5. EORs involved in diseases
Some EORs may show no or very low expression in healthy tissues.
Nevertheless, they can show high expression in cancer and diseased
tissues, which might represent such kinds of EORs as potential
biomarkers for pathogenesis.
5.1 Breast Cancer
OR2B6, a tissue-specific EOR in breast cancer, has been found 73% and
80% expression of OR2B6 in breast carcinoma cell lines and carcinoma
tissues respectively. The upregulated expression of OR2B6 was found in
blood platelets of tumors from breast cancer patients. OR2B6 can also
express mutually with a histone gene HIST1H2BO and build a fusion
transcript together in carcinoma tissues. However, healthy tissues
hardly show any expression of OR2B6 that makes it to be a probable
biomarker for breast cancer. OR2B6 can also express in several carcinoma
tissues, but not as remarkable as breast carcinoma tissues (Weber et
al., 2018a). OR2W3 and OR2T8 are also highly up-regulated. These
three EOR genes are “over-expressed” in breast cancer tissues
(Masjedi, Zwiebel & Giorgio, 2019).
5.2 Bladder Cancer
Bladder cancer tissues show a significant expression of OR10H1 compared
to the normal bladder. The triggering of this receptor can change the
morphology of cytoskeletons that can be identified β-Catenin,
T-cadherin, and β-actin staining. Stimulation by sandranol blocks cell
migration and proliferation, implies cell cycle arrest and leads to a
limited extent- apoptosis. Sandranol inhibits adenylyl cyclase and thus
reduces cAMP levels which evoke an increase of intracellular
Ca2+ concentration (Weber et al., 2018b).
5.3 Lung cancer
RSK1 silencing increases tumor metastasis in non-small-cell lung cancer
(NSCLC) tissues in humans. This cell shows very strong positivity about
the expression of OR2J3. This receptor also can start the release of
Ca2+ from intracellular Ca2+ stores
(Kalbe et al., 2017). Functional imaging and immunohistochemical
studies show quite stable and high expression of OR51E1 in lung cancer
cells. Due to the extensive membrane localization, OR51E1 can be
considered as a novel therapeutic target against available Somatostatin
receptors (SSTRs). Moreover, some tumor patients do not respond to SSTRs
based diagnosis (Giandomenico et al., 2013)
5.4 Colorectal Cancer
In 2017, Weber et al. found extraordinarily over-expression of
OR51B4 in colorectal cancer tissues confirmed by shRNA mediated
knockdown. In HCT116 cells, Troenan can stimulate anti-proliferation,
anti-migration, and pro-apoptosis by PLC activation and intracellular
calcium level changes. This results in phosphorylation levels changes of
p38, mTOR and Akt kinases (Weber et al., 2017). Cancer initiating
cells (CICs) in colon expresses OR7C1, which results in higher
tumorigenicity. Peptide specific cytotoxic T lymphocyte (CTL) antigen
for OR7C1 is toxic to CICs (Morita et al., 2016).
5.5 Myelogenous leukemia
By next-generation sequencing, a recent study shows that chronic
myelogenous leukemia (CML) cell express OR at a high rate, specifically
OR51B5. Isononyl alcohol activates OR51B5 and increase intracellular
Ca2+ level in acute myelogenous leukemia (AML)
patients. OR51B5 can inhibit cell proliferation in both AML and CML
patients by reducing the phosphorylation of p38MAPK (Manteniotis et
al., 2016b).
OR2AT4 can increase the phosphorylation of p38-MAPK that leads to
leukemia. This receptor expresses highly in human myelogenous leukemia
(Manteniotis et al., 2016a)
5.6 Hepatic diseases
OR1A2, which is paralogous to OR1A1, is a potentially expressed
olfactory receptor in hepatic cancer cells. (-)citronellal activates
OR1A2 which increases cytosolic Ca2+ level via the
cAMP-dependent pathway and reduces cell proliferation by p38MAP
phosphorylation (Meβberg et al., 2015). As a null variant, OR1B1
gene influences liver cell metabolism by reducing serum cholinesterase
activity. It aids to effect significantly in liver autoimmune disease
(Koyano et al., 2008).
5.7 Retinitis Pigmentosa
Definite mutation in OR2W3 gene is highly related to an ocular disease
called Retinitis pigmentosa (RP) (Ma et al., 2015). RP is an inherited
autosomal dominant retinal disease, which is rare, reported only one of
3000 to 5000 peoples (Zhang & Huang, 2015). OR2W3 is located in the
photosensitive outer membrane of cone cells. As it does not fuse with
Trim58 transcript, it can be concluded that it possibly has a
physiological function in the human retina (Sharon, Kimchi & Rivolta,
2016).
