Peroxisome proliferator–activated receptors (PPARs) form a family of nuclear hormone
receptors involved in energy hemostasis and lipid metabolism (1,2) and include three
isotypes encoded by different genes: PPARα (chromosome 22q12–13.1), PPARβ/δ (chromosome
6p21.2–21.1), and PPARγ (chromosome 3p25). PPARα was the first discovered and causes
cellular peroxisome proliferation in rodent livers (3), giving this receptor family
its name. Upon activation, PPARs interact with retinoid X receptor to create heterodimers,
which bind to a specific DNA sequence motif termed peroxisome proliferator response
element (4). Peroxisome proliferator response element usually appears in promoter
regions and is constructed from repeats of nucleotide sequence AGGTCA separated by
a single nucleotide.
PPARα is widely expressed in tissues with high fatty acid catabolic activity: brown
fat, heart, liver, kidney, and intestine (5). Upon activation by endogenous fatty
acids and their derivatives, PPARα mediates fatty acid catabolism, gluconeogenesis,
and ketone body synthesis, mainly in liver (6–9). In rodents, PPARα activation also
influences immune modulation (10,11) and amino acid metabolism (12), reduces plasma
triglyceride, reduces muscle and liver steatosis, and ameliorates insulin resistance
(IR) (13,14). Pharmacologic PPARα activation is achieved by fibrates (7) and results
in reduced (30–50%) triglyceride and VLDL levels by increasing lipid uptake, lipoprotein
lipase–mediated lipolysis, and β-oxidation (15). This is accompanied by a modest increase
in HDL cholesterol (5–20%), secondary to transcriptional induction of apolipoprotein
A-I/A-II synthesis in liver (15). In man, the primary effect of PPARα is to reduce
plasma triglyceride concentration; effects on plasma free fatty acid (FFA) concentration/FFA
oxidation, muscle/liver fat content, and muscle/hepatic insulin sensitivity have not
been demonstrated with current PPARα agonists such as fenofibrate (16,17). Fibrates
are used to treat severe hypertriglyceridemia and combined hyperlipidemia (18–20).
Clinical trials to establish a role for PPARα agonists (fenofibrate, gemfibrozil)
in primary or secondary cardiovascular prevention in patients with hypertriglyceridemia
or diabetes have been disappointing (21,22). Clinically significant effects of fibrates
on glucose homeostasis, IR, and insulin secretion in man have not been demonstrated
(16,17,23).
PPARβ/δ is expressed ubiquitously, correlating with the level of cellular proliferation
exhibited in different tissues (24). In rodents, PPARβ/δ activation exerts metabolic
effects in skin, gut, skeletal muscle, adipose tissue, and brain (25,26). Several
PPARβ/δ agonists are in clinical trials because of their beneficial effects on dyslipidemia
(27,28) and other components of metabolic syndrome (29,30).
PPARγ has two splice variants, PPARγ1 and PPARγ2, differing by 30 amino acids in the
N′ terminal end. While PPARγ1 is widely expressed in tissues (skeletal muscle heart,
liver) at low levels, both are highly expressed in adipose tissue (31,32). PPARγ is
considered the “master” regulator of adipogenesis (33). PPARγ overexpression in cultured
fibroblasts transforms them into adipocytes (34), while selective adipose deletion
of PPARγ results in lipodystrophy and IR (35–37). Dominant negative PPARγ mutations
are associated with lipodystrophy (in the limbs and gluteal region), dyslipidemia,
hypertension, and severe IR (38–40). PPARγ polymorphisms (specifically, Pro12Ala)
are associated with increased risk of developing type 2 diabetes (T2DM) (41–43). PPARγ
agonists, thiazolidinediones (2,44,45), are potent insulin sensitizers, enhance insulin
secretion, improve glucose tolerance, and are the focus of this review.
THIAZOLIDINEDIONES: PAST TO PRESENT
Troglitazone was the first thiazolidinedione approved by the U.S. Food and Drug Administration
(FDA) and shown to improve insulin sensitivity and β-cell function in T2DM, impaired
glucose tolerance (IGT), and nondiabetic individuals (46–50). Troglitazone also was
shown to improve endothelial dysfunction in obesity and T2DM (49,51), induce ovulation
in PCOS (52), and effectively treat lipodystrophy (53). Troglitazone also caused fat
redistribution from visceral to subcutaneous adipose tissