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8/13/2019 Isolated Islets in Diabetes Research
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Isolated islets in diabetes research
R. Bhonde, R.C. Shukla, M. Kanitkar, R. Shukla, M. Banerjee & S. Datar
Tissue Engineering & Banking Laboratory, National Centre for Cell Science, Pune, India
Received November 8, 2006
This review highlights some recent developments and diversified applications of islets in diabetes
research as they are rapidly emerging as a model system in biomedical and biotechnological research.
Isolated islets have formed an effective in vitro model in antidiabetic drug development programme,
screening of potential hypoglycaemic agents and for investigating their mechanisms of action. Yet
another application of isolated islets could be to understand the mechanisms of cell death in vitro
and to identify the sites of intervention for possible cytoprotection. Advances in immunoisolation
and immunomodulation protocols have made xeno-transplantation feasible without
immunosuppression thus increasing the availability of islets. Research in the areas of pancreatic
and non pancreatic stem cells has given new hope to diabetic subjects to renew their islet cell massfor the possible cure of diabetes. Investigations of the factors leading to differentiation of pancreatic
stem/progenitor cells would be of interest as they are likely to induce pancreatic regeneration in
diabetics. Similarly search for the beta cell protective agents has a great future in preservation of
residual beta cell mass left after diabetogenic insults. We have detailed various applications of
islets in diabetes research in context of their current status, progress and future challenges and
long term prospects for a cure.
Key words Cytoprotection - hypoglycaemics - islets - regeneration - transplantation
Islets of Langerhans are organelles present within
the pancreas and are mainly responsible for the
production of insulin, glucagon, somatostatin andpancreatic polypeptide upon stimulation. The primary
focus of islet research is, however, the cure and/or
better management of diabetes mellitus which results
from a loss of insulin secretion from beta cells present
within the islets of Langerhans. This review seeks to
take a bird’s eye view of the contribution of islets as a
model system in diabetes research beyond
transplantation.
Since their discovery in 18691, islets have been
viewed as a possible in vitro system for a syndrome
that cannot be mimicked very effectively using celllines. They are miniature organ systems, retaining their
architecture, differentiated state and ability of insulin
secretion upon stimulation, independent of nervous
control. Isolation of islets2,3 has promoted studies related
to understanding the pathophysiology of type I and II
diabetes, transplantation, screening of hypoglycaemic
drugs and probing into diabetes causing mechanisms,
to device effective means of prevention.
425
Indian J Med Res 125, March 2007, pp 425-440
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Islets in pharmacological research
Insulin secretion enhancers: Isolated islets, in vitro,
respond to glucose stimulation and hence haveimmensely contributed to the study of various
pharmacological aspects and for screening of
promising antidiabetic agents. A number of
hypoglycaemic drugs act as insulin secretagogues,
in corroboration with this, we have reported that islets
can serve as an in vitro model for antidiabetic drug
screening4. Our model offers several advantages as
it is simple and economical in terms of reduction in
the number of animals used as well as the amount of
drug required for testing. A number of plants, in
traditional medicine, are claimed to have anti-
diabetic properties and isolated islets have been used
extensively for checking their properties and
plausible modes of action. For example, extract of
Tinospora crispa5, extract of Gymnema sylvestre6,
bittergourd fruit juice7, leaf extract of Urtica diocia8,
aqueous extract of Scoparia dulcis9 and aqueous
extract of Teucrium polium10 have been shown to
exhibit insulin secretagogue activity. Apart fromplant/natural extracts, several synthetic drugs have
been tested for their insulin secreatagogue property
and for determination of their mechanism of action,
using isolated pancreatic islets in vitro.
It is understood that glucose stimulates insulin
secretion in the pancreatic cell by means of a
synergistic interaction between at least two signaling
pathways. In the K (ATP) channel-dependent
pathway, glucose stimulation increases the entry of
extrinsic Ca2+ through voltage-gated channels by
closure of the K (ATP) channels and depolarization
of the beta cell membrane. The resulting increase in
intracellular Ca2+ stimulates insulin exocytosis.
While in the GTP-dependant pathway, intracellular
Fig. 1. Signaling cascade of glucose stimulated insulin secretion. The figure summarizes the signaling pathway triggered by glucose
in beta cells for insulin exocytosis. Agents in blue represent secretagogues while agents in red represent insulin secretion inhibitors.
