Authors: Nimrah Inam  ( Department of Paediatrics, Aga Khan University Hospital )
Ayesha Hameed  ( MBBS Graduate, Jinnah Medical and Dental College )
Lubna Vohra  ( Department of Breast Surgery, Aga Khan University Hospital )
Sana Zeeshan  ( Department of Breast Surgery, Aga Khan University Hospital )

One of the bone’s morphogenic protein (BMP) antagonists, Gremlin-1 or GREM-1, can bind directly to BMPs. GREM-1 can act in either BMP-dependent or -independent pathways, according to research. It reinforces organogenesis, tissue differentiation, and organ fibrosis. Recent research from numerous studies has demonstrated the significance of GREM-1 in the initiation, progression, and even metastasis of different cancers, including breast, cervical, gastric, and colorectal cancers. This review highlights the function of GREM-1 in the development of breast cancer and its effect on the cellular procedures and signalling pathways involved in carcinogenesis.

 

Keywords: Bone Morphogenetic, Carcinogenesis, Organogenesis, Colorectal Neoplasms, breast cancers, stem cells.

 

DOI: 10.47391/JPMA.AKUS-25

 

Introduction

 

Breast cancer is the most frequently found cancer in women worldwide, with significant advancements in its diagnosis and treatment in past decades¹. Breast cancer is among the types with a high risk of relapse and metastasis, but early detection responds positively to the treatment.². Estrogen and estrogen receptors are commonly seen in the progression of breast cancer, which is treated by giving selective estrogen receptor modulators^3,4. The most overexpressed receptor is the HER2 protein, which is the biomarker used in the treatment of HER2-positive breast cancer³. Despite breast cancer being the most common type of cancer worldwide, it still lacks research on targeted therapies for triple-negative cancers (lack of HER2, estrogen, and progesterone receptors)⁵. Unfortunately, identifying metastasis and recurrence-associated molecular markers of breast cancer remains obscure, thereby necessitating the need to identify therapeutic targets and mechanisms of action associated with breast cancer metastasis⁶

Bone morphogenetic proteins (BMPs) are tumour suppressor or oncogenes which plays a vital role in bone formation and other biological processes such as organogenesis and tissue differentiation⁷. Disadvantageously, the BMP antagonists bind directly to the BMP ligands and inhibit the BMP-induced signalling pathways⁸.

Gremlin-1 or GREM1, a BMP antagonist, has a vital role in organ development, organ fibrosis , tissue differentiation and bone formation9. On the other hand, it is also involved in inflammation¹⁰, and BMP-dependent or independent cancer^11,12. GREM-1 is involved in breast cancer growth and is associated with a poor survival rate¹¹ (Figure 1).

 

 

It is also found to be responsible for causing metastasis¹³. Epithelial-mesenchymal transition (EMT) is a phenomenon in which epithelial cells lose contact with each other and become mobile mesenchymal cells which promote cell migration and tumour growth under the influence of GREM-1¹⁴. It is found that GREM-1 also mimics the effects of vascular endothelial growth factor (VEGF) on the VEGF receptor-2 (VEGFR-2) in endothelial cells¹⁵. Recent clinical research has shown the correlation between GREM-1 and poor prognosis in various cancers, such as breast and colorectal cancer, where GREM-1 provides a route to the activation of PI3K/AKT/mTOR and antagonistic BMP2 signalling pathways¹⁶. Similarly, GREM-1 has a functional part in regulating EMT and the sensitivity of hepatoma cells to drugs like sorafenib in hepatocellular carcinoma¹⁷.

In this review, we have highlighted GREM-1 and its correlation with breast cancer and stem cell maintenance via different pathways to better understand its mechanism of action. 

 

Methods

 

The literature search was done on PubMed, Google Scholar, Scopus, and Embase. Mesh terms used for the search were “GREM-1”, “Gremlin-1”, “breast cancer” and “breast neoplasm. We screened 290 articles, published during 2010 to 2022 and identified 25 which had focused on the role of GREM-1 in breast cancer stem cells.  The review focused on questions like the role of GREM-1 in different pathways, progression, and microenvironment of breast cancer stem cells, recent advancements, and GREM-1 as a marker for prognosis.