5.8 Neurological Disorders
Neurodegenerative and neuropsychiatric disorders evaluation in recent
days proves the relation with dysregulated OR gene expression. Several
ORs (OR2L13, OR1E1, OR2J3, OR52L1, and OR11H1) have been identified to
be down-regulated in the early stages of Parkinson’s disease
pathogenesis (Garcia-Esparcia et al., 2013; Grison et al.,2014), which may lead to their undeniable importance in the development
of the disease. Patients samples with Alzheimer’s disease (AD),
Creutzfeldt-Jakob disease, and progressive supra-nuclear palsy present
differentially regulated OR gene expression (Ansoleaga et al.,2013). Among the verified ORs in AD patients, half showed altered
expression in the cortical region. OR11H1 supposed to be up-regulated,
while OR4F4, OR52L1, and OR10G8 expression are reduced and linked up
with disease development (Ansoleaga et al., 2013; Woodward et
al., 2017).
Downregulated ORs in cerebral regions leads to chronic schizophrenia
(Ansoleaga et al., 2015) and traumatic brain injury (Zhao et
al., 2013). For biomarker analysis, the detection of both OR4M1
and OR11H1 can have a potential diagnostic feature in the near future
(Maßberg & Hatt, 2018). Down-regulation of OR2L13, OR2T33, OR2J3,
OR52L1, OR10G8, OR11H1 and OR4F4 in frontal cortex has been detected in
the early stages in Parkinson’s disease (Grison et al., 2014).
Olfactory receptor gene cluster on 14q11.2 region containing OR4M1,
OR4N2, OR4K2, OR4K5, and OR4K1 shows modifications in their expression
at the earlier age of Alzheimer’s disease (Ansoleaga et al.,2013).
5.9 Urological disorders
OR51E1 and OR51E2 are the most distributed and expressed ectopic OR.
They make themselves noble biomarkers by expressing higher in cancer
tissues than healthy tissues. They were first introduced as a
significant OR, expression restricted to prostate adenocarcinoma and
named as PSGR (OR51E2) and PSGR2 (OR51E1) (Xia et al., 2001;
Xu et al., 2006). PSGR locates in chromosome 11p15 encoding 320
amino acid-containing proteins (Xu et al., 2000). Its mRNA level,
correlated with prostate-specific antigen (PSA), raises remarkably
prostate intraepithelial neoplasia (PIN) and Prostate cancer (PCa)
(Xu et al., 2006). As it is available in human urine sediment, it
can be used as an alternative biopsy for prostate cancer (Rigau et
al., 2010). According to Cao et al., PSGR can boost up
cell proliferation invasion and later on metastasis in prostate cancer
(Cao et al., 2015). The elevated expression refers to an early
alteration of PCa while low expression reveals poor prognosis. Both
OR51E1 and OR51E2 can be linked with PCa marker alpha-methyl-CoA
racemase (AMACR) (Wang et al., 2006). Therefore, a dual marker
can be a good identifier to ensure the development of prostate cancer.
Besides, PSGR can be fused up with erythroblast transformation specific
(ETS) transcription factor (ETV-1) chimerically and show important
positive effects on the elevation of prostate cancer (Weng et
al., 2005; Barros-Silva et al., 2013).
5.10 Others
OR51E1 can also be a potential biomarker for the detection of
somatostatin receptor-negative lung carcinoids (Giandomenico et
al., 2013) and small intestinal neuroendocrine carcinomas
(Leja et al., 2009; Cui et al., 2013). A recent study
mentioned that peptide derived from OR51E2 can behave as
tumor-associated antigen (TSA), detected by CD8+ T-cells, in various
cancer cells including melanoma (Gelis et al., 2017).
6. Possible mechanisms of
EORs
Even EORs are not involved in smell and neural perception; they share
the same structure with ORs in the nasal epithelium as well as their
mechanisms. The signaling of EORs is hypothesized by the fragmentary
involvement of cAMP, recommending heterotrimeric G-protein (Golf
protein) as a strong stimulatory that widely expressed in human tissues
(Flegel et al., 2013; Busse et al., 2014).
6.1 The initial process for EORs activation
When a ligand binds to the OR, the 7 transmembrane receptor initiates
and converts its conformation based on the interaction between GPCR and
G-protein. The Gα subunit presumes a triggered conformation upon
GTP-binding and dissociates from receptor and Gβγ. When GPCR activates,
Gα releases from GDP. “Empty pocket” of G-protein and the receptor
bridges with a high-affinity complex, which can be described with
“action at a distance” hypothesis elaborated by Oldham & Hamm (2008).
