In the course of
any life, somatic cells can be the subject of these changes, and in
some cases the cell may exhibit function(al) changes, or even steps
which may lead to malignant diseases.
The host has a range
of possible responses to pathogens, and these can be altered by nutrient
aberrations (deficiencies, excesses or imbalances) as well as by toxic
substances entering living systems or at times being produced by pathogens.
There are circumstances
where organisms can exist in pathogenic and non-pathogenic forms, and
others where more than one pathogen influences the disease entity.
How do we explain
different responses to infective organisms.
Interest has often
been focussed upon different immune mediated mechanisms to the same
pathogen.
Why, for example,
do some people exposed to a virus such as hepatitis B or C end up with
a grumbling hepatitis gradually giving rise to cirrhosis, or liver cancer,
while others do not?
Why is one response
to mycobacterium leprae, lepromatous leprosy, and another neuromatous
leprosy?
What does it mean
when organisms such as cytomegalovirus, chlamydia pneumoniae, or periodontal
pathogens can be found in ather omatous plaque (found through studies
of samples of the artery wall obtained during coronary surgery?)
Inflammatory bowel
diseases such as Crohn's disease and ulcerative colitis, tissue diseases
like rheumatoid arthritis and so called reactive arthropathy, systemic
lupus erythematosus and other collagen diseases, type 1 diabetes mellitus
and multiple sclerosis, are all examples of diseases which appear to
be examples of these specific host (with its particular genetic makeup),
environment and antigen interactions.
There is an increasingly
strong case that chronic fatigue syndromes are varieties of such responses.
In fact the Newcastle
(New South Wales, Australia) University research group have evidence
of 5 and possibly 6 subgroups of CFS.
The reader will
have choices to try to explore the complex but stimulating glimpse of
modern biology.
I have included
enough material for the person with some biological knowledge to learn
or update their knowledge of fundamental components and connections
with important mechanisms in health and disease.
Cell signalling
has been identified as crucial in coordination of living cellular and
subcellular systems.
Early in the study
of neurology, such signalling was identified and now has been greatly
elaborated.
Neuro-transmitters
and the receptors which they can occupy were determined, and the downstream
cellular changes were mapped.
The molecule which
can occupy a receptor site is called a ligand.
In the study of
endocrinology there was an early emphasis on the hormones from the glands
like the pituitary, thyroid, parathyroid, pancreatic islet cells, adrenals,
and gonads (ovaries and testes)
When brain influences
were added, neuro-endocrinology became a more encompassing term.
We could perhaps
divide hormones/signallers into
(1) Amino acid
derivatives,
(2) Small neuropeptides
like GnrH and TRH
(3) Protein sized
peptides like insulin, luteinizing hormone and parathyroid hormone,
(4) Steroid hormones
like cortisol and oestrogens,
(5) Vitamin derivatives
like retinoids (vitamin A) and vitamin D
The following material
is included to give readers who are intrigued by biological mechanisms,
opportunities to grasp the amazing interplay between cells and things
which influence them.
It is exceedingly
difficult to know how much detail should be provided here, in view of
a mammoth amount of details in comprehensive literature.
Every thing that
I now describe has a fuller elaborated depth.
These descriptions
also can give us an idea of the gap in knowledge base between the majority
of practising clinicians and the world leaders in this research.
Ion transport
There exist ligand-gated ion channels.
I will not develop
concepts about ion channels here, except to say that all cells are involved
with solute transport,
These systems are
typified by Na+, K+-ATPases and a Cl-, HCO3-exchanger.
Membrane transporters
are integral membrane proteins and can be classified as carriers, channels
and pumps.
Receptors and their
ligands
Transmembrane
receptors.
It has been exciting
to discover that a huge number of transmembrane receptors consist of
specific proteins capable of binding to guanosine linked with phosphate
as guanosine triphosphate (GTP)
The heterotrimeric
GTP binding proteins (G proteins) are used in a switch” on”
and “off” response possible because of “on” being
GTP-bound and “off “being GDP-bound states.
G protein coupled
receptors (GCRs) have been cloned and identified in terms of structure
and function.
Categories of
ligands for GCRs are
Biogenic amines:
acetylcholine, gamma-amino benzoic acid (GABA), adrenaline and noradrenaline,
glutamate, histamine and serotonin.
Lipid derivatives:
leucotrienes, platelet activating factors, prostacyclins and prostaglandins,
and thromboxanes.
