the development of novel and effective treatment approaches for PMOP.

In the present study, a bioinformatics analysis was performed based on a gene expression profile of 12 tissue samples from ovariectomized (OVX) mice and 11 normal tissue samples from sham surgery mice. Differentially expressed gene (DEG) investigation, function and pathway enrichment studies, as well as protein-protein interaction network (PPI) analyses, were performed. Furthermore, TF-target gene regulatory networks and chemical-gene interaction networks were investigated. The present study aimed to systematically investigate potential genes and TFs associated with the progression of PMOP, which may aid in elucidating the molecular mechanism underlying PMOP. Additionally, potential chemical compounds that may be employed for the treatment of PMOP were also identified. The present study may provide insights into PMOP that may aid the development of novel and effective therapies for PMOP.

Materials and methods

Data resource

Gene expression profile data (accession no. GSE68303) were downloaded from the Gene Expression Omnibus (GEO) database (http://www.ncbi.nlm.nih.gov/geo/) (13). This dataset was produced on a GPL6885 Illumina MouseRef-8 v2.0 expression beadchip platform (Illumina, Inc., San Diego, CA, USA). A total of 12 tissue samples from OVX mice (OVX group) and 11 normal tissue samples from sham surgery mice (control group) were included in this dataset. The OVX and sham surgical operations were conducted by Calabrese et al (13). Briefly, mice were anesthetized using isoflurane and surgery was performed using a dorsal approach to excise the ovaries, and the abdominal and skin incisions were subsequently closed. At 4 weeks following surgery, the uterine weights were measured to confirm successful OVX surgeries (13).

Data preprocessing and differential expression analysis

Normalization of gene expression profile data was performed using the Robust Multichip Averaging (RMA) method (14) in Affy package version 1.44.0 (15) in R (version 3.0.0; (http://bioconductor.org/biocLite.R). DEGs were identified using unpaired t-tests using the Linear Models for Microarray Data package (limma version 3.32.2; http://www.bioconductor.org/packages/release/bioc/html/limma.html) (16) by comparing gene expression values between the OVX group and the control group. Following t-tests, the Benjamini-Hochberg procedure was performed to adjust P-values. Finally, P<0.05 and |log2 fold change|>0 was selected as the threshold for identification of DEGs. A heat map was constructed using pheatmap package version 1.08 (17) in R.

PPI network construction

The Search Tool for the Retrieval of Interacting Genes/Proteins (STRING; version 10.0; http://www.string-db.org/) database provides comprehensive coverage and access to experimental and predicted information concerning PPIs (18). Biological General Repository for Interaction Datasets (BioGRID; version 3.4; https://wiki.thebiogrid.org/) is an open access database that contains information on genetic and protein interactions obtained from primary biomedical literature for numerous model organisms and humans (19). Mentha (http://mentha.uniroma2.it/about.php) is a resource used to browse integrated protein-interaction networks (20). Furthermore, the Human Protein Reference Database (HPRD; release no. 9; www.hprd.org/) is a centralized platform for the visual representation and integration of information pertaining to domain architecture, post-translational modifications, interaction networks and disease association for each protein in the human proteome (21). In the present study, these four databases were used to predict PPIs.

Based on the above databases, a PPI network was constructed using upregulated and downregulated DEGs, and visualized using Cytoscape software (version 3.2.0) (22). Topological index of a network (degree centrality) is defined as the number of links incident upon a node (23). The score of each node was determined by degree centrality, where an increased score indicates a more important location within the network.

Investigation of modules in the PPI network

It has been previously demonstrated that genes from the same module in a PPI network serve similar roles and are implicated in the same biological functions (24). Analysis of modules in PPI networks was performed using ClusterONE package in Cytoscape software (22) using the cut-off value of P<0.001.

