Stanniocalcin Protein Expression in Female Reproductive Organs: Literature Review and Public Cancer Database Analysis

A search of the PubMed database was conducted for articles in the English language using the advanced search builder tool, focusing on the following key terms “stanniocalcin,” “STC1,” “STC2,” “female reproduction,” “endometrium,” “endometriosis,” “polycystic ovary syndrome,” “polyps,” “preeclampsia,” “pregnancy complications” “implantation,” “decidualization,” “ovarian cancer,” “endometrial cancer,” “cervical cancer,” “bioinformatic tools,” and “the cancer genome atlas.” The selection criteria were meticulously applied by the authors after being carefully analyzed regarding their relevance, importance, and impact. References were also sourced directly from the articles included in this manuscript.

STC proteins 1 and 2 function as paracrine factors in humans, playing multifaceted roles across various physiological and pathological processes. This review highlights the challenges posed by the ubiquity and contradictory role of STC1 and STC2 in gynecological and malignant conditions, which complicate their interpretation and limit their potential as biomarkers. Given their ubiquitous differential expression of STC1 and STC2, further investigation is warranted to elucidate their precise involvement in female reproduction, infertility, and pregnancy-associated pathologies, including implantation failure, endometriosis, PCOS, uterine polyps, compromised endometrial decidualization, abnormal placental development, and preterm birth in larger clinical study settings. However, dysregulation of these proteins in both tissue and circulation in gynecological endocrine conditions with comorbidities such as diabetes and obesity complicates the attribution of these alterations specifically to gynecological pathologies or associated issues. Given the fluctuating expression of STC across the menstruation cycle, future studies could focus on well-stratified disease groups, including menopausal women, to gain deeper insights into STC regulation. In menopausal women, drastic changes in calcium metabolism could impact STC expression levels, which might also vary with conditions such as diabetes, bone pathologies, or obesity. Understanding this dysregulation is crucial, as it may also act as a defense mechanism against the adverse effects of these pathologies including cancer. To explore the potential ubiquitous role of STC proteins as diagnostic and prognostic biomarkers in gynecological conditions, artificial intelligence-directed advanced histology technologies and in vitro disease models using induced pluripotent stem cells may offer promising opportunities over traditional histology and animal models respectively, particularly for complex diseases such as endometriosis and PCOS ( 182-184 ). Given the contradictory role of STC in cancer progression depending on tissue type, more extensive studies are needed in the context of gynecological cancers, involving larger patient cohorts and comprehensive clinical and genetic characterization. In the current review, we took preliminary steps by utilizing novel bioinformatic tools encompassing public cancer data repositories, like the TCGA, to investigate the importance of STC1 and STC2 gene expression in gynecological cancers in predicting patient prognosis. To date, data highlighting the pivotal role of STC in gynecological cancers are scarce. By utilizing intuitive bioinformatic tools and public repositories, researchers can continue to explore and understand the diagnostic and predictive values of STC genes across various cancer types, leading to improved patients’ outcomes in gynecological malignancies.

Ovarian cancer (OvCa) is the second most lethal gynecological malignancy, with approximately 239 000 new cases and 152 000 fatalities reported worldwide annually ( 147 , 148 ). The majority of patients experience poor prognosis, with over 75% diagnosed at an advanced metastatic stage, resulting in a global 5-year relative survival rate typically falling between 30% and 40% ( 149 , 150 ). The high mortality and poor prognosis associated with OvCa present significant challenges for clinicians, stemming from delayed diagnosis, advanced metastasis, and resistance to chemotherapy ( 151 ). With substantial experimental evidence, STC1 was initially identified as an HIF-1 target gene in OvCa cell lines, playing a pivotal role in reprogramming ovarian tumor metabolism, unlike other cancers ( 53 ). Significantly elevated STC1 protein level in serum and tissue samples from patients with OvCa, including cell lines, suggests its role as a potential diagnostic biomarker ( 74 , 75 ). This heightened expression is in turn associated with aggressive OvCa progression, influencing