Peptides: Adreno-cortico
tropic hormone (ACTH) angiotensin II, bombesin, bradykinin, C5A, calcitonin,
cholecystokinin (CCK), chorionic gonadotrophin (CG), cortico-releasing
hormone (CRH), endothelins, follicle stimulating hormone (FSH), glucagon,
growth hormone releasing factor (GnRh) luteinizing hormone (LH), melanocyte-stimulating
hormone (MSH), neuropeptide Y, neurokinin 1(substance P), NK2, NK3,
opioids, oxytocin, secretin, somatostatin, thrombin, thyroid stimulating
hormone (TSH), thyrotropin releasing hormone(TRH) and vasoactive intestinal
peptide)
Sensory stimuli:
Light, odorants, taste stimuli.
Other: Adenosine,
AMP, ADP, ATP, cyclic AMP, cannabinoids, and Calcium ions.
This list is incomplete
and new examples are being added as research unfolds.
There are other
major receptor types.
Tyrosine kinase
linked receptors (RTKs) do not have an “on-off” action, but
activated tyrosine kinase receptors assemble signalling complexes.
Examples of ligands
acting on receptor tyrosine kinases (RTKs) are epidermal growth factor
(EGF), insulin, platelet derived growth factor (PDF), fibroblast growth
factor, and nerve growth factor (FGF).
Often RTKs are phosphorylated
with specific functional effects.
Activated RTKs can
generate binding sites, and recruit proteins for signal transductions.
There are genes
that code for Ras proteins and regulation of mitogen signalling is initiated
by most RTKs.
Ras proteins are
a distinct group of GTP binding proteins distinct from heterotrimeric
G proteins. Ras is a central regulator of cell growth and in some cells
controls differentiation.
Ras mutations are
important in malignant diseases.
Activation of p42
and p44 mitogen-activated protein kinases (MAPKs) leads to trans migration
of the MAPK to the cell nucleus, where it can phosphorylate targets
such as transcription factors (jun, myc, NF-IL6, and ATF-2
There are other
tyrosine-kinase associated receptors which are used by a large number
of plasma membrane receptor proteins.
This includes relatively
simple single membrane-spanning receptors such as CD4, and receptors
for IL2 and interferon alpha.
With more complex
receptors there are functions such as T lymphocyte antigen receptors
where non-variable domains of the MHC type II molecules from antigen
presenting cells are ligands.
Another tyrosine
kinase associated receptor type is responsive to interferons. These
interferons are not mitogenic, but inhibit the growth of several types
of cells by activating intracellular signalling molecules, which then
evoke gene activations.
In this I am alluding
to the details of how cytokines are ligands for their receptors and
we will develop more about this when we come to the section on immunology.
In essence both
of the above systems involve a ligand, an integral membrane receptor
and components which amplify or specify the signal and response.
Substances found
in nature may block a particular GPR.
Nuclear hormone
receptors. (NHRs)
The active thyroid
hormone tri-iodothyronine (T3) acts by binding to a particular kind
of intranuclear receptor with high affinity and specificity.
This gives rise
to transcriptional regulation of target genes.
With parsimony in
biology other functions may exist beyond receptor-mediated events.
(E.g T3 may have
nongenomic effects such as activating kinases or calmodulin. It may
even have effects on the multidrug resistance P glycoprotein)
There are nearly
100 known members of this family.
In this group of
nuclear receptors we find
Androgen receptors
(AR)
Glucocorticoid
receptors (GR)
Oestrogen receptors
(ER)
Progestogen receptors
(PR)
Peroxisome proliferator-
activated receptors (PPAR)
Retinoic acid
receptors (RAR)
Retinoid X receptors
(RXR)
Thyroid hormone
receptors (TR)
Vitamin D receptors
(VDR)
If ligands are not
known for some NHRs, those members may be called “orphan receptors”(e.g.steroidogenic
factor-1 (SF-1), dosage sensitive sex reversal factor (DAX-1), hepatic
nuclear factor 4 alpha (HNF4a)
GRs are largely
in cytoplasm, whereas TRs are always intranuclear.
After binding, the
GRs translocate to the nuclei.
The arrangements
of all of these NHRs are similar.
All have * Amino-terminal
A/B domains
*Central DNA binding
domains with 2 zinc fingers (DBD)
*Hinge regions
with nuclear localization signal carboxy terminal ligand binding domain
(LBD) which mediate transcription control.
