INTRODUCTION:

Cancer is diseases that cause an alteration in cells of the body which result in them to divide in an uncontrolled way. Cancer cells have a distinct feature to form lump or mass, also called a tumor¹. Breast cancer has highest prevalence among all cancers which causes death in women globally². Approximately 231,840 invasive and 60,290 non–invasive breast cancer cases were detected in 2015. (Table 1). Many factors contribute initiation and progression of breast cancer. Breast cancer risk is directly proportional to the amount of estrogen exposed³. The frequency of breast cancer has increased due to elevated pollution, change in physical activity, reproductive patterns, and obesity. The attributes of cancer include unrelenting proliferative signaling by suppressing growth factor resulting continuous replication. It also induces angiogenesis and activates invasion and metastases⁴.

Phenotype with high aggressiveness is due to the acquisition of genetic mutation. The relative risk of breast cancer is more than 10 if associated genes are BRCA1, BRCA2, and p53. However, impact on the hereditary gene is only 0.1% as a result of variations in these genes. High-penetrance genes have very low impact on breast cancer as compared to random genetic mutation. Development of breast cancer also depends on polymorphism occurring in the least expressed gene⁵.

Table 1: Breast Cancer Incidence accordance of Age in United States (2015)

Age Group	Non -invasive Cases	Invasive Cases
Less than 40	1,650	10,500
Between 40 and 80	54780	192380
More than 80	3,860	28,960
All ages	60,290	231,840

ERα Breast cancer:

Estrogen Receptors (ER) are transcription factors which come under the superfamily of nuclear receptors⁶. ERα is expressed by distinct genes present on chromosome 6 and ERβ is expressed on chromosome 14^7-9. Initiation and proliferation of breast cancer is mainly associated with ERα but receptors of both ER are present in normal breast cancer. ERβ function in breast cancer is still unidentified¹⁰. ER positive breast cancer is the widespread metastatic incurable disease. Estrogenic signaling is the most critical oncogenic pathways known in breast cancer but specific therapy for controlling endocrine resistance is yet to be discovered¹¹. For endocrine therapy, the degree of expression of estrogen receptor plays a very crucial role as it is directly associated with estrogen dependent growth¹². Progression of hormonal carcinogenesis depends on both the estrogen receptor subtypes (ERα and ERβ) by exerting specific functions. In breast cancer, ERα is related with unusual proliferation inflammation and with the expansion of malignancy. Expression of the ERα-regulated gene is altered by ERβ in order to resist the action of ERα on cell proliferation. It also opposes properties of migration and invasion in breast cancer cells. High level of ERβ expression has been associated with enhanced survival¹³. Gene expression mediated by ER affect multiple factors which account for the negative and positive effects of estrogens and anti-estrogens. Disturbance in ER subtype-specific expression has been discovered in alternative stages of cancer, with the levels of ERα and ERβ has been reduced in most cancers as the disease progresses. ERα is the most prominent biomarker in breast cancers in endocrine therapy. Apart from the role of ER signaling in binding property with ligand, other activity such as ubiquitin ligases, growth factor signaling, chaperones and methylases are under examination¹⁴.

Significance of LMTK3:

Lemur tyrosine kinase 3 (LMTK3), a member of receptor tyrosine kinase (RTK) family, is a key protein associated with breast cancer growth. It has been hypothesized to Darwinian positive selection. It downregulates the protein kinase C (PKC) activity and phosphorylates AKT (Ser473), which enhance the binding effect of forkhead box O3 (FOXO3) to the ESR1 promoter. It has a substantial role in breast cancer development as its increased expression associated with poor overall and disease-free survival¹⁵. LMTK3 is overexpressed in triple negative breast cancers and it is associated with focal adhesion formation, actin cytoskeleton remodeling, and increased cell motility. High expression of LMTK3 allows the dispersal and invasion of cancerous cells into the matrigel and collagen. On the edge of migrating cells, it escalates formation of focal adhesion and actin protrusions¹⁶.

Interaction of FOXO:

FOXO is a major protein belonging to the O-subfamily which act as a transcription factor and regulates various gene expression involved in biological processes. The important feature of breast cancer cells is nuclear exclusion which is altered by oncogenic PI3K–AKT signaling. FOXO alters the function of estrogen activity and facilitates a specificity in hormone independent breast cancer¹⁷. FOXO3a, one of the members of FOXO family, interacts with forkhead box M1 (FOXM1) located in the ERα promoter for regulating the expression of ERα^18-20. Activation of FOXOs results in suppression of a variety of cell types which promotes apoptosis in cells²¹. In vivo FOXO3a and ER interaction plays an important role in suppressing tumorigenesis and cell growth which is mediated by estrogen-dependent breast cancer²².

VEGF in the Regulation of Breast Cancer:

In breast cancer, VEGF is regarded as most important mediators of tumor angiogenesis. Estrogen interacts VEGF at the transcriptional level by modulating its expression in breast cancer cell²³. It acts as an angiogenic enhancer which induces the proliferation of new blood vessel. The level of VEGF is high in patients which are unable to respond to hormone therapy or those who have an early recurrence. In response to estrogen or progesterone, hormone dependent tumors secrete VEGF which exhibit strong interaction of steroid hormone which promotes growth in breast cancer. VEGF is a key factor in tumor expansion and its survival²⁴. In serum deprived form, it up- regulates the expression of Bcl-2, which is one of the regulator protein in order to protect the cell from undergoing apoptosis. Reducing VEGF levels can inhibit the growth of tumor cell by suppressing the expansion of endothelial cell facilitating tumor cells death. It has been stated that Bcl-2 induced by VEGF may have an extensive role than simply acting as an anti-apoptotic protein. In MCF-7 breast cancer cells, ER induces VEGF which is responsible for cell survival and Bcl-2 activator. High production of VEGF also contributes to the procurement of anti-hormonal resistance^25,26. VEGF is primarily a mitogen which is specific to an endothelial cell whose proliferative response depend on MAPK/ERK signaling pathway. Trimodal therapy may be considered as a beneficiary method in breast cancer as it can target VEGF production, VEGF receptors and the MAPK/ERK signaling pathway altogether. Other advantages of this therapy include anti –hormone resistance and blocking proliferation of tumor which lacks steroid receptors²⁷.