proliferation, migration, cell cycle regulation, and antiapoptotic processes ( 75 ). Moreover, Yang et al demonstrated that elevated STC1 in the malignant stroma modulates the tumor microenvironment, promoting metastasis through EMT and Akt phosphorylation ( 55 ). Another study by Bajwa et al uncovered increased STC1 localization in mesothelial cells, the primary site of metastasis, evident in both ex vivo and in vivo conditions ( 76 ). In fact, elevated STC1 in conjunction with angiopoietin-like 4 (ANGPTL4) directly modulates the tumor microenvironment, enhancing aggressive OvCa progression ( 76 ). In addition, single-cell RNA sequencing data also confirmed the prominent role of STC1 in advanced peritoneal metastasis, lipid metabolism, and resistance to cisplatin chemotherapy, possibly through the integrin β6 (ITGβ6)/PI3K/Forkhead box C2 (FOXC2) signaling axis in vitro ( 77 ). Based on these findings, STC1 is considered a potential therapeutic target, particularly for patients with cisplatin chemoresistant OvCa ( 77 ). Sevoflurane, an anesthetic, has shown favorable effects in inhibiting tumor proliferation, invasion, and migration in various tumors, including glioma, lung, colon, and EnCa, by downregulating STC1 expression ( 152 , 153 ). This downregulation reduces migration and invasion while enhancing apoptosis via Akt/mTOR/p70S6k signaling pathways and MMP9 activity. Consistently, overexpression of STC1 reversed this inhibitory effect in OvCa models both in vivo and in vitro, elucidating the potential of sevoflurane as an anticancer drug targeting STC1 activity ( 78 ). Despite the contradictory regulatory role of STC in cancer progression, a study by Law et al indicated the role of STC2 as a positive regulator in OvCa progression in vitro ( 62 ). As an HIF-1 target gene, STC2, when overexpressed, induces EMT under hypoxic conditions, consequently enhancing migration and invasion. These metastases are possibly mediated via ERK1/2 signaling pathways along with heightened levels of reactive oxygen species in OvCa in vitro models ( 79 ). A later retrospective cohort study also validated these prior findings, demonstrating that high-grade serous cancer, a lethal form of OvCa, exhibited remarkedly increased STC2 levels, positively correlating with clinicopathological factors and poor OS ( 80 ). Gene expression analysis also confirmed a direct association between STC2 and high motility gene group A2 (HMGA2) genes, contributing to aggressive OvCa progression by promoting the EMT process ( 80 ). Taken together, these data indicate the prognostic role of both STC proteins in OvCa progression. EnCa is the sixth most common gynecological malignancy, primarily localized in the epithelium of the uterine inner lining, with over 400 000 new cases and over 800 000 deaths per year globally ( 154 , 155 ). Critical risk factors for EnCa include altered steroid receptors, inflammation, obesity, family history, and older age ( 156 ). Type I EnCa, often low-grade and E2-driven, is prevalent in premenopausal or perimenopausal women. In contrast, type II EnCa, a more aggressive nonendometrioid tumor, is common in postmenopausal women, independent of E2 levels, aligning with the high copy number molecular subtypes ( 157 ). Despite the presence of STC1 in gene expression data ( 158 ), its role in EnCa progression remains unclear. In our previous study with a tissue microarray cohort of hysterectomy specimens from 832 EnCa cases, 99.15% of the cases showed positive STC1 staining primarily in the endometrial epithelium, highlighting potential tumor microenvironment modulation via the EMT process ( 81 ). Interestingly, decreased STC1 expression was linked to aggressive clinicopathologic features such as high-grade tumors, deep myometrial invasion, lymphovascular space invasion, and large tumor size. Despite the moderate association between low STC1 and the DNA mismatch repair deficiency subgroup, no association has been reported with disease-specific survival, suggesting a protective role for STC1 in EnCa progression, conflicting with findings from other studies ( 77 , 159 , 160 ). Furthermore, weak STC1 expression was also observed in women with obesity and type 2 diabetes mellitus who also had EnCa ( 81 ). Consistent with our data, elevated STC1 expression has been observed in low-grade compared with high-grade endometrioid EnCa, suggesting a potential role of STC1 as a tumor differentiation marker ( 82 ). Regarding STC2, Aydin et al reported the positive staining of STC2 protein expression with a prevalence of 73.5% in endometrioid-type EnCa samples. However, increased STC2 expression was significantly linked to grade 2 to 3 tumors and to an increased likelihood of disease recurrence ( 83 ). Moreover, their multivariate