Certain NHRs such
as GRs and ERs can allow activation of cross-talk by activating or repressing
signal transduction pathways.
Genetic poly morphisms
have been defined for most of these receptors.
Most NHRs bind to
DNA as dimers. Each monomer recognizes aDNA motif referred to as a “half-site”
Steroid receptors
(GR,ER,PR,AR) bind to DNA as homodimers.
Consistent with
this 2 fold symmetry their DNA sites are “palindromic.”
TRs, retinoid receptors,
PPARs and VDRs bind to their DNA sites as heterodimers in combination
with RXRs.
Their DNA half sites
are arranged as direct repeats.
Receptor specificity
is determined by
(1) Sequence of
half sites
(2) Orientation
of the half sites and
(3) Spacing between
half sites.
I here elaborate
on one of these NHRs
Peroxisome proliferator
activated receptors (PPAR)
Nuclear peroxisome
proliferator-activated receptors (PPARs) are ligand-activated transcription
factors belonging to the nuclear receptor superfamily. As transcription
factors, PPARs regulate the expression of numerous genes and affect
glycaemic control, lipid metabolism, vascular tone and inflammation.
This subfamily consists
of three isotypes, alpha (NR1C1), gamma (NR1C3), and beta/delta (NRC1C2)
with a differential tissue distribution.
PPAR alpha is expressed
primarily in tissues with a high level of fatty acid catabolism such
as liver, brown fat, kidney, heart and skeletal muscle.
PPAR beta is ubiquitously
expressed.
PPAR gamma has a
restricted pattern of expression, mainly in white and brown adipose
tissues, whereas other tissues such as skeletal muscle and heart contain
limited amounts.
Activation of the
subtype PPAR-gamma improves insulin sensitivity.
Furthermore, PPAR
alpha and gamma isotypes are expressed in vascular cells including endothelial
and smooth muscle cells and macrophages/foam cells.
PPARs are activated
by ligands, such as naturally occurring fatty acids, which are activators
of all three PPAR isotypes.
In addition to fatty
acids, humolones from hops (agonists for a and g), several synthetic
compounds, such as fibrates (alpha), angiotensin receptor blockers (g),
and thiazolidinediones (rosiglitazone and pioglitazone), bind and activate
PPAR alpha and PPAR gamma.
Rosiglitazone, which
acts on the alpha receptor, has some adverse effects on lipids, but
pioglitazone, which acts on the gamma receptors, does not adversely
affect lipids.
There are cautions
about their uses in view of reports of adverse effects on heart and
bone.
As well, apigenin,
chrysin, and kaempferol significantly stimulated PPAR gamma transcriptional
activity in a transient reporter assay. In addition, these three flavonoids
strongly enhance the inhibition of inducible cyclooxygenase and inducible
nitric oxide synthase promoter activities in lipopolysaccharide-activated
macrophages, which contain the PPAR gamma expression plasmids.
There are also PPAR
agonist actions with curcumin.
In order to be transcriptionally
active, PPARs need to heterodimerize with the retinoid-X-receptor (RXR).
Upon activation,
PPAR-RXR heterodimers bind to DNA specific sequences called peroxisome
proliferator-response elements (PPRE) and stimulate transcription of
target genes. PPARs play a critical role in lipid and glucose homeostasis,
but lately they have been implicated as regulators of inflammatory responses.
A role for PPAR
gamma in inflammation has also been reported in monocyte/macrophages,
where ligands of this receptor inhibited the activation of macrophages
and the production of inflammatory cytokines (TNF alpha, interleukin
6 and 1beta), although part of the anti-inflammatory effects of these
ligands seems to be mediated by a mechanism not involving PPAR gamma.
All these findings
suggest a role of PPARs in the control of the inflammatory response
with potential therapeutic applications in inflammation-related diseases.
Actions of PPAR
agonists will probably play important roles in many inflammatory diseases.
We also have to
be prepared for possible adverse effects, as both rosiglitazone and
pioglitazone appear to be risky to aggravate heart failure when it is
present, and both may increase the risk of fractures, through an adverse
effect on bone.
Protection from
hepatic fibrosis is another recent hope, as Xu, Fu and Chen demonstrated,
for the first time, that curcumin dramatically induced the gene expression
of PPAR-gamma and activated PPAR-gamma in activated HSC. Blocking its
trans-activating activity by a PPAR-gamma antagonist markedly abrogated
the effects of curcumin on inhibition of cell proliferation.