Enrichment analysis of the DEGs

Multifaceted Analysis Tool for Human Transcriptome (MATHT, http://www.biocloudservice.com) is an online tool that provides a comprehensive set of functional annotation tools for investigators to understand the biological roles served by large lists of genes. Gene Ontology (GO, http://www.geneontology.org/) provides structured, controlled vocabulary and classification that includes several domains of molecular and cellular biology, and is available for the scientific community to annotate genes, gene products and sequences (25). Furthermore, Kyoto Encyclopedia of Genes and Genomes (KEGG) is a knowledge base for the systematic analysis of gene functions in terms of networks of genes and molecules (26). In the present study, GO-Biological Process (GO-BP) function and KEGG pathway enrichment analyses of DEGs were performed utilizing the MATHT based on Fisher's method. P<0.05 was considered to indicate a statistically significant difference.

TF-target gene regulatory network construction

The regulation of gene expression by TFs is important. Analysis of TF binding sites is required for the investigation of gene regulation systems. In the present study, a transcriptional regulatory network was constructed based on the Overrepresentation Enrichment Analysis (ORA) method using the WebGestalt database (http://www.webgestalt.org/option.php) (8). The TF-target gene regulatory network was visualized using Cytoscape software (version 3.2.0). False discovery rate (FDR) <0.05 was considered the threshold.

Chemical-gene interaction network construction

The Comparative Toxicogenomics Database (CTD) provides manually curated information concerning chemical-gene/protein interactions, and chemical-disease and gene-disease associations (27). The interactions between chemicals and genes were determined using the CDT. Subsequently, the chemical-gene interaction network was constructed using Cytoscape software (version 3.2.0).

Results

DEGs in OVX samples compared with control samples

As large amounts of data are included in a gene expression profile, the original data was analyzed and filtered. A total of 784 upregulated and 729 downregulated DEGs were identified. A heat map of upregulated and downregulated DEGs was constructed (Fig. 1) from DNA microarray data, reflecting the gene expression values between the control and OVX groups.

Figure 1. Heat map presenting differentially expressed genes between the ovariectomized group and control group. Red, upregulated genes; Green, downregulated genes; SHAM, control group; OVX, ovariectomized group.

PPI network and module analysis

To identify potential interactions between DEGs, a PPI network was constructed based on protein interactions between DEGs. The results identified 552 nodes (genes) in upregulated DEGs, including protein tyrosine phosphatase receptor type C, protein tyrosine phosphatase non-receptor type 6, spleen tyrosine kinase, proto-oncogene vav, phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit γ isoform and γ-actin 11. In addition, 493 nodes were identified in downregulated DEGs, including polo-like kinase 1, cell division cycle 6 (CDC6), minichromosome maintenance complex component (MCM)3, ribonucleoside-diphosphate reductase subunit M2, MCM7 and baculoviral inhibitor of apoptosis repeat-containing 5 (BIRC5; data not shown).

A total of 2 upregulated and 6 downregulated modules were identified in the PPI network with P<0.001 using Cytoscape software. There were 14 nodes and 91 interactions in one of the upregulated modules (module a; P=1.173×10−7; Fig. 2A) and 9 nodes and 27 interactions in the other upregulated module (module b; P=5.498×10−4; Fig. 2B). The following number of nodes and interactions were observed in the 6 downregulated modules: 32 nodes and 254 interactions in module a (P=0; Fig. 3A), 31 nodes and 239 interactions in module b (P=8.384×10−9; Fig. 3B), 13 nodes and 78 interactions in module c (P=3.342×10−7; Fig. 3C), 13 nodes and 41 interactions in module d (P=5.799×10−4; Fig. 3D), 12 nodes and 34 interactions in module e (P=8.300×10−4; Fig. 3E) and 14 nodes and 36 interactions in module f (P=9.507×10−4; Fig. 3F).

Figure 2. Two modules of upregulated differentially expressed genes. (A) Module a with 14 nodes and 91 interactions. (B) Module b with 9 nodes and 27 interactions.

Figure 3. Six modules of downregulated differentially expressed genes. (A) Module a with 32 nodes and 254 interactions. (B) Module b with 31 nodes and 239 interactions. (C) Module c with 13 nodes and 78 interactions. (D) Module d with 13 nodes and 41 interactions. (E) Module e with 12 nodes and 34 interactions. (F) Module f with 14 nodes and 36 interactions.