Gq protein: member of G family of proteins, IP3: Inositol triphosphate.
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Ca2+ is elevated by GTP-dependent proteins and
augments the Ca2+-stimulated release (Fig. 1).
Secretagogues and insulin secretion inhibitors act at
intermediate steps of these signaling pathways andinfluence the process of insulin exocytosis. Several
researchers have investigated this intricate mode of
known secretagogue action using isolated islets as
an in vitro model. To quote a few; imidazoline
antagonists of alpha 2-adrenoreceptors increase
insulin release in vitro by inhibiting ATP-sensitive
K+ channels in pancreatic cells11. Giannaccini et
al12 have evaluated the properties of sulphonylurea
receptors (SUR) of human islets of Langerhans. They
studied the binding affinity of various oral
hypoglycaemic agents to the receptor and also tested
insulinotropic action of the drugs on intact human
islets. This binding potency order was parallel with
the insulinotropic potency of the evaluated
compounds12 . Masuda et al13 have shown an
insulinotropic effect of Triglitazone (CS-045) and
have shown its mode of action to be distinct from
glibenclamide (a sulphonylurea drug). A-4166, a
derivative of D-phenylalanine, evokes a rapid and
short-lived hypoglycaemic action in vivo. It has been
shown to act via the tolbutamide binding sites 14.Louchami et al15 showed S21403, a meglitinide
analogue to be a novel insulinotropic tool in the
treatment of type 2 diabetes, as it affected cationic
fluxes and the drugs secretary responses displayed
favourable time course of prompt, and not unduly
prolonged, activation of cells. Mears et al16
demonstrated that tetracaine (an anaesthetic)
stimulates insulin secretion by release of intracellular
calcium and for the first time elucidated the role of
intracellular calcium stores in stimulus-secretion
coupling in the pancreatic cells. JTT-608, is a
nonsulphonylurea oral hypoglycaemic agent which
stimulates insulin release at elevated but not low
glucose concentrations by evoking PKA-mediated
Ca2+ influx17.
Insulin secretion inhibitors: Besides insulinotropic
studies, several investigators have used isolated islets
to study drugs that inhibit insulin secretion and their
probable mode of action. Several drugs have
inhibitory effect on insulin secretion by cells and
this is an important aspect, which needs to be
considered while using the said drug. Cyclosporine,
a widely used immunosuppressant, induces inhibitionof insulin release in isolated rat islets18,19. Paty et al20
demonstrated the inhibitory effects of several
immunosuppressive drugs on insulin secretion. Thus
low dose immunosuppressive drug protocols should
be used in clinical islet transplantation and patients
using this drug have to be carefully monitored for
signs of deficient insulin secretion. Tacrolimus is
known to cause post-transplant diabetes mellitus.
Using isolated rat pancreatic islets, Uchizono et al21
studied its mechanism of action and found that it
interferes with the process of insulin exocytosis and
that protein kinase C (PKC)-mediated (Ca2+
dependent and independent) and Ca2+-independent
GTP signaling pathways may be involved. L-
asparaginase inhibits glucose-induced insulin
secretion in a dose-dependent manner by decreasing
total cAMP in isolated rat islets22. Metz et al23 have
used selective inhibitors of GTP synthesis and proved
that they impede exocytotic insulin release from
intact rat islets. The study provides first direct
evidence that GTP is required for insulin release23.Hydrochlorothiazide is also known to inhibit insulin
release. This inhibition is not mediated by reduced
chloride fluxes but rather by inhibition of calcium
uptake24.
Many antidiabetic drugs act on peripheral tissues
and have no direct effect on pancreatic islets. One
such drug is metformin, which affects glucose and
free fatty acid metabolism in peripheral insulin target
tissues. However, Patane et al25 exposed rat
pancreatic islets to high glucose and free fatty acid
(FFA) levels in vitro mimicking the in vivo
environment in presence and absence of metformin.