Orphan nuclear receptors
belong to this gene super-family but their target genes and physiological
function are still being studied.
The “orphans”
belonging to the PPAR, LXR and FXR family function as lipid and bile
acid sensors while PXR and CAR function as xenobiotic sensors.
Small-molecule modulators
of LXR and FXR control key genes involved in cholesterol and lipid metabolism.
PXR is a highly promiscuous xenosensor that responds to xenobiotic ligands
(antibiotics, statins, glucocorticoids) and induces the CYP 3A gene,
thereby playing a role in hepatic protection and bile acid metabolism.
A related receptor
from the gene subfamily, CAR, displays high ligand selectivity and modulation
of its activity in humans may significantly alter metabolism of drugs
and other xenobiotics.
The role of the
ER relatives, the ERRs will become more apparent as ligands are identified
and linked to target genes and physiological function. These targets
offer multiple opportunities for therapeutic intervention with small-molecule
drugs, in diseases related to neuronal function, inflammation, lipid
homeostasis, metabolic function and cancer.
Homocysteine (Hcy)
induces nuclear factor-B (NF-B). On the other hand, a negative correlation
between high levels of Hcy and peroxisome proliferator-activated receptor
(PPAR) expression has been demonstrated
Also, PPAR agonists
inhibit the metalloproteinase activation in macrophage.
There is even evidence
that intestinal flora influence PPAR expression in intestinal locations.
A good beginning
for human life is breast feeding!
There is evidence
that human milk is an exact fit for the best nutrition of the brain.
It is best if the
mother is nutrient replete!
This “best
fit” holds for amino acids, fatty acids, minerals and glyconutrients.
It is also optimized
for infant digestion and nutrient availability.
(Consider the unpleasant
smell of formulas when regurgitated with the pleasanter smell of regurgitated
human milk!)
No one has succeeded
in making a better feeding formula, because even if one equaled the
fatty acid and amino acid and mineral balance, it is not yet possible
to provide the immune factors in the balance of the natural human milk.
Human milk contains
n-3 and n-6 LCPUFA (long chain polyunsaturated fatty acids), which are
absent from many infant formulas. During neonatal life, there is a rapid
accretion of AA (arachidonic acid) and DHA (docosahexaenoic acid) in
infant brain, DHA in retina and of AA in the whole body. The DHA status
of breast-fed infants is higher than that of formula-fed infants when
formulas do not contain LCPUFA.
Breast-feeding reduces
enteric infection and may reduce chronic disease in later life. Although
human milk contains significant secretory immunoglobulin A (sIgA), most
of its protective factors are constitutively expressed.
Multifunctional
milk components are nutrients whose partial digestion products inhibit
pathogens.
Cytokines, cytokine
receptors, TLR agonists and antagonists, hormones, anti-inflammatory
agents, and nucleotides in milk modulate inflammation.
Human milk is rich
in glycans (complex carbohydrates): As prebiotics, indigestible glycans
stimulate colonization by probiotic organisms, modulating mucosal immunity
and protecting against pathogens. Through structural homology to intestinal
cell surface receptors, glycans inhibit pathogen binding, the essential
first step of pathogenesis.
Bioactive milk components
comprise an innate immune system of human milk whereby the mother protects
her nursing infant.
Specific transfer
factors (about 50 peptide length) convey immunity to microorganisms
that the mother has met.
Lactoferrin consists
of a series of glycoproteins with activity against staphylococci, gut
bacterial pathogens, candida, and viruses such as hepatitis C. As well
it has been shown to heal experimentally induced colitis. It has about
40% bioavailability.
Actions include
depriving some bacteria of needed iron, breaking up biofilm which colonies
of bacteria produce to be less susceptible to body chemicals. It also
enhances neutrophil phagocytic activity.
Interactions between
human milk glycans, intestinal microflora, and intestinal mucosa surface
glycans underlie ontogeny of innate mucosal immunity, pathobiology of
enteric infection, and inflammatory bowel diseases.
Less infection means
less risk to the brain.
Unless many factors
are taken into account(eg mother’s warmth and care, sensory stimulation,(provision
of crucial inputs which enrich but do not overload the child) as well
as avoiding things which cause fear and distress to the child, we can
only say that breast milk is the best feeding material for at least
the first 6 months and possibly more.
I will elaborate
on cytokines and their receptors in a later chapter.
Do you marvel at
these patterns?