analysis data highlighted both STC2 expression and tumor grade as independent predictors of disease recurrence. While EnCa samples with high STC2 expression exhibited significantly poorer recurrence-free survival (RFS), OS remained the same regardless of STC2 expression levels. These findings underscore the elevated role of STC2 expression as a negative prognostic factor, suggesting a heightened risk of recurrence in endometrioid EnCa ( 83 ). However, none of the above-mentioned studies explored the underlying mechanism or signaling pathways linked to the involvement of STC1/STC2 in EnCa progression. A recent study by Wang et al, however, did provide mechanistic insights into the critical role of STC2 in EnCa progression ( 84 ). The study identified STC2 as an E2-responsive gene, similar to findings for breast cancer ( 161 ). According to their data, type I E2-dependent EnCa tissues showed a higher expression of STC2 than type II E2-independent EnCa tissues. In addition, E2 treatment increased STC2 expression by promoting cellular proliferation and inhibiting apoptosis in EnCa cell lines ( 84 ). Conversely, STC2 knockdown reduced cell viability and proliferation while promoting apoptosis in E2-treated cell lines. Furthermore, the loss of STC2 suppressed E2-stimulated tumor growth in vivo, suggesting that STC2 deficiency inhibits E2-stimulated proliferation and tumor growth by activating phosphorylated-AMP–activated protein kinase (AMPK) signaling, particularly in type I EnCa ( 84 ). These findings may validate the correlation between STC and hormonal regulation in EnCa progression. Cervical cancer (CeCa) ranks as the fourth most common cause of malignancy and mortality among women worldwide and is among the top 3 malignancies affecting reproductive women under 45 years of age, with variations depending on demographic factors ( 162 , 163 ). The prognosis for CeCa is poor due to a lack of understanding of the underlying cellular mechanisms in advanced metastatic or recurrent stages ( 164 ). In CeCa, the progression from cervical intraepithelial neoplasia to malignancy is linked to the persistent infection of human papillomavirus (HPV) ( 165 ). In contrast to OvCa, STC1 exhibits downregulation in tissues diagnosed with CeCa compared with noncancerous cervical tissue ( 15 ). This downregulation correlates with increased cell growth, migration, and invasion when STC1 is knocked down, while overexpression negatively regulates such cellular activities in CeCa cell lines. Furthermore, the interaction of the NF-κB p65 protein directly bound to the STC1 promoter activates the expression of STC1 in CeCa cells, indicating suppressed cell proliferation and invasion through NF-κB p65 activation ( 15 ). A follow-up study by this team revealed that heightened STC1 expression facilitated cellular apoptosis via the NF-κB phosphor-P65 (Ser536) pathway, regulated by PI3K/AKT, IκBα, and IKK signaling cascades. Conversely, silencing STC1 was found to attenuate the proliferation of both in vivo and in vitro models ( 85 ). In addition, decreased STC1 expression was noticed in advanced stages of CeCa, validating the notion that low STC1 expression is a marker of advanced-stage disease ( 85 ). Interestingly, the CeCa cell line treated with trichostatin A, an anticancer drug, showed significantly high STC1 levels with accelerated rates of apoptosis and autophagy ( 86 ). Moreover, STC1 plays a critical role in controlling the PRMT5/STC1/TRPV6/JNK axis in trichostatin A–mediated effects on CeCa cells, as evidenced by the increase in transient receptor potential cation channel-subfamily V-member 6 (TRPV6) and the decrease in p-JNK protein levels upon STC1 inhibition ( 86 ). Intriguingly, STC2 expression was reported to be much higher in tumors of patients with CeCa than in surrounding normal cervical tissues. Elevated STC2 expression is associated with shorter OS, whereas lower expression correlates with longer OS and progression-free survival after radiotherapy. Moreover, increased STC2 expression is also linked to lymph node metastasis, indicating its role as a prognostic marker in postradiotherapy follow-up in patients with CeCa ( 87 ). Consistent with these data, findings from another study also reported high STC2 expression and a positive correlation between STC2 and cellular proliferation in tissue and cell lines. Elevated STC2 levels were also reported in cisplatin-resistant CeCa cell lines, indicating resistance to platinum-based chemotherapy drugs in vitro. Additionally, silencing or overexpressing STC2 modulated cellular proliferation and apoptosis. Conclusively, the team also discovered the pivotal regulatory role of STC2 involving MAPK signaling pathways between cisplatin-sensitive and resistant CeCa cells ( 88 ).

The Cancer Genome Atlas (TCGA) offers a comprehensive genomic and proteomic molecular landscape of various cancer types, including OvCa, EnCa, and CeCa ( 166 ). Analyzing these multidimensional, diverse data necessitates sophisticated computational and bioinformatics tools. Fortunately, several web-based tools are available to assist researchers in understanding the diagnostic and prognostic implications of STC in gynecological cancers using TCGA data. Moreover, considering HPV infection as the primary risk factor for the majority of CeCa cases, including other anogenital carcinomas ( 167 ), the TCGA may offer a unique opportunity to explore the potential link between HPV infection and STC protein expression through multivariable survival analysis and explainable artificial intelligence ( 168-170 ). For instance, platforms like cBioPortal and UCSC Xena allow for an interactive exploration of multidimensional cancer genomic datasets, facilitating comprehensive analyses of genetic alterations, gene expression, and clinical data ( 171 , 172 ). Furthermore, KMplotter and GEPIA/GEPIA2 can provide crucial insights into the correlation between gene expression and patient survival, aiding in the assessment of the prognostic significance of STC gene expression ( 173 , 174 ). Additionally, tools like the UCSC Cancer Genomics Browser and UALCAN/UALCAN2 offer sophisticated visualization and analysis capabilities, allowing researchers to better understand STC gene alterations within the context of cancer phenotypes ( 175 , 176 ). Furthermore, GENI and MEXPRESS enable gene set enrichment analysis and visualization of expression, DNA methylation, and clinical data, further enriching our understanding of STC biology ( 177 , 178 ). Lastly, ExplORRNet integrates miRNA expression analysis and patient survival assessments, shedding light on the intricate regulation of STC and its prognostic implications in gynecological and other cancers ( 179 ). By leveraging these diverse and powerful bioinformatics tools, researchers can explore the diagnostic and predictive roles of STC across various cancer types, utilizing the wealth of information provided by these cancer databases. To demonstrate the utility of the aforementioned tools, we analyzed STC1 and STC2 expression and clinicopathological characteristics in OvCa, EnCa, and CeCa using web-based tools, including GEPIA and KMplotter ( 173 , 174 ). Since the number of normal tissues available for gynecological cancer comparisons in TCGA was insufficient, normal tissues were included as a reference from the Genotype-Tissue Expression (GTEx) resource ( 180 ). Our gene expression analysis revealed significant upregulation of both STC1 and STC2 in OvCa cases ( P < .05), underscoring their potential relevance in these malignancies, in line with previous research ( Fig. 2 ) ( 50 , 77 ). Contrary to the literature ( 81 , 82 ), both STC1 and STC2 expression were found to be significantly higher in EnCa cases than endometrial tissue without tumors. On the other hand, STC1 showed a downward trend in CeCa ( Fig. 2A ) ( 15 , 85 ), while STC2 displayed higher expression patterns, consistent with the literature ( Fig. 2B ) ( 87 , 88 ). However, no statistical significance was observed for either STC for CeCa ( Fig. 2 ). TCGA-based gene expression levels of STC1 and STC2 . (A) STC1 ; (B) STC2 in OvCa, EnCa, and CeCa cancers. The number of tumor tissues (T) Is represented by red bars, while the number of normal tissues (N) Is shown in grey bars. A red star indicates statistical significance with P < .05. Abbreviations: OvCa, ovarian cancer; EnCa, endometrial cancer; CeCa, cervical cancer. Generated using GEPIA website. Finally, our findings revealed significant associations between STC gene expression and survival outcomes of patients. While a thorough survival analysis utilizing TCGA was conducted for all 3 cancer types, only statistically significant associations ( P ≤ .05) for survival are depicted and discussed in this review ( Fig. 3 ). In contrast, to demonstrate the distinct expression patterns of STC1 and STC2 across all 3 cancer types, nonsignificant associations ( P > .05) for survival are presented elsewhere (Fig. S1 ( 181 )). In OvCa, STC1 expression showed a significant association with OS, with lower expression levels correlating with reduced OS (hazard ratio [HR] 0.69, 95% CI 0.52-0.93, P = .014) ( Fig. 3A ), whereas lower STC2 levels were associated with longer RFS in OvCa (HR 0.67, 95% CI 0.47-0.96, P = .027) ( Fig. 3B ). In EnCa, although the association between STC expression and RFS outcomes did not reach statistical significance (HR 0.59, 95% CI 0.35-1.02, P = .055), the findings were close to significance, suggesting potential prognostic implications for EnCa, with lower STC1 expression associated with shorter RFS time ( Fig. 3C ). Thus, further investigation is needed to understand the precise prognostic role of STC in EnCa. In CeCa, higher STC1 expression was correlated with reduced OS (HR 2.24, 95% CI 1.38-3.65, P = .00085) ( Fig. 3D ). On the other hand, although STC2 expression in CeCa showed a significant association with OS time (HR 1.71, 95% CI 1.08-2.73, P = .022), the interpretation of this association requires caution, as the difference in expression cannot be clearly observed by the survival curve ( Fig. 3E ). However, the higher expression of STC2 in CeCa is correlated with longer RFS time (HR 2.87, 95% CI 1.31-6.27, P = .0056) ( Fig. 3F ). Overall, our TCGA-based expression and survival analysis highlights the potential of STC gene expression as a prognostic marker in CeCa and OvCa ( 20 , 47 , 65 ). TCGA-based Kaplan–Meier survival plots of STC1 and STC2 . (A) OS of STC1 in OvCa; (B) RFS of STC2 in OvCa; (C) RFS of STC1 in EnCa; (D) OS of STC1 in CeCa; (E) OS of STC2 in CeCa; (F) RFS of STC2 in CeCa cases. STC expression levels are shown as high (red) and low (black) accompanied by the number of cases (n). The median follow-up for OS and RFS was 200 months for both EnCa and CeCa, while that for OvCa was 150 months due to data availability. Abbreviations: OS, overall survival; RFS, recurrence-free survival; OvCa, ovarian cancer; EnCa, endometrial cancer; CeCa, cervical cancer; HR, hazard ratio; statistical significance with P = .05. Generated using KM plotter website.