They found that metformin can restore a normal
secretary pattern in islets whose secretary function
has been impaired by chronic exposure to elevated
FFA or glucose levels. Recently, Marchetti et al26
cultured islets isolated from type 2 diabetes subjects
in presence of metformin and found that several
functional and survival defects of T2D islets were
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ameliorated by metformin. Thus in diabetic patients,
metformin (in addition to its peripheral effects) may
have a direct beneficial effect on the cell secretory
function. On the contrary, Leclerc et al27 have shownthe ability of metformin to activate AMP-activated
protein kinase in human islets and inhibit insulin
secretion. This inhibitory effect needs to be
considered with respect to the use of this drug for
the treatment of type II diabetes.
We examined the effect of commonly used
antibiotics such as gentamycin, penicillin,
streptomycin, tetracycline, neomycin, erythromycin
and chloramphenicol on isolated islets viability,
functionality and induction of oxidative stress if any.
Our results revealed the innocuous nature of the
antibiotics used at pharmacological concentrations,
suggesting their safety whenever prescribed for
combating infections and also during islet isolation
procedures28.
Cytoprotection of islets
Another primary area of research greatly
facilitated by usage of islets as a model system is inunraveling the mysteries of beta cell death and ways
of preventing the same.
cell death: The precise mechanisms of cell death
in vivo, leading to diabetes, remain unclear. However,
extensive studies, using islets as a model system show
that there are many molecules including Fas Ligand
(FasL) and cytokines such as interleukin-1 (IL-1),
tumour necrosis factor alpha (TNF) and interferon
gamma (IFN ) that cause release of other cytokine
mediators that have potential to damage cells in
vitro and in vivo29. cell death appears to be
ultimately caused by receptor mediated mechanisms
and/or by secretion of cytotoxic molecules like
granzymes and perforin. In addition, toxic molecules
such as reactive oxygen species (ROS: superoxide
radicals, hydroxyl radicals, and nitric oxide) play a
significant role in islet cell death by inducing DNA
damage. DNA damage, in cells, leads to poly (ADP
ribose) polymerase (PARP) activation which
increases NAD consumption, depletion of which
compromises ATP production in cells30. It is apparent
that a number of different mechanisms of cell death
are operative in destruction of islets (Fig. 2).
Inhibi tion of ox idants : Normally, cells counter
oxidative stress by expression of ROS scavenging
enzymes like catalase (CAT), glutathione peroxidase
(Gpx) and superoxide dismutase (SOD). cells,
however, have extraordinarily low levels of ROS
scavenging enzymes31 . The correction of this
deficiency, in vitro, by overexpression of cellular
enzymes like SOD may lead to protection of cells
against oxidative stress induced cell damage/
death32. Studies, wherein mitochondrial form of Mn-
SOD was overexpressed in cells are shown to have
protected isolated islets against oxidative damage in
vitro33. It has also been shown that adenoviral
overexpression of glutamyl cyesteine ligase catalytic
subunit, a primary regulator of de novo synthesis of
glutathione (GSH) in mammalian cells and central
to the antioxidant capacity of the cell, protects
pancreatic islets against oxidative stress, in vitro34.
In vitro stress in islets is often produced by using
cell specific toxins like streptozotocin (STZ) andalloxan. STZ induces islet necrosis by employing
effector molecules like nitric oxide (NO) and ROS.
Pro-inflammatory cytokines also employ NO as an
effector molecule for necrosis and / or apoptosis
induction 35 . Hence a plausible means for
cytoprotection of islets could be scavenging of NO
or inhibition of iNOS (inducibe nitric oxide synthase)
which synthesizes NO. iNOS inhibitors can be used
for cytoprotection of islets in vitro as inhibition of
iNOS would inhibit formation of NO, preventing islet
cell death indirectly. It has been reported that a
combination of an iNOS inhibitor and a free radical
scavenger, guanidinoethyldisulphide restored IL-1
induced suppression of islet insulin secretion in
vitro36. An imidazole compound called Efaroxan has
also been shown to impart complete protection
against IL-1 induced toxicity37. It has recently been
shown that silymarin, a polyphenolic flavonoid that
has a strong antioxidant activity, prevented IL-
1+IFN- -induced NO production and -cell
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dysfunction in human islets. These cytoprotective
effects of silymarin appeared to be mediated through
the suppression of c-Jun NH2-terminal kinase and
Janus kinase/signal transducer and activator of transcription pathways38. In vitro treatment of islets
with exogenous antioxidants is another viable option.