 

GREM-1 and BMP correlation

 

BMPs (Bone morphogenetic proteins), the subfamily of TGF- β (Transforming Growth Factor Beta), are extracellular proteins that play an important role in organogenesis and tissue differentiation⁸. BMPs act on cognate transmembrane serine/ threonine kinase receptors which result in the accumulation of activated R-SMADs (SMAD 1/5/8) (Receptor -Regulated Suppressor of mothers against decapentaplegic) that causes further transcriptional responses¹⁸ (Figure 2).

 

 

BMP antagonists such as Noggin¹⁹, Coco²⁰, and GREM-1²¹ disrupt BMP signalling in cancers like breast cancer²². GREM-1 inhibits BMP 2, BMP4, and BMP7¹⁵. After using In Situ hybridization (ISH), levels of GREM-1 were detectable in CAFs (fibroblast-like cells). In the study they were not detectable in cancer-adjacent normal tissues, adjacent cancer-free breast tissues, or epithelial cells of breast cancers²². , Intriguingly enough in a 2019 study, GREM-1 was found to be elevated in metastatic cancer cells even with increased co-expression of a BMP protein in tumour cells, exhibiting  a direct link to decreased relapse-free survival (RFS) in ER-negative breast cancer cells (HR = 1.51 (1.2–1.9), p-value = 0.00037)^13,21, ER-positive (HR = 1.19 (1.01–1.4), p-value = 0.035), human EGF receptor (HER)2+ and triple− (in all subtypes combined HR = 1.32 (1.18–1.47), p-value = 6.9e− 07), making GREM-1 a poor prognostic marker in all breast cancer subtypes²². Additionally, GREM-1 was one of the eight elevated genes found in invasive breast carcinoma patients compared to ductal carcinoma in situ patients²³, recommending that cancer cells form GREM-1 on their own and are of an invasive phenotype¹³.

 

Role of GREM-1 in maintaining cancer cell stemness

 

Epithelial-origin cancer cells attain the undifferentiated form of stem cells by GREM-1 and also involved in the EMT process²². CD44+/high CD24−/low cells are known to stem population markers of breast cancer cells²⁴. GREM-1 increases the expression of YAP, TAZ, SOX2, and OCT4 transcriptional regulators, thereby conserving breast cancer stemness and enhancing Mammo sphere formation²². GREM-1 generates epithelial to mesenchymal transition, a process where epithelial cells transition to mobile mesenchymal cells causing metastasis and cancer cell invasion¹⁴ which aids in sustaining stem cell properties²⁵.

Many in vitro studies done in the past concluded that breast cancer stem cells are responsible for radiation and chemotherapy resistance²⁴. The reason possibly is a decrease in pro-oxidants in CD44+/CD24− cells and the neoadjuvant trial design, which   hypothesised that breast cancer stem cells are the reason for cytotoxic chemotherapy resistance²⁵. After neoadjuvant chemotherapy, the number of atmospheres forming cells and CD44+/CD24− expressing cells increases²⁴.

GREM-1 and VEGFR-2 Angiogenesis is augmented by the VEGF  which helps  endothelial cellular proliferation, differentiation and enhancing microvascular permeability and vasodilation²⁶. Sometimes GREM-1 mimics the role of VEGF and binds with VEGFR-2, which leads to a cascade of intracellular events which causes neovascularization and angiogenesis^27,28. Thus, further research studies are required on the Gremlin-1/VEGFR2 axis as it is considered an excellent therapeutic target²⁹.

