The most recurrent and considered second most frequent cause of cancerrelated death in women is the breast cancer worldwide. In breast cancer cases patients are usually diagnosed in the beginning at the curable stage. However, its treatment remains a great clinical challenge. A number of studies have been carried out for the treatment of breast cancer which includes the targeted therapies and increased survival rates in women. Essential PI3K/mTOR signaling pathway activation is observed in most breast cancers. The cell growth and tumor development in this case involves phosphoinositide 3 kinase (PI3K)/ Akt/mammalian target of rapamycin (mTOR) pathway. It has been observed, through preclinical and clinical trials, that there are a number of other inhibitors of PI3K/Akt/mTOR pathway, which either alone or in combination with other agents can be used for treatment of cancer. Pre-clinical studies have confirmed that P13K, Akt and mTOR inhibitors achieve anticancer effects by targeting different levels of the PI3K/Akt/mTOR pathway. This chapter evaluates the role of mTOR along with some of its inhibitors and the PI3K/Akt/mTOR pathway in the pathogenesis and prevention of breast cancer.

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Chapter 10
PI3Kinase/AKT/mTOR Pathway in Breast
Cancer; Pathogenesis and Prevention with
mTOR Inhibitors
Varruchi Sharma*
*Department of Biotechnology & Bioinformatics, Sri Guru
Gobind Singh College, Chandigarh, India;
Sushil Kumar Upadhyay** & Anil K Sharma**
**Department of Biotechnology, Maharishi Markandeshwar
(Deemed to be University), Mullana-Ambala, Haryana, India
ABSTRACT
The most recurrent and considered second most frequent cause of cancer-
related death in women is the breast cancer worldwide. In breast cancer cases
patients are usually diagnosed in the beginning at the curable stage. However, its
treatment remains a great clinical challenge. A number of studies have been
carried out for the treatment of breast cancer which includes the targeted therapies
and increased survival rates in women. Essential PI3K/mTOR signaling pathway
activation is observed in most breast cancers. The cell growth and tumor
development in this case involves phosphoinositide 3 kinase (PI3K)/
Akt/mammalian target of rapamycin (mTOR) pathway. It has been observed,
through preclinical and clinical trials, that there are a number of other inhibitors
of PI3K/Akt/mTOR pathway, which either alone or in combination with other
agents can be used for treatment of cancer. Pre-clinical studies have confirmed that
P13K, Akt and mTOR inhibitors achieve anticancer effects by targeting different
levels of the PI3K/Akt/mTOR pathway. This chapter evaluates the role

2. PI3Kinase/Akt/Mtor Pathway In Breast Cancer; Pathogenesis and .... 185
of mTOR along with some of its inhibitors and the PI3K/Akt/mTOR pathway
in the pathogenesis and prevention of breast cancer.
Keywords: Phosphoinositide 3 kinase, PI3Kinase, Mammalian target of
rapamycin, mTOR, Breast cancer, Signaling pathway.
INTRODUCTION
The mammalian target of rapamycin (mTOR) is an atypical serine/threonine
(S/T) protein kinase which is a central controller of cell growth, proliferation and
metabolism by forming and signaling through two protein complexes, mTORC1 and
mTORC2. mTOR complex 1/2 (mTORC1/2) are evolutionarily conserved from yeast
to mammals[1]. The introduction of mammalian target of rapamycin (mTOR)
inhibitors marked a pivotal development in the treatment of various cancers including
breast cancer. These inhibitors inhibit serine/threonine-specific protein kinase that
belongs to the family of phosphatidylinositol-3 kinase (PI3K) related kinases (PIKKs).
Oncogenic activation of the PI3K/AKT/mTOR pathway can occur through a variety
of mechanisms; this often includes mutation and/or amplification of genes encoding
RTKs, subunits of PI3K (e.g., p110β, p110α, p85β and p85α; encoded by PIK3CB,
PIK3CA, PIK3R2 and PIK3R1 respectively), AKT (AKT1), or activating isoforms of
RAS. Loss-of-expression or function of PTEN, through deletions, mutations or
epigenetic silencing, is also common. Rapamycin was first discovered in 1975 and it is
a macrolide which is produced by the microorganism Streptomyces hygroscopius and
used as an antifungal. Rapamycin had also been demonstrated to have an
immunosuppressant property[2]. In 1980s, rapamycin was also found to have
anticancer activity as evaluated by the Developmental Therapeutic Branch of the
National Cancer Institute (NCI)[3,4]. After this, rapamycin derivatives known as
rapalogs were developed having similar therapeutic effects as rapamycin but with
improved hydrophilicity and can be used for oral and intravenous administration[5].
Rapalogs are the first generation mTOR inhibitors, have been proven effective in a
range of preclinical models[6,7]. Due to partial mTOR inhibition, rapalogs are not
sufficient for achieving a broad and robust anticancer effect, at least when used as
monotherapy[8,9]. The inhibition of mTORC1 by rapalogs fails to suppress a negative
feedback loop that results in phosphorylation and activation of AKT[10]. Due to these
limitations, the second generation of mTOR inhibitors was developed[11]. The second
generation of mTOR inhibitors is known as ATP-competitive mTOR kinase inhibitors.
mTORC1/mTORC2 dual inhibitors are developed to compete with ATP in the
catalytic site of mTOR[12]. They inhibit the kinase-dependent functions of mTORC1
and mTORC2 and therefore[13], block the feedback activation of PI3K/AKT signaling,
unlike rapalogs that only target mTORC1[14]. In addition, some naturally occurring
compounds have been found to down regulate mTOR signaling. The chapter discusses
the role of PI3K/ AKT/mTOR pathway in the pathogenesis of breast cancer and
preclinical and in vitro findings with the respect to PI3K/AKT/mTOR inhibitors have
also been presented[15,16].