There are many known antioxidants like vitamin C,
vitamin E39, Scoparia dulcis a traditional antidiabetic
plant40 that are known to protect islets against
oxidative stress induced dysfunction and death.
Studies with islets have shown that the experimental
drug, bis-o- hydroxycinnamoyl methane, an analogue
of naturally occurring bis- demethoxycurcumin,
enhances the antioxidant defense against
ROS, thus protecting cells from death41. It has been
demonstrated that polyenoylphosphatidylcholine
(PPC), a phosphatidylcholine rich phospholipid
extracted from soybean, protects cells against STZ
induced toxicity and also plays an important part in
maintaining their insulin synthesis and secretion for
normal glucose homeostasis42.
PARP and NF B inhibition : Poly (ADP ribose)
polymerase (PARP) is a major effector molecule in
the oxidative stress induced or cytokine induced cell
death pathway. It is known that STZ injections cause
extensive necrosis in islets of Parp + / + mice while
the extent of necrosis was markedly lower in Parp- / -
islets43. It has also been shown that inhibition of
Parp-1 by synthetic inhibitors like 3-aminobenzamide
resulted in protection against necrosis. A potent
inhibitor of Parp, 5-iodo-6amino-1, 2 benzopyrone
(INH2BP), was found to protect rat islets and beta
cell line RIN-5F from cytokine induced damage44.
Fig. 2. Mechanisms of islet cell death. Flow chart depicts apoptotic and necrotic beta cell death cascades along with possible modes
of intervention. Causes/agents of beta cell death are indicated in red while agents/strategies for prevention of beta cell damage are
indicated in blue. Red arrows stand for possible sites of intervention. IL-11: Interleukin 11, IKK: Inhibitor of kappa kinase, iNOS:
inducible nitric oxide synthase, NO, Nitric oxide, STZ: Streptozotocin, ROS: Reactive oxygen species, PARP: Poly (ADP-Ribose)
Polymerase, CAT: Catalase: SOD, superoxide dismutase; GSH: glutathione peroxidase, NF-kB: Nuclear Factor kappa B. This figure
has been compiled and constructed by authors by referring to data cited in references 34-38.
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NF-B activation is an important event in
inflammation, cellular death signaling and is often
activated by oxidative stress45. One of the key steps
in activation of the NF-B pathway is thestimulation of the I-B kinases46. Inhibition of NF-
B activation could be an effective means of
controlling islet cell death in vitro. Studies have
shown that IL-11, a regulatory cytokine, has been
effective in inhibiting NF-B activation in islets
leading to prevention of multiple low doses STZ
(MLD- STZ) induced diabetes in vivo47. Rehman
et al48 have demonstrated that adenoviral gene
transfer of the NF-B inhibitor I-B to isolated
human islets resulted in protection from IL-1
mediated dysfunction and apoptosis. Mouse islets
when transduced in s i tu by infusion of the
transduction peptide prior to isolation lead to 40
per cent of peptide transduction of cells. Delivery
of the IKK inhibitor transduction fusion peptide
(PTD-5-NBD) in situ to mouse islets resulted in
improved is let function and viabil i ty af ter
isolation48. Other studies involving rhIL-1 have
suggested a cytoprotective role of the recombinant
cytokine against alloxan induced toxicity in
diabetic rat islets49.
Apart from exogenously induced islet damage,
islets also suffer from damage due to endogenous
hyperglycaemia in vivo. It is known that high glucose
concentration causes apoptosis in cultured human
pancreatic islets of Langerhans. Data suggest that in
human islets, high glucose modulates the balance of
pro- and anti-apoptotic Bcl proteins towards cell
apoptosis50. Hyperglycaemia also causes production
of IL-1 by islet cells leading to cytotoxicity in
human pancreatic islets51. Chronic exposure to free
fatty acids alone or with hyperglycaemic conditions
lead to pancreatic cell death possibly employing
oxidative stress as the mechanism for cell
destruction52,53. Hence, blockage of multiple
pathways, rather than a single pathway, leading to
cell death may be necessary to fully protect cells
from destruction in vitro54. It is relatively easy to
study mechanisms of cell death, in its intricate
detail, along with modes of preventing the same and
its effect on diabetogenesis employing isolated islets
rather than animal experimentation.