The conversion of GTP from GDP results in dissociation of Gβγ dimer from
Gα that further starts intracellular signaling as “second messenger”
(Sprang, 2007). This second messenger can initiate or inhibit other
elements of cell mechanisms. For instance, Phospholipase C enzyme can
hydrolyze phosphatidylinositol 4,5-bisphosphate (PIP2) to
1,2-diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). DAG can
actuate protein kinase C isoforms. IP3 can connect with receptors which
lead to calcium release in the cytosol. A large number of the second
messenger including cyclic AMP, cyclic GMP, calmodulin, and kinases can
be modulated by G proteins. When GTP hydrolyzes into GDP by GTPase
activity of the Gα subunit, inactive Gαβγ is formed by the
re-association of Gα-GDP and Gβγ. Evidence implicates that there is no
physical disassociation of G-protein from the complex (Frank et
al., 2005; Digby et al., 2006).
An activated G-protein can co-localize with regulatory factor Resistance
to inhibitors of cholinesterase 8B (Ric8B) in olfactory sensory neurons.
Although there is no solid information for specific functional
appearance, there are some limited proof and proposed data of G-protein
subunit activation in cancer cells (Sanz et al., 2014). ORs
require definite cofactors to develop membrane localization of
receptors. The important co-factors are Receptor transporter proteins 1
and 2 (RTP1 and RTP2) and receptor expression enhancing protein (REEP1).
They are found in neurons cytoplasm doing down-regulation of brain’s
signaling molecules (Krautwurst, Yau, & Reed, 1998; Abaffy, Matsunami,
& Luetje, 2006; Li & Matsunami, 2011; Peterlin, Firestein & Rogers,
2014)
6.2 cAMP-induced calcium flux
Most of the ectopic ORs trigger a cAMP-induced calcium influx. It
generally happens from outside the cells representing the canonical
pathway cascade in olfactory sensory neurons (OSNs). The necessary
subunits expressions to build the canonical heterotetrameric cyclic
nucleotide-gated (CNG) channel (CNGA2, CNGA4, and CNGB1), particularly
CNGA2, has not been identified in most tissues of the human body. CNG
channel is a very important channel in peripheral human tissues and
cells (Flegel et al., 2013). CNGA1, mainly activated by cGMP
along with cAMP, is the native rod protein capable of forming functional
homomeric channels. CNGA1 can perform as a CNG channel (Kaupp et
al., 1989). CNGA3 channel is a cone photoreceptor native to sperm
cells. Along with the CatSper channel, CNGA3 expressed functionally
indicating its possible involvement in OR-mediated sperm chemotaxis
(Busse et al., 2014; Maßberg et al., 2015)
Though the Ca2+ entering channels are still mostly
unidentified, some researches have shown the involvement of TRP
channels, CRAC channels, voltage-gated L-type Ca2+channels, or spermatozoa-specific CatSper channels (Brown et al.,2019). There is a possibility of determining Ca2+influx through TRPM family members by using particular channel blockers
2-APB (Kalbe et al., 2016b; Flegel et al., 2016;
Manteniotis et al., 2016b).
Intracellular Ca2+ increment in human airway smooth
cell is induced by OR1D2 and OR2AG1 through a cAMP-dependent pathway.
OR1A1 induces cAMP response element-binding protein (CREB) without cAMP
induction and intracellular Ca2+.
6.3 MAPK downstream cascade
Ectopic OR activation can lead to regulate downstream protein kinase
cascades, precisely MAPK, in various cellular signaling dependent or
independent of the canonical pathway. These protein kinases presume to
be the main downstream modulators of several cellular processes (Kim et
al., 2015; Gelis et al., 2016)
These ORs mediated signaling in physiological systems are distinctly
relied on the OR ligands structure and concentration, morphology and
biochemistry of the regarding cellular systems, the heterotrimeric G
protein subunits, and the involvement of other less regulatory scaffold
proteins(Rodriguez et al., 2014; Wiese et al., 2015; Wu et
al., 2015). By reducing early phosphorylation of p38
mitogen-activated protein kinases (p38-MAPK), OR2AT4 can hinder cell
growth and through the phosphorylation of p44/42-MAPK can decrease cell
apoptosis in acute myelogenous leukemia (AML) patient (Manteniotis et
al., 2016a).
6.4 Others
Some ORs can activate the tyrosine kinase Src (sarcoma) signaling
pathway. Without activating any G-protein, this cascade can raise the
Ca2+ level in the cell via transient receptor
potential channel V6 (TRPV6). Some evidences show that some specific
ligands such as β-ionone can trigger PI3K/AKT downstream signaling via
Gβγ stimulation. OR51E1 shows involvement in AR-mediated signaling
through Src kinase (Spehr et al., 2011; Maßberg et al.,2016).
In essence, ORs activated by their specific odorants can transduce
intracellular signal cascade by several mechanisms.