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mTOR PATHWAY
mTORC1 and mTORC2 are the two structurally and functionally
different complexes of mTOR Both of these complexes play very important
role in the pathway at various levels (Fig. 1). mTORC1 comprises of mainly
five components: mammalian lethal with Sec13 protein 8 (mLST8) (The mLST8
erasure does not change mTORC1 activity in vivo), Raptor (the regulatory-
associated protein of mTOR) Raptor enrolls substrates for mTOR and the
complex formation gets regulated[17], proline rich AKT substrate 40 kDa
(PRAS40), Deptor (DEP -domain-containing mTOR-interacting protein) and
mTOR[18,19]. mTORC2 complex consists of six different proteins: mTOR,
Rapamycin-insensitive companion of mTOR (Rictor), mSIN1, mLST8, Protor-
1, Deptor. Some of the main components: mTOR, mLST8 and Deptor are
shared commonlyamong both mTORC1/C2 complexes[1,20,21].
PI3K/AKT/mTOR pathway is a cell cycle regulation pathway that is a
key regulator of cell metabolism, growth, proliferation and cell survival. The
activation of the pathway starts with different cellular processes like
angiogenesis; formation of tumor etc. PI3K/AKT complex activates mTORC1
and is inhibited by the complex TSC1/TSC2 while mTORC2 is activated by
growth factors. mTORC1 regulates ribosomal formation and protein synthesis
through the phosphorylation followed by inactivation of the repressor of
mRNA translation 4EBP1 and phosphorylation and activation of S6K[22]. The
pathway is regulated by various growth factors, ATP, amino acids and
Oxygen levels[23,24]. When AKT is phosphorylated then it triggers the
mTORC1 signaling. The second messenger PtdIns (3,4,5) P3 is prodcued by
Class I PI3K, upon generation of the second messenger binds to the pleckstrin-
homology (PH) domain of AKT and PDK1. PtdIns (3,4,5) P3 to the PH domain
of AKT brings the kinase to the cell membrane formed by its activation by
phosphorylation of PDK1 at Thr308 position and by phosphorylation of
mTORC2 at Ser473 position. PTEN negatively controls AKT activation, as it
transform PtdIns (3,4,5) P3 to PtdIns(4,5) P2, resulting in reduced recruitment
of AKT to cell membrane [25,26] .

4. PI3Kinase/Akt/Mtor Pathway In Breast Cancer; Pathogenesis and .... 187
PI3K/AKT/mTOR INHIBITORS
There is a class of PI3K/AKT/mTOR inhibitors, which has a significant
role in the treatment of disease. NVP-BKM120 is a new generation of Class
1 PI3K-specific inhibitor which act upon NF-κB expression and PI3K/AKT
signaling. NVP-BKM120 is quite effective in reducing AKT phosphorylation.
PARP inhibitor alone reduces tumor growth. NVP-BEZ235 is a novel and orally
available dual PI3K/mTOR inhibitor [27]. Jolkinolide B induces apoptosis in
MDA-MB-231 cells. N-Hydroxyphthalimide (NHPI) has a capability of being
selective anti-proliferative effect on human breast carcinoma BT-20 cells
[23,28,29]. ZSTK474 is a specific and new class I phosphatidylinositol 3-kinase
inhibitor that induces G1 arrest and autophagy in human breast cancer MCF-
7 cells[30]. AZD8835, another inhibitor which is a potent and selective inhibitor
of PI3Kα and PI3Kδ.SZC015[31].Another study reported an inhibitor which
induces both apoptosis and autophagy in MCF-7 breast cancer cells[32,33].
Melittin (MEL) is a major peptide constituent of bee venom, in studies it has
been observed that this has the capability of inhibition of EGF-induced
invasion and migration of breast cancer cells[34]. ARQ 092 and ARQ 751 (next
generation AKT inhibitor) are selective, allosteric, pan-AKT[35,36].
Trisubstituted-Imidazoles pyridine has the property of targeting oncogenic
PI3K/Akt/mTOR Signaling Pathway and induced apoptosis in human breast
cancer cells[37,38]. INK128 is a novel and selective small molecule active-site
mTORC1/2 dual kinase inhibitor[24].
CONCLUSIONS
Current Chapter has mainly focused on providing a detailed information of
various components and their role in the regulation of PI3K/AKT/mTOR
pathway. Efforts have been made to understand the structure- function
relationship and regulation/deregulation of mTOR, along with modern
generation inhibitors for PI3K/AKT/mTOR pathway besides targeted therapies.
The mTOR inhibitor studies have clearly revealed their role in antitumor activities
as a result of either the activation of different components as well as some
alterations at the gene expression levels. Moreover, it will help building in a
precise and accurate platform for the personalized medicines paving a way for
understanding dreadful diseases like cancer for their proper management. The
mTOR inhibitor development needs to be further encouraged which might prove
useful to control malignancies.
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