Isolated islets for transplantation
Apart from studying strategies for prevention or
abrogation of cell death, isolated islets are widely
used for transplantation. Extensive studies have been
conducted wherein islets were transplanted into a
hyperglycaemic host and then checked for reduction
in hyperglycaemia. Till date it remains the most
successful means of achieving normoglycaemia in
humans55. Allo- or xeno- tranplantation of whole
pancreas is possible56, but it requires major surgery,
hence transplantation of islets57,58 or insulin producing
beta cells59,60 would be a more viable option.
Any successful transplantation depends on three
things, viz:
(i) Primary non function: Primary nonfunctioning
(PNF) of islets accounts for the bulk of graft losses.
Macrophages, the main effector cells in PNF,
release proinflammatory cytokines like IL-1 and
TNF-, which in turn recruit free radicals to mediatea nonspecific inflammatory response61. Lee et al62
have shown that the blockade of monocyte chemo-
attractant protein-1 (MCP-1) binding to CCR2, in
conjunction with sub therapeutic
immunosuppression, leads to islet allograft survival.
Hence, interruption of the leukocyte recruitment
through chemokine receptor targeting may be of
therapeutic benefit. Transfection of cytoprotective
genes to isolated pancreatic islets may contribute
to enhanced survival in transplant settings, e.g., the
overexpression of erythropoietin gene protects islets
from destruction and does not compromise islet
functionality63. It has been shown by Riachy et al64
using cell lines and human islets, that 1, 25-(OH)
2D3, the active metabolite of vitamin D3, was able
to induce and maintain high levels of A20, an anti-
apoptotic protein known to block NF-B activation,
thus promoting islet cell survival by modulating the
effects of inflammatory cytokines, which contribute
to cell demise64. Disruption of islet extracellular
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matrice during pancreatic digestion leads to
induction of apoptotic pathways, thus increasing
cumulative PNF. Targeting the apoptotic pathway
by adenovirus-mediated gene transfer of the anti-apoptotic Bcl-2 gene exerts a major cytoprotective
effect on isolated islets. This was experimentally
proved with isolated macaque pancreatic islets. Bcl-
2 transfection ex vivo protected these islets from
apoptosis65.
(ii) Abrogation of immune rejection of graft and
recurrence of autoimmunity: The next hurdle, graft
rejection, can be dealt by the administration of
immunosuppressive drugs in the host, and/or
immunoisolation of the graft, or immunomodulation
of the graft or host or both 66 . Immuno-
compromisation enables allogenic and xenogenic
islet transplantation in preclinical, non human
primate models 67. Isolated islets are known to
reverse diabetes in immunocompromised nude mice
rendered diabetic by STZ. Although attractive, an
immunosuppression regime leaves the host
susceptible to other infections68, and these drugs
have adverse effects on insulin secretion by
cells 22,23,58 . A better alternative toimmunocompromization is to make the host tolerant
to the graft. Many different strategies have been
developed to achieve transplantation tolerance. In
one approach used by Oluwole et al 69 the
intravenous administration of genetically
engineered host dendritic cells (DCs) expressing
allo-MHC peptides, along with transient ALS
immunosuppression, resulted in induction of graft
tolerance. Similarly, induction of mixed chimerism
via bone marrow (BM) cells transplantation from
normal donors into autoimmune non obese diabetic
(NOD) mice has been shown to reverse insulitis and
prevented the development of diabetes and induces
tolerance to donor islet cells 70-74. This approach
however leaves the host susceptible to the graft
versus host disease (GVHD). Like this, Liang et al75
have described a radiation free regimen for
induction of chimerism, donor-specific tolerance,
reversal of insulitis, and resistance to diabetes
development in NOD mice model.
It is understood that complete T cell activation
requires two signals; T cell receptor (TCR)
interaction with peptide- MHC complex presented
by antigen presenting cells (APCs). This signal mustthen combine with another co-stimulatory signal,
mediated by interaction between distinct cell surface
molecules of APCs and T cells76. In absence of co-
stimulation, T cells undergo anergy and become non-
responsive. The B7/CD28/CTLA4 co-stimulatory
pathway plays a critical role in the regulation of T-
cell activation transplant rejection and autoimmunity.
