Imp

1. Hormone signaling Pathways
and Cancer
By
Ansa Farooq

2. Introduction:
• The term hormone refers to those chemical messengers that travel
through the bloodstream between cellular sites and regulate
physiology and behavior.
• As hormones are the only way outside of the nervous system for our
organs to communicate, they are involved in almost all physiological
processes including digestion, respiration, lactation, reproduction,
response to stress, movement, growth, and many others.

3. • Given their ubiquitous roles in cellular physiology, it is not surprising
they are also critical to both tumorigenesis and tumor progression.
Many common cancer types are driven by inappropriate hormone
signaling, including prostate, breast, endometrium, ovary, thyroid,
testes, and bone cancers (Henderson et al, 1982).

4. • The relationship between prostate enlargement and hormones produced
by the testes has long been recognized. Although the chemical nature of
androgens was not known, it was reported in 1895 that surgical
castration (orchiectomy) of elderly men with prostate enlargement,
presumably due to benign prostatic hyperplasia, resulted in rapid atrophy
of prostatic tissue.
• Following the isolation of “testosterone” as the most potent androgenic
compound in the testes in 1935, Huggins and Hodges later demonstrated
the efficacy of surgical orchiectomy for the treatment of metastatic
prostate cancer, for which Huggins received the Nobel Prize in Physiology
or Medicine in 1966. Similarly, a link between estrogen and breast cancer
growth was established at the end of the 19th century, when Beatson
demonstrated that removal of ovaries (oophorectomy) helped in the
treatment of metastatic breast cancer in some premenopausal women.

5. • However, a molecular basis for this observation was not forthcoming
until the 1960s with the discovery of the estrogen receptor, followed
by the demonstrated expression of estrogen receptors in some
human breast tumors by Elwood Jensen.

6. • Evidence for a direct link between the action of sex steroids, and
a causal role in the carcinogenic process leading to breast and
prostate tumors, was first provided by Robert Noble.
• He reported that prolonged exposure to estrogen, androgen, or
combinations of the 2 led to breast and prostate cancers in rats.
More recently, the successful use of the antiestrogen, tamoxifen,
to reduce the incidence of breast cancer in high risk women,
supports a direct link between estrogen action and breast tumor
formation.
• These hormones are fundamental not only to the development of
normal mammary and prostate glands, but also to dysplastic and
neoplastic processes that occur in these tissues.

7. BASIC MECHANISMS OF HORMONE ACTION
• Hormones can be classified generally into 2 broad groups:
• (a) nonsteroidal (amino acids, peptides, and
polypeptides), which usually require cell membrane
localized receptors
• that regulate second messenger molecules such as
cyclic adenosine monophosphate (cAMP) to mediate
their action

8. b) steroidal, which bind directly to intracellular receptors
to mediate their action.
• Breast and prostate cancer are dependent primarily on estrogen and
androgen steroid hormones, respectively, for their growth and
viability.
• Other examples of steroid hormones include glucocorticoids,
mineralocorticoids, and progestins such as progesterone.
• The bioavailability of steroid hormones at the site of action depends
on several factors, including synthesis, transport via the blood, access
to target tissue, metabolism in target tissue, and expression of
receptors within the target cell.

9. •Breast and prostate cancers are the most commonly
occurring cancers in Western society, and are the
second leading cause of cancer death (next to lung
cancer) in women and men, respectively. Both of
these cancers arise in tissues that require steroid
hormones (estrogens and androgens) for their
development, growth, and function. Human cancers
occur in other hormone dependent tissues, such as
the uterus, ovary, and testis as well.

10. Steroid Hormone Receptors
• The majority of steroid hormone that enters the cell is derived from
the small non bound fraction in the circulation, which can enter by
passive diffusion.
• Upon entry into the cell, the steroid or its metabolic derivative (eg,
DHT) binds directly to a predominantly cytoplasmic (androgen
receptor) or nuclear (estrogen receptor) steroid receptor protein.
• The steroid receptor complex undergoes an activation step involving a
conformational change and shedding of heat shock (including HSP70,
HSP90, and HSP40) and other chaperone proteins, which are
necessary to maintain the receptor in a competent ligand binding
state(Aranda and Pascual, 2001).