Functional enrichment analysis for DEGs

To further elucidate the roles of DEGs, GO functional and KEGG pathway enrichment analyses were performed. The upregulated DEGs were primarily involved in the ‘sensory perception of smell’ (GO accession no. 0007608), including olfactory receptor (OLFR)594, OLFR1247, OLFR846, OLFR218 and OLFR1389 genes, and KEGG pathway ‘olfactory transduction’ (entry no. mmu04740), including OLFR594, OLFR1247, OLFR846, OLFR218 and OLFR1389 genes (Table I). Furthermore, the downregulated DEGs were involved in ‘DNA replication initiation’ (GO accession no. 0006270), including cyclin E2, CDC6, MCM7, and origin recognition complex subunit 5 and 6 genes, ‘cell cycle’ (GO accession no. 0007049), including fizzy and cell division cycle 20-related 1 (FZR1), kinetochore-associated 1, BIRC5, checkpoint kinase 2 and MCM3 genes, and ‘sensory perception of smell’ (GO accession no. 0007608), including OLFR1249, OLFR239, OLFR177, OLFR1098 and OLFR703 genes. Downregulated DEGs were primarily involved in KEGG pathways of ‘cell cycle’ (entry no. mmu04110), including E2F transcription factor 1, anaphase-promoting complex subunit 2, CDC6, FZR1 and extra spindle pole bodies-like 1 genes, ‘DNA replication’ (entry no. mmu03030), including MCM7, DNA polymerase α subunit 2, MCM3 and MCM5 genes, and ‘glutathione metabolism’ (entry no. mmu00480), including microsomal glutathione S-transferase 3, glutathione peroxidase (GPX)1, glutathione S-transferase κ1, GPX4 and GPX8 genes. Detailed functional analyses of downregulated DEGs in 6 modules are presented in Table II.

Table I. Results of GO function and KEGG pathway enrichment analyses of upregulated differentially expressed genes in postmenopausal osteoporosis (all the significant terms or the top 5 most significant terms of the KEGG pathways or GO_BP were listed).

Table I.

Results of GO function and KEGG pathway enrichment analyses of upregulated differentially expressed genes in postmenopausal osteoporosis (all the significant terms or the top 5 most significant terms of the KEGG pathways or GO_BP were listed).

A, GO function and KEGG pathway enrichment analyses of DEGs in the upregulated module a
ID	Analysis type	Process	Count	P-value	Genes
mmu04740	KEGG pathway	Olfactory transduction	13	5.33×10−11	OLFR594, OLFR1247, OLFR846, OLFR218, OLFR1389, OLFR1303, OLFR1209, OLFR980, OLFR1034, OLFR1500, OLFR716, OLFR205, OLFR669
GO: 0007608	GO_BP	Sensory perception of smell	14	2.41×10−16	OLFR594, OLFR1247, OLFR846, OLFR218, OLFR1389, OLFR1303, OLFR1209, OLFR980, OLFR1034, OLFR1500, OLFR716, OLFR205, OLFR504, OLFR669
GO: 0007186	GO_BP	G-protein coupled receptor signaling pathway	14	4.50×10−14	OLFR594, OLFR1247, OLFR846, OLFR218, OLFR1389, OLFR1303, OLFR1209, OLFR980, OLFR1034, OLFR1500, OLFR716, OLFR205, OLFR504, OLFR669
GO: 0050907	GO_BP	Detection of chemical stimulus involved in sensory perception	4	3.83×10−4	OLFR1247, OLFR218, OLFR1303, OLFR1209
B, GO function and KEGG pathway enrichment analyses of DEGs in the upregulated module b
ID	Analysis type	Process	Count	P-value	Genes
mmu04130	KEGG pathway	SNARE interactions in vesicular transport	6	2.17×10−11	SNAP29, STX6, STX16, VAMP4, GOSR2, STX1B
mmu04721	KEGG pathway	Synaptic vesicle cycle	3	1.30×10−3	CPLX2, STXBP1, STX1B
GO: 0016192	GO_BP	Vesicle-mediated transport	6	1.18×10−8	STX6, STX16, STXBP1, VAMP4, GOSR2, STX1B
GO: 0015031	GO_BP	Protein transport	7	3.18×10−8	COG3, SNAP29, STX6, STX16, STXBP1, GOSR2, STX1B
GO: 0006810	GO_BP	Transport	8	7.62×10−7	COG3, SNAP29, STX6, CPLX2, STX16, STXBP1, GOSR2, STX1B
GO: 0090161	GO_BP	Golgi ribbon formation	3	9.40×10−6	STX6, STX16, VAMP4
GO: 0031629	GO_BP	Synaptic vesicle fusion to presynaptic active zone membrane	3	1.55×10−5	SNAP29, STXBP1, STX1B