Adams et al77 have used LEA29Y (BMS-224818), a
mutant of CTLA4-Ig along with repamycin and IL-
2R to effectively prevent the rejection of islet
allograft in a preclinical primate model.
Administration of co-stimulatory blockade (anti-
CD40L) has been reported to induce mixed
chimerism in NOD mice78,79. Pearson et al80 have
reported that an allelic variant of Idd3 gene is
responsible for prolonged islet allograft survival by
co-stimulatory blockade in NOD mice. MHC
antigens, expressed on APC of donor tissue, stimulate
a higher T cells response as compared to host APCs.
As passenger (donor) APCs are largely responsible
for co-stimulatory activity, graft immunomodulatorystrategies aim at depleting them from the islet grafts81.
These strategies include in vitro culture of graft at
suboptimal temperature for extended time82,83, UV-
B irradiation84, cryopreservation85,86 mitomycin C
treatment87, ICAM-1 specific monoclonal antibody
treatment88, co-transplantation of islets with sertoli
cells89 etc.
Xenotransplantation: The existing shortage of donor
islets makes it necessary to pool islets from different
donors or look for alternative islet sources. Foetal,
neonatal and adult porcine islets along with bovine,
murine, canine, avian and piscean islets have been
tested for this purpose and porcine islets have been
found to be an acceptable source for alternative islets.
It was observed that neonatal porcine pancreatic cell
clusters (NPCCs) contain mature endocrine cells and
bring about sustained normalization of blood glucose
levels when transplanted into kidney capsule of
diabetic nude mice. Rapid return to diabetic state was
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observed after removal of islet grafts90,91. Recently,
Garkavenko et al92 have reported a follow up study
in human diabetic patients receiving porcine islets
for 9 yr. The finding that none of the patientsdeveloped viral infection hold promises for
xenotransplantation. Similarly, canine, bovine and
porcine islets have been successfully used for
xenotransplantation in a diffusion based bio hybrid
artificial pancreas93,94. Chick embryo pancreatic
transplants have shown reversal in experimental
diabetes of rats without immunosuppression 95.
Transient reversal of experimental diabetes in mice
has also been reported by transplantation
of chicken pancreatic islets96. Xenotransplantation of
fish islets into the non-cryptorchid testis has also
been carried out. Cryopreservation of principal islets
of teleost fish and their xenotransplantation has also
been studied97,98. Immunoisolation of islets by micro-
encapsulation is of great clinical potential in the
treatment of diabetes with xenotransplantation of
islets99-102. Lim and Sun, first described the alginate
micro-encapsulation of islets 10 2. Since then
immunoisolation has been regarded as the
technological key to xenotransplantation without
immunosuppression. The encapsulated isletscan be transplanted in, and retrieved from, the
peritoneal cavity with minimal invasive surgery.
Immunoisolation has facilitated the transplantation,
and consequent reversal of hyperglycaemia, from rat
to mice103,104, monkey to rats105, porcine tomurine, dog
to mice106, and even across a large species barrier
i.e., from rabbit to cynomolgus monkey107. An ideal
membrane for immunoisolation should be
biocompatible, non immunogenic, non cytotoxic,
differentially permeable to glucose, insulin, oxygen
and other growth factors required for prolonged
survival of graft and impermeable to
immunoglobulins, immune effector cells and their
recruiting cytokines108. Various natural biopolymers
like and synthetic materials have been used
extensively as immunoisolation material. We have
tested cellulose macrocapsules and molecular
dialysis membrane109,110, polyurethan111, chitosan-
PVP112 and chitosan-alginate113,114 microcapsules for
islet storage and transplantation purposes. Polymeric
biomaterials such as alginate115, agarose116, polyamide
4-6 membranes11 7, poly (ethylene glycol)
diacrylate118, polyvinylchloride acrylic copolymer119,
AN69120, polyurethane-polydimethylsiloxane121, poly(2-hydroxyethylmetacrylate) 122 have also been
proposed as immunobarriers.