Importance of HER2 (ErbB2):

The HER2 (ErbB2) encodes receptor tyrosine kinase, is a member of epidermal growth factor receptor which belongs to the superfamily of transmembrane receptors²⁸. It is the regarded as a key biomarker in the phosphotidylinositol-3-OH kinase (PI3K)/Akt/mTOR signal transduction pathway. It is the major signaling pathway by the erbB family of receptors which control various cellular processes like proliferation, differentiation, and apoptosis²⁹. HER2 occur in one –fourth of the total human breast cancer, predicting poor clinical outcome^30,31. Due to its availability of extracellular portion and its role in breast cancer pathogenesis, HER2 is known as a promising candidate for targeted antibody therapy³². Patients who were diagnosed of HER2 presence have a disease with more aggression, greater involvement of lymphatic nodes, decreased expression of ER and elevated endocrine treatment resistance³³. One-third of total patients having breast cancer overexpress human epidermal growth factor receptor 2 (ErbB2 or HER2) which promotes metastasis, aggressive tumor phenotype and decreases survival rate of patients^34-36. Discovery of specific molecular factors controlling HER2 signaling is crucial for more effective and targeted therapy against breast cancer with positive HER2³⁷. Amplified HER2 breast cancers have elevated the amount of sensitivity to particular cytotoxic chemotherapeutic agents although resistance persists for certain hormonal agents³⁸. Breast cancers which have HER2 gene copy which can be goes up to 50 copies can increase the HER2 protein expression 40 to 100 times^39,40. Overexpression of HER2 protein under breast cancers is achieved by amplifying HER2 gene. Breast Cancer with HER2-positive is more intrusive, however, prognosis in patients has been enhanced by targeted therapy⁴¹.

Disposition of Tumor Suppressor Gene:

A tumor suppressor gene is an anti-oncogene that guards a cell against cancer. The mutation of this gene cause functional loss, consequently the cell form cancer with the assistance of other genetic changes⁴². Tumor suppressor genes are growth-inhibitory proteins involved in a variety of cellular processes like replication, recombination, signal transduction, cell cycle, aging differentiation and repair⁴³. Loss of tumor suppressor gene leads to deregulated cell proliferation⁴⁴. Growth and progression of the tumor are due to progressive changes at the level of DNA. Tumor suppressor gene can oppose tumorigenic activities at various levels and may block specific stages in the development progress⁴⁵.

Intrinsically disordered proteins and their function:

Intrinsically disordered proteins (IDPs) plays a crucial part in the cellular signaling which allows polypeptide to initiate different interactions with significance change. The main function of IDPs is to participate in accumulating signaling complex and also initiate self-assembly of cytoplasmic and nuclear organelles. IDPs enhances complexity to the regulatory network by participating in post translational modification and splicing. It also provides mechanism for tissue specific signals⁴⁶.Intrinsic disordered proteins (IDPs) are proteins which have non -existing 3D structure entirely or partially in solution^47-49. This structure loss has significance advantages^50-55. It provides a surface area which is large as compared to globular proteins of the same length. IDPs interact with numerous other proteins by conformational plasticity, exposure of short conserved protein motifs and structural motifs susceptible to interaction which enable regulation of their stability and function in a cell by diverse post translational modifications. IDPs interact with their targets with least affinity and high specificity as they fold on binding which allows them to assimilate rapidly and initiate a signaling process, however, it dissociates easily at the same time when the task is completed⁵⁶.

CONCLUSION:

Breast cancer is the prime cause of death outcome among women and it is designated as most common malignancy. It is also frequently investigated diseases whose inherent biological features are still not well clarified. Most of the breast cancers are incurable which possess major concern globally. Numerous therapeutic strategies have been suggested, such as molecular targeted and endocrine therapy to treat breast cancer. Molecular pathways that control the sensitivity toward hormone in breast cancer cells is one of the important approaches to improving the efficacy of endocrine therapy. Identifying genetic variation that regulate the function of estrogen receptor can assist in the development of strategies targeting breast cancer. Understanding the intrinsic factors underlying the procurement of breast cancers allows for the prediction of outcome as well as it helps in identifying alternative novel and unique therapeutic strategies to suppress or break the endocrine resistance. Designing treatment strategies that precisely target apoptosis and cell cycle checkpoints in cancer cells by characterizing the mutation in genetic determinant in breast cancer without affecting normal cells eventually leads to declined death rate from cancer.

ACKNOWLEDGEMENTS:

The authors take this opportunity to thank the management of VIT University for providing the facilities and the encouragement needed to carry out this work.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

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Received on 28.06.2017 Modified on 17.07.2017

Research J. Pharm. and Tech 2018; 11(8): 3543-3546.

DOI: 10.5958/0974-360X.2018.00654.6