[i] GO, Gene Ontology; GO_BP, GO Biological Process; KEGG, Kyoto Encyclopedia of Genes and Genomes; DEGs, differentially expressed genes.

Table II. Results of GO function and KEGG pathway enrichment analyses of downregulated DEGs in postmenopausal osteoporosis (all the significant terms or the top 5 most significant terms of the KEGG pathways or GO_BP were listed).

Table II.

Results of GO function and KEGG pathway enrichment analyses of downregulated DEGs in postmenopausal osteoporosis (all the significant terms or the top 5 most significant terms of the KEGG pathways or GO_BP were listed).

A, GO function and KEGG pathway enrichment analyses of DEGs in the downregulated module a
ID	Analysis type	Process	Count	P-value	Genes
mmu04110	KEGG pathway	Cell cycle	14	4.52×10−19	E2F1, ANAPC2, CDC6, FZR1, ESPL1, CHEK2, MCM3, MCM5, CCNE2, MCM7, PLK1, TFDP2, ORC5, ORC6
mmu03030	KEGG pathway	DNA replication	4	1.07×10−4	MCM7, POLA2, MCM3, MCM5
mmu04114	KEGG pathway	Oocyte meiosis	4	3.11×10−3	CCNE2, ANAPC2, PLK1, ESPL1
mmu04115	KEGG pathway	P53 signaling pathway	3	1.40×10−2	CCNE2, RRM2, CHEK2
mmu04914	KEGG pathway	Progesterone-mediated oocyte maturation	3	2.29×10−2	ANAPC2, FZR1, PLK1
GO: 0006270	GO_BP	DNA replication initiation	9	2.01×10−17	CCNE2, CDC6, MCM7, ORC5, ORC6, POLA2, MCM3, MCM10, MCM5
GO: 0006260	GO_BP	DNA replication	12	6.83×10−17	CDC6, MCM7, GINS3, RRM2, CHTF18, ORC5, ORC6, POLA2, MCM3, MCM10, MCM5, CDT1
GO: 0007049	GO_BP	Cell cycle	16	1.41×10−14	E2F1, ANAPC2, CDC6, FZR1, KNTC1, BIRC5, CHEK2, MCM3, MCM5, CDT1, CCNE2, MCM7, PLK1, TFDP2, CHTF18, ZW10
GO: 0051301	GO_BP	Cell division	10	8.49×10−9	CCNE2, ANAPC2, CDC6, FZR1, PLK1, BIRC5, CHEK2, KNTC1, MCM5, ZW10
GO: 0007067	GO_BP	Mitotic nuclear division	9	1.59×10−8	ANAPC2, CDC6, FZR1, PLK1, KNTC1, BIRC5, ESPL1, CHEK2, ZW10
B, GO function and KEGG pathway enrichment analyses of DEGs in the downregulated module b
ID	Analysis type	Process	Count	P-value	Genes
mmu04110	KEGG pathway	Cell cycle	13	7.81×10−18	CCNE2, E2F1, ANAPC2, CDC6, FZR1, MCM7, PLK1, ORC5, ORC6, ESPL1, CHEK2, MCM3, MCM5
mmu03030	KEGG pathway	DNA replication	4	7.87×10−5	MCM7, POLA2, MCM3, MCM5
mmu04114	KEGG pathway	Oocyte meiosis	4	2.31×10−3	CCNE2, ANAPC2, PLK1, ESPL1
mmu04115	KEGG pathway	P53 signaling pathway	3	1.15×10−2	CCNE2, RRM2, CHEK2
mmu04914	KEGG pathway	Progesterone-mediated oocyte maturation	3	1.90×10−2	CCNE2, RRM2, CHEK2
GO: 0006270	GO_BP	DNA replication initiation	9	1.09×10−17	CCNE2, CDC6, MCM7, ORC5, ORC6, POLA2, MCM3, MCM10, MCM5
GO: 0006260	GO_BP	DNA replication	12	2.82×10−17	CDC6, MCM7, GINS3, RRM2, CHTF18, ORC5, ORC6, POLA2, MM3,C MCM10, MCM5, CDT1
GO: 0007049	GO_BP	Cell cycle	14	2.90×10−12	CCNE2, E2F1, ANAPC2, CDC6, FZR1, MCM7, PLK1, KNTC1, CHTF18, BIRC5, CHEK2, MCM3, MCM5, CDT1
GO: 0051301	GO_BP	Cell division	9	9.13×10−8	CCNE2, ANAPC2, CDC6, FZR1, PLK1, KNTC1, BIRC5, CHEK2, MCM5
GO: 0007067	GO_BP	Mitotic nuclear division	8	2.15×10−7	ANAPC2, CDC6, FZR1, PLK1, KNTC1, BIRC5, ESPL1, CHEK2
C, GO function and KEGG pathway enrichment analyses of DEGs in the downregulated module c
ID	Analysis type	Process	Count	P-value	Genes
mmu04740	KEGG pathway	Olfactory transduction	12	3.84×10−10	OLFR239, OLFR177, OLFR1143, OLFR556, OLFR1098, OLFR703, OLFR1265, OLFR151, OLFR392, OLFR979, OLFR1140, OLFR1153
GO: 0007608	GO_BP	Sensory perception of smell	13	3.85×10−15	OLFR1249, OLFR239, OLFR177, OLFR1098, OLFR703, OLFR1265, OLFR151, OLFR1143, OLFR556, OLFR392, OLFR1153, OLFR1140, OLFR979
GO:0007186	GO_BP	G-protein coupled receptor signaling pathway	13	4.80×10−13	OLFR1249, OLFR239, OLFR177, OLFR1098, OLFR703, OLFR1265, OLFR151, OLFR1143, OLFR556, OLFR392, OLFR1153, OLFR1140, OLFR979
D, GO function and KEGG pathway enrichment analyses of DEGs in the downregulated module d
ID	Analysis type	Process	Count	P-value	Genes
mmu00480	KEGG pathway	Glutathione metabolism	8	2.23×10−14	MGST3, GPX1, GSTK1, GPX4, GPX8, GCLM, GSTM5, GSTP1
mmu00980	KEGG pathway	Metabolism of xenobiotics by cytochrome P450	4	4.41×10−5	MGST3, GSTK1, GSTM5, GSTP1
mmu00982	KEGG pathway	Drug metabolism-cytochrome P450	4	4.83×10−5	MGST3, GSTK1, GSTM5, GSTP1
mmu05204	KEGG pathway	Chemical carcinogenesis	4	1.31×10−4	MGST3, GSTK1, GSTM5, GSTP1
GO: 0006749	GO_BP	Glutathione metabolic process	6	9.25×10−11	GPX1, GSTK1, GPX4, GCLM, GSTM5, GSTP1
GO: 0045454	GO_BP	Cell redox homeostasis	5	7.37×10−8	GPX1, GLRX5, TXNDC8, TXNRD2, PRDX3
GO: 0006979	GO_BP	Response to oxidative stress	5	1.32×10−6	GPX1, GPX4, GPX8, PRDX3, GCLM
GO: 0055114	GO_BP	Oxidation-reduction process	6	4.58×10−5	GPX1, TXNDC8, GPX4, TXNRD2, GPX8, PRDX3
GO: 0042744	GO_BP	Hydrogen peroxide catabolic process	2	9.25×10−3	GPX1, PRDX3
E, GO function and KEGG pathway enrichment analyses of DEGs in the downregulated module e
ID	Analysis type	Process	Count	P-value	Genes
mmu03008	KEGG pathway	Ribosome biogenesis in eukaryotes	4	1.18×10−5	RCL1, UTP6, CIRH1A, GNL3
GO: 0006364	GO_BP	rRNA processing	3	2.09×10−3	RCL1, UTP6, CIRH1A
GO: 0006396	GO_BP	RNA processing	2	3.97×10−2	RCL1, UTP6
GO: 0042254	GO_BP	Ribosome biogenesis	2	4.82×10−2	RCL1, UTP6
F, GO function and KEGG pathway enrichment analyses of DEGs in the downregulated module f
ID	Analysis type	Process	Count	P-value	Genes
mmu03008	KEGG pathway	Ribosome biogenesis in eukaryotes	4	1.18×10−5	RCL1, UTP6, CIRH1A, GNL3
GO: 0042254	GO_BP	Ribosome biogenesis	3	1.28×10−3	RCL1, NOA1, GNL3
GO: 0006364	GO_BP	rRNA processing	3	2.54×10−3	RCL1, UTP6, CIRH1A
GO: 0006396	GO_BP	RNA processing	2	4.35×10−2	RCL1, UTP6

[i] GO, Gene Ontology; GO_BP, GO Biological Process; KEGG, Kyoto Encyclopedia of Genes and Genomes; DEGs, differentially expressed genes.

TF-target gene regulatory network analysis

To further investigate the association between TFs and their target genes, the TF-target gene network was constructed using WebGestalt software. Using FDR<0.05 as a cut-off value, the results demonstrated that there were 187 upregulated DEGs, including high-mobility group protein, forkhead box protein P1, sodium-hydrogen antiporter 3 regulator 1 and G protein-coupled receptor kinase-interactor 2, and certain TFs, including protein ETS1, ETS-2 and other ETS family members, GA-binding protein, myc-associated factor X, Spi-1 proto-oncogene/PU1, SP1 TF (SP1) and CDC5 in this network (Fig. 4).

Figure 4. Transcription factor-target gene regulatory network. Blue hexagon represents a transcription factor, red circle represents an upregulated gene, arrows indicate the direction of regulation.

Chemical-gene interaction network analysis

Based on the CTD database, the present study revealed a total of 79,334 chemical-gene interactions identified in osteoporosis patients or in postmenopausal women. Among these interactions, a total of 850 interactions were identified between upregulated genes and chemicals (375 upregulated genes and 15 chemicals; Fig. 5). There were 781 interactions between downregulated genes and chemicals (375 downregulated genes and 14 chemicals; Fig. 6). Certain chemicals, including estradiol, resveratrol, quercetin, calcitriol, genistein and raloxifene were identified as key chemicals in upregulated or downregulated DEG-chemical networks.

Figure 5. Upregulated gene-chemical interaction network. Red circle represents the upregulated gene, blue triangle represents a chemical.

Figure 6. Downregulated gene-chemical interaction network. Green square represents a downregulated gene, blue triangle represents a chemical.

Discussion

PMOP, frequently associated with skeletal disorders among elderly women, represents a medical and economic burden (1). It has been demonstrated that estrogen serves a role in the regulation of bone turnover (5,28). However, the role of estrogen deficiency in the mechanisms underlying PMOP based on gene expression patterns has not been previously investigated. In the present study, the upregulated DEGs in modules were enriched in ‘sensory perception of smell’ function and ‘olfactory transduction’ pathway. OLFR family genes were enriched in the upregulated modules. Downregulated DEGs in modules were enriched in ‘DNA replication initiation’ function and ‘cell cycle’ pathway. A total of 8 TFs, including SP1 and ETS1, were associated with PMOP. Furthermore, estradiol and resveratrol were key chemicals in the chemical-gene interaction network.