11. • The receptor DNA complexes, in turn, associate dynamically
with coactivators and basal transcriptional components to
enhance the transcription of genes, whose messenger RNAs
(mRNAs) are translated into proteins that elicit specific
biological responses.
• It is likely that receptors that are not bound to their ligands, or
those bound to antagonists (Fig. 20–4), form complexes with
corepressors to inhibit the transcription of specific genes
(Perissi and Rosenfeld, 2005).
• After dimerization, and nuclear transport in the case of some
steroid receptors like the androgen receptor, the activated
receptor dimer complex binds to specific DNA motifs called
hormone responsive elements (HREs) found in the promoters of
hormone regulated genes (Aranda and Pascual, 2001).

12. • All steroid receptors are members of a so called superfamily of more
than 150 proteins. All are ligand responsive transcription factors that
share similarities with respect to their structural homology and
functional properties.
Each member of the steroid receptor family possesses a modular
structure composed of the following:
1. An N terminal region containing ligandin dependent transcriptional
activating functions (collectively referred to as activating function1 or
AF1);
2. A centrally located DNA binding domain of approximately 65 amino
acids having 2 zinc fingers (see Chap. 8, Sec. 8.2.6);
3. A hinge region that contains signal elements for nuclear localization;
and
4. A ligand binding domain in the C terminal region of the protein
containing a ligand dependent transcriptional activating function (called
AF2).

13. Role of coactivators and corepressor in regulation
of steroid receptor action
• In the presence of agonistic ligands (eg, estradiol for estrogen receptor [ER]
and DHT for androgen receptor [AR]), the steroid receptorDNA complexes
associate dynamically with coactivators, which, in turn, recruit other proteins,
including cointegrator complexes, that contact and stabilize the basal
transcription unit resulting in enhanced transcription of target genes.
• In the presence of antagonistic ligands (eg, tamoxifen for ER and flutamide for
AR), the receptors are in a different conformational state (steroid receptor
ligand analog) and the receptorDNA complexes dynamically associate with
corepressors (CoR) which destabilize basal transcription units and result in
reduced transcription of target genes. HRE, hormoneresponsive element.

14. • DNA damage response and repair (DDR) is a tightly controlled process
that serves as a barrier to tumorigenesis.
• Consequently, DDR is frequently altered in human malignancy, and can be
exploited for therapeutic gain either through molecularly targeted
therapies or as a consequence of therapeutic agents that induce
genotoxic stress.
• In select tumor types, steroid hormones and cognate receptors serve as
major drivers of tumor development/progression, and as such are
frequently targets of therapeutic intervention.
• Recent evidence suggests that the existence of crosstalk mechanisms
linking the DDR machinery and hormone signaling pathways cooperate to
influence both cancer progression and therapeutic response.

15. These underlying mechanisms and their
implications for cancer management is as
• Steroid hormones are systemically circulating small molecules that elicit
autocrine, paracrine, and endocrine functions in physiology and pathology,
most often through the binding to and regulation of cognate nuclear
receptor (NR).
• 1 Steroid hormones influence distinct and important cancer-associated
phenotypes, including but not limited to proliferation, apoptosis,
migration, and invasion
• 2. Steroid-induced biological outcomes occur in a context-dependent
manner in multiple malignancies.
• 3 and prostate (PCa) cancers
• 4. As such, therapy for selected hormone-dependent cancers focuses on
diminishing availability of ligand or direct antagonism of NRs.

16. While the biological implication remains uncertain, steroid hormones (e.g.
estrogens, androgens, glucocorticoids) have also been associated with
induction of genotoxic stress via multiple mechanisms, such as formation
of DNA adducts, or generation of reactive oxygen species (ROS) .
These effects can be exacerbated by both genetic aberrations, such as
through mutation of important DNA repair genes, as well as chemical
perturbations , or through use of nuclear receptor antagonists.
Conflicting data exists as to whether hormones are carcinogenic or cancer-
protective, and the discrepancies are context-, model-, and hormone-
specific.
However, steroid hormones have been selectively shown to promote
transformation, as well as generate complex genomic rearrangements
through induction of double-strand breaks (DSBs) that are associated with
tumorigenesis .

17. Evidence has emerged demonstrating that multiple functions of NRs are
influenced both by DNA damage as well as components of the DDR
machinery. Conversely, NRs also influence DDR gene expression and
function. This interplay between NR and the DDR machinery has
potentially strong tissue- and hormone-dependent implications for
hormone-sensitive malignancies.

18. Hormonal Regulation of Upstream DNA Damage
Signaling
• The tumor suppressor p53 (TP53) gene encodes p53, which is the most
mutated gene in human malignancies. p53 is considered the ‘guardian of
the genome’, sensing DNA damage and other abnormalities, and serving as
a decision point for DNA repair or apoptosis [36]. Estrogen and
progesterone signaling have been reported to activate the p53 axis in three
distinct ways:
• (i) directly via activation of the estrogen receptor α (ERα) and
increased TP53 mRNA expression ;
• (ii) by estrogens serving as a regulator function to modulate both p53 levels
and activity.
• (iii) combined estrogen/progesterone treatment resulting in activation of
p53 function.

19. The downstream biological effects of an estrogen/ progesterone
combination include decreased proliferation, increased apoptosis, and
reduced tumor formation in a Trp53 heterozygous murine mammary
model. In sum, estrogen and progesterone signaling promote the activity
of p53, indicating that female sex hormones have the potential to
positively regulate tumor suppressor function.
Conversely, male sex hormones have been implicated in diminishing p53
function. Androgen signaling through the androgen receptor (AR) inhibits
p53 function in models of hepatocellular carcinoma (HCC) [40], leading to
reduced apoptosis and increased proliferation.
It has been reported that dexamethasone (a glucocorticoid receptor
agonist) treatment when in combination with the chemotherapy drug
cisplatin (CDDP) reduces efficacy of treatment in models of non-small cell
lung cancer (NSCLC) via attenuation of p53 activity.

20. The similar actions of androgens and glucocorticoids on the function of p53
may be attributed to the knowledge that AR and glucocorticoid receptor
(GR) are more evolutionarily related to each other than to the ER [1],
suggesting an evolutionary pressure for this dichotomous regulation of p53
function. Although PR and AR are more evolutionarily related than GR and
AR[42], suggesting that ER function may be dominant to PR function with
respect to p53 regulation.
Furthermore, glucocorticoids are often used in concert with DNA damaging
regimens to reduce side effects of chemotherapy. Therefore, the negative
impact of GR signaling on p53 activity may have broader implications on
the strength and duration of a therapeutic response to chemotherapy in
any tumor type, and not simply restricted to hormone-dependent
malignancies.

21. Taken together, these data indicate that ER and progesterone receptor
(PR) signaling positively regulate p53, while AR and GR signaling negatively
regulate p53 function. While it is unclear what the implications of these
regulatory events are for tumor phenotypes and/or malignant
progression, these functional interactions may be relevant when
combining hormone therapy and DNA damaging therapeutic strategies in
p53-positive tumors.

22. HORMONAL REGULATION OF DNA REPAIR
There are five basic categories of DNA damage repair. To date, the
majority of experimental evidence demonstrates that steroid hormones
primarily regulate double-strand break (DSB) repair through non-
homologous end joining (NHEJ) and homologous recombination (HR).

23. • In breast cancer, ligand-independent activation of the ER has been
demonstrated frequently and may contribute to hormone
independence and antiestrogen resistance. Some of the more common
alterations in breast tumors are upregulated and abnormally regulated
growth factor pathways (EGFR, IGFR, HER2/NEU/ERBB2) and/or their
intracellular signal transduction molecules (RAS/RAF, MAPK,
phophatidylinositol3 kinase [PI3K], AKT.
Alternate Pathways of Signal Transduction in
Breast Cancer

24. • The expression and/or activity of many of these kinases are often
increased in breast tumors compared to normal breast tissue, and
specific phosphatases that deactivate these kinases are expressed at
higher levels in some breast cancer cell lines with altered responses to
estrogen. Thus an altered phosphorylation profile of the ER and/or its
coregulators may underlie the progression to hormone independence
and endocrine resistance (Xu et al, 2009; Skliris et al, 2010)
• Growth factors such as IGFI, heregulin and EGF can activate ER in the
absence or presence of estrogen through mechanisms involving
phosphorylation of coregulators or of the ER itself (Musgrove and
Sutherland, 2009). Similar phosphorylation mechanisms are almost
certainly operational for the ligand-independent activation of ER by
protein kinase A, protein kinase C, pp90rsk1, and protein kinase B (Ali
and Coombes, 2000; Clarke et al, 2001).

25. Alternate Pathways of Signal Transduction in
Prostate Cancer
• In prostate cancer, there is an increase in paracrine stimulation by growth
factors produced in prostatic stroma with eventual autocrine production
and stimulation by the prostate cancer epithelial cell (Rennie and Nelson,
1998). For example, in BPH, epithelial cells express EGFR but not its ligand
• TGFα, whereas prostate stroma produces TGFα but not EGFR. However, in
many prostate epithelial tumor cells, coexpression of EGFR and TGFα
• Is observed, indicating a shift from paracrine to autocrine stimulation (Leav
et al, 1998). Whether the molecular mechanisms responsible for the shift
in this regulatory loop are a result of mutational or adaptive processes is
unknown.
• However, by simply adapting the amount of the tyrosine kinase inhibitor
genistein in the diet, the expression of EGFR in the EGFR/TGFα system
could be manipulated in a rat prostate cancer model.

26. Estrogen Receptor and Androgen Receptor
Amplifications and Mutations
• In breast cancer, de novo hormone independence is often associated
with absence of ER. However, a substantial proportion of ER+ breast
tumors are either resistant de novo to endocrine therapy or acquire
resistance after treatment with endocrine therapies despite the
continued expression of ER.
• The presence of ER mutations and/or amplifications in breast cancer
is infrequent. Although mutated ERα is quite rare in human breast
cancers, variant ERs generated by alternative RNA splicing of both
ERα and ERβ genes are common. Changes in expression of some of
these variants have been documented in breast tumors, but only
limited correlation with endocrine resistance has been reported

27. • Unlike breast cancer, androgen independent human prostate cancer is
seldom associated with absence of AR. Immunohistochemical analysis
of AR in biopsies from virtually every stage and grade of prostate
cancer show that AR is retained regardless of hormone sensitivity.
Only in some multiplypassaged androgenindependent human
prostate cancer cell lines, such as Du145 and PC3 cells, is there lack of
detectable or functional AR. Because loss of AR expression is not
normally associated with the malignant phenotype, attention has
focused on more subtle mutational events in the AR gene that could
give rise to alterations in AR activity

28. Summary
• Both breast and prostate cancer require long-term exposure to steroid hormones to
develop. A direct causal link to overstimulation with estrogens and androgens has
been demonstrated in animal tumor models; conversely, a protective effect has been
observed from treatments that block the action of these hormones (eg, prepubertal
castration or administration of antihormonal drugs).
• Estrogens and androgens share many features in their mechanism of action: both are
synthesized from common precursors (eg, cholesterol); both are carried mainly by
the same protein in the blood (ie, SHGB [sex hormonebinding globulin]); and both
bind to structurally related intracellular receptors. Both types of hormonereceptor
complexes, in turn, bind to comparable regulatory DNA sequences in the promoters
of genes and interact with similar sets of coregulator proteins to activate or repress
gene expression.
• Approximately 70% of breast carcinomas are ER+ and approximately 50% of these
will respond to endocrine therapy, giving an overall response rate of approximately
35% to 40%. By comparison, most prostate cancers are AR+ and most (~90%)
respond to hormonal therapy. Therefore, ER assays are performed routinely in
women with breast cancer and only ER+ tumors are hormonally treated, whereas no
receptorbased selection process is applied to prostate tumors.

29. • Treatment modalities used to kill hormone dependent breast or prostate tumor cells
are based on the same principles of either blocking the synthesis of the steroid
hormone or blocking their activity in the target cell. Unfortunately, endocrine therapy
for locally advanced or metastatic disease is not curative as the tumors progress from
hormone dependence to hormone independence.
• Potential mechanisms leading to hormone independence include ascendancy of
alternative signal transduction pathways; receptor gene mutations or amplifications;
ligand independent receptor crosstalk with growth factors or kinases; autocrine
synthesis of androgens; and upregulation of cell survival genes. Whether the
hormone independent phenotype is caused by the outgrowth of preexisting clones
with these genetic abnormalities or by adaptive/epigenetic changes is unknown.
• There are no proven treatments to delay or prevent progression to hormone
independence in either prostate or breast cancer. However, there are several ongoing
prospective clinical trials to test the efficacy of intermittent androgen suppression on
delay of progression in men with prostate cancer.
• Increased understanding of steroid hormone action and the use of techniques such as
DNA and protein microarrays are driving the development of better markers for the
prediction of treatment response, the risk of developing invasive cancer, and the
identification of alternative targets for the treatment and prevention of breast and
prostate cancers.