Despite these successes the hurdles to be
conquered are monumentous. Theoretically it would
be best if an individual could simply regenerate its
own pancreas/islets.
cell replication and/or regeneration
Pancreatic islet cell growth can be mediated
by two separate mechanisms123. Either new islets can
generate from budding of the pancreatic ductal
epithelium 124-128 or from intra islet precursor
cells129,130, i.e., islet-neogenesis and by replication of
existing islet beta cells123,131 (Fig. 3).
Is let neogenesis : Neogenesis of islets primarily
occurs during foetal and perinatal stages of
development132, but has also been observed in the
regenerating adult pancreas133,134. In a population of well differentiated adult pancreatic islet cells, the
number of cells actually undergoing cell division
is small, measured to be between 0.5 to 2 per cent135.
Although the growth potential of the pancreatic islet
cells is limited, glucose (nutrients), c-AMP, and
certain polypeptide growth factors have been
reported to exert modest stimulatory effects on cell
growth and replication136 . Several authors have
reported differential effects of various growth factors
on islet neogenesis phenomenon. Movassat et al137,
have investigated the effects of keratinocytes growth
factor (KGF), in vitro, on cell differentiation from
undifferentiated pancreatic precursor cells. However
KGF does not help in cell replication137. Similarly
vascular endothelial growth factor (VEGF- ligand of
foetal liver kinase-1), has been shown to play a role
in the development of foetal rat islet-like structures
in vitro, possibly by stimulating the maturation of
endocrine precursor cells in the pancreatic ductal
epithelium138. Epidermal growth factor (EGF), an
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activator of the MAP kinase pathway, increases the
mass of pancreatic epithelial cells but the absolute
number of developing endocrine cells decreases. On
the other hand, inhibition of MAPK pathway byPD98059 in the precursor cells leads to decreased
proliferation of epithelial cells but endocrine cell
differentiation was activated. Hence, MAPK pathway
determines the final mass of developing endocrine
tissue139.
cell replication: Sjoholm et al140, reported that
lithium treatment stimulates rat cell replication
and long term insulin secretion in vitro. The
relationship between cell replication and insulin
release was further investigated using neonatal rat
pancreatic monolayer cell cultures and the study
demonstrates the importance of glucose utilization
for both of these cell processes141. In another study
authors have demonstrated that even after complete
destruction of cells by STZ treatment in vitro,
foetal pancreatic cells retain the ability to regenerate cells142. The potential for large scale production
of endocrine tissue in vitro has been indicated,
however, more investigation needs to be carried out
on the various signals and pathways involved in
pancreatic development. An attempt to transduce
NPI (neonatal pancreatic islet) with gene of interest
i.e., PDX-1, allowed researchers to determine the
effects on islet maturation. The authors believed that
these transduced NPIs provide an effective tool to
study islet growth and maturation143. Transfection
of cells with tyrosine kinase receptors144 and
human islets with chimeric signaling receptors145
leads to ligand dependent cell proliferation. This
Fig. 3. Islet beta cell birth. Figure depicts different ways of islet beta cell neogenesis. Inset 1 to 4 depicts various stages in islet
generation from pancreatic duct epithelium stem cell monolayer (1), intense zone of activity (2), islet formation (3) and fully formed
mature islet (4). Further inset sequence represents trans-differentiation of islets from non-pancreatic stem cells viz. bone marrow,
acinar cells and hepatic progenitor cells. Insets 5: monolayer, 6: islet-like cell clusters and 7: Budding and maturation of islets.
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strategy has potential to reduce the quantity of
human islets required for treatment of patients with
type 1 diabetes.
Conclusion
It is apparent that isolated islets form a handy
model system due to ease of isolation and
maintenance. Being miniature organ systems, they
do not require a nervous control and manipulations
like transfection studies related to signaling
pathways, insulin stimulation and secretion assays
are easy to perform in an in vitro system. Along with
ease of handling, studies based on isolated islets can
be extrapolated and data corroborate with related in
vivo findings, with high efficiency, thus supporting
the 3R principle of ‘Reduction, Refinement and
Replacement’ of animals in biomedical research.
These factors have led to islets being popularly used
as a compatible model system for diabetes and related
research. All above mentioned approaches are
considered in context of their current status, progress,
future challenges or limitations, and long-term
prospects for a cure. Although definitive success is
still at the horizon, the advances reviewed herepredict the future to be bright.
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