Estrogen has an important role in bone metabolism/remodeling, and estrogen deficiency leads to bone loss, as identified in maxillary alveolar bone (29). Therefore, the reduced estrogen levels in post-menopausal women may influence the progression of bone formation (29). Bone morphogenetic proteins (BMPs) serve roles in the process of bone formation and maintenance (30). Shou et al (31) indicated that the survival of OLFR neurons was regulated by BMPs in vitro. OFLR family genes, which are expressed in the cell membranes of OFLR neurons, are members of the class A rhodopsin-like family of G protein-coupled receptors (32). Wineland et al (33) demonstrated that the regulation of BMP levels was crucial to the development and maintenance of OLFR neurons and that overexpression of BMPs led to reduced numbers of OLFR neurons. In the present study, ‘sensory perception of smell’ and ‘olfactory transduction’ were the most enriched GO functions and KEGG pathways for upregulated DEGs, respectively. The upregulated DEGs that were associated with the most enriched function and pathway all belonged to the OLFR family and included OLFR846, OLFR218 and OLFR1389 genes, which suggests that the OLFR family may serve an important role in the process of PMOP. Therefore, based on the above results, it may be hypothesized that estrogen deficiency may induce low expression of BMPs, which further leads to the overexpression of OLFR genes. However, researching concerning the association between estrogen, the OLFR family and bone development has been neglected in recent years, and further investigation is necessary to confirm the pathological mechanism underlying PMOP. Furthermore, 50 years ago, estrogen was reported to alter the duration of DNA synthesis and the cell cycle in mice (34). Recently, Javanmoghadam et al (35) indicated that estrogen receptor α regulated the cell cycle in a ligand-dependent manner. In the present study, downregulated DEGs were primarily enriched in ‘DNA replication initiation’ function and ‘cell cycle’ pathway. These results indicate that estrogen deficiency may lead to the reduced expression of genes enriched in ‘DNA replication initiation’ and ‘cell cycle’ functions and pathways, respectively, which may further influence the process of bone formation.

SP1 is a protein encoded by the SP1 gene in humans (36). SP1 functions as an osteogenic cell fate-determining factor by regulating gene expression at the transcriptional level (37). Based on a DNA microarray analysis, Xie et al (11) demonstrated that SP1 regulated the majority of upregulated DEGs from primary osteoporosis tissue samples and normal bone tissue samples. Furthermore, a large-scale gene analysis demonstrated that SP1 polymorphism was associated with reduced bone mineral density and may predispose females to incident vertebral fractures (38). ETS1, which is a member of the ETS family of transcription factors, is a protein encoded by the ETS1 gene in humans (39). Ring finger protein 11 is expressed in bone cells during osteogenesis and is regulated by ETS1 (40). Almeida et al (41) indicated that in bone stromal cell-derived lesions, protein kinase A regulates caspase 1 via ETS1. In a tissue engineering study, Sutter et al (42) revealed differential expression of ETS2 in tissue engineered bone constructs in vitro, demonstrating an association between ETS2 and osteogenesis. However, direct evidence of associations between these genes and PMOP is rare. In the present study, SP1, ETS1 and ETS2 were identified as key TFs in the TF-target gene network, which further indicated that these TFs may serve roles in the process of PMOP. The identification of the enriched TFs may improve the understanding of the mechanisms underlying PMOP.

Furthermore, several chemicals, including estradiol and resveratrol, were identified in the present study. Estradiol is a medication and a naturally occurring steroid hormone (43). Estradiol is used in hormone replacement therapies for the treatment of moderate to severe menopausal symptoms, including osteoporosis (44). Resveratrol is a polyphenolic phytoalexin that also exhibits osteoprotective and chondroprotective properties (45). Preclinical evidence from rat models of osteoporosis demonstrated that resveratrol may be used as a therapeutic agent for bone loss (46). A recent study demonstrated that oral gavage of rats with resveratrol at 1 mg/kg/day may potentially prevent bone defects (47). Therefore, these chemicals identified in the present study may be used for treatment of PMOP in the future.

However, certain limitations are associated with the present study. No animal, molecular or cellular biological experiments were conducted in the present study. The microarray data was generated by research by Calabrese et al (13), but whether the OVX and sham surgical operations were performed under identical conditions is not known.

In conclusion, functions, including ‘sensory perception of smell’ and ‘DNA replication initiation’, as well as pathways, including ‘olfactory transduction’ and ‘cell cycle’, may serve roles in the process of PMOP. Furthermore, the OLFR family genes, and SP1 and ETS1 TFs, may be involved in the progression of PMOP. Additionally, chemicals, including estradiol and resveratrol, may in the future be used for the treatment of PMOP. The results of the present study should be further confirmed by a study with a larger sample size, and by in vitro and in vivo experiments. A greater number of potential molecular pathways and genes underlying the progress of PMOP should be identified.

References