 

GREM-1 and Matrix Metalloproteinases (MMPs)

 

MMPs are endopeptidases whose level increases in most cancers, causing tumour proliferation, survival, angiogenesis, and metastasis by breaking down the ECM (extra-cellular matrix) and then finally modulating the tumour stroma¹⁵. There are many types of MMPS of which MMP13 is noted to be involved in the metastasis of breast cancer cells by Golgi membrane protein 1¹⁵. Correction analysis revealed a positive relation between GREM-1 and MMP13²¹. Sufficient literature is available indicating that the GREM-1/MMP13 axis hampered the breast cancer tumour^6,21,30,31. GREM1 expression was compared with breast cancer and normal breast tissue using the Oncomine microarray database and other genes associated with angiogenesis, and metalloendopeptidase activity. The p-value calculated was 2.35 × 10−4 with a median rank of 27²¹.

 

Role of GREM-1 in glycolysis of breast cancer cells

 

Grem 1 overexpressing breast cancer cells have been found to increase glucose uptake and lactate production via glycolysis³², which leads to the lowering of the pH within the tumour microenvironment³³, considered to be a hallmark of cancer³⁴.

GREM-1 increases the expression of HK2³². Moreover, it stimulates the STAT3 transcription factor by the ROS-Akt signaling pathway³². Th.en ROS-Akt-STAT3 axis stimulates the GREM-1 which increases the activity of HK2 in breast cancer cells and there is increased uptake of glucose³².

 

GREM-1 and ER-negative breast cancer

 

It has been shown that overexpression of GREM-1 in ER-negative breast cancer is linked to worse survival¹¹. Few studies suggest that increased levels of GREM-1 had an association with oestrogen receptor expression¹³. It was found that all cell lines releasing GREM-1 lacked oestrogen receptor signalling¹³. In a recent study, overexpression of MMP 13 had a direct impact on reduced metastasis-free survival to a large extent in ER-negative breast cancer²¹.

 High mRNA levels of CRIM-1, GREM-1, and SMAD 6 (extracellular BMP antagonist) correlated with decreased relapse-free survival in ER-negative breast cancer¹³. In contrast, low mRNA levels of BMPER, CHRDL1, CRIM1, and SOSTDC1 correlated with reduced relapse-free survival in ER-positive breast cancer, suggesting that GREM-1 expression is linked to aggressive tumour progression.¹³ In a study done by Park et al, GREM-1 mRNA was closely linked to RFS and distant metastasis-free survival (DMFS) in ER-negative breast cancer patients. ER- negative breast cancer patients were found to have high expression of Grem 1 supporting poor prognosis in this particular subtype (shown in table 1)¹¹ .

 

 

Conclusion and Future Direction

 

Research on GREM-1 is of great global importance since breast cancer is the most common cancer in women with variable survival outcomes. Multiple studies have highlighted GREM-1 as the predominantly overexpressed mRNA in most cancer development with a dominant part in stemness and progression of breast cancer cells. It is directly linked to enhancing multiple pathways necessary for metastasis and cancer cell survival. With all the advancements and clinical trials, the crucial details of GREM-1 remain to be explored and understood better. From the pharmacological aspect, GREM-1 is a possible pharmacodynamics target for many cancers, including breast cancer, due to its multifocal role on a molecular level. However, no drug is designed to target GREM-1 for breast cancer. Further functional research is much needed to fully understand its role in chemotherapy and radiation resistance to increase patients’ survival and quality of life.

 

Disclaimer: None to declare.

 

Conflict of Interest: None to declare.

 

Funding Disclosure: None to declare.

 

Abbreviations

 

BMP: Bone Morphogenetic Protein

 

GREM1: Gremlin-1

 

VEGF: Vascular Endothelial Growth Factor

 

STAT: Signal Transducer and Activator of Transcription

 

EMT: Epithelial-Mesenchymal Transition

 

RFS: Relapse-free survival

 

DMFS: Distant metastasis-free survival

 

ERR α: Estrogen-Related Receptor α

 

TGF- β: Transforming Growth Factor Beta

 

R-SMAD: Receptor -Regulated Suppressor of mothers against decapentaplegic

 

ISH: In Situ hybridization

 

CAF: Cancer-associated fibroblast

 

ECM: Extra-cellular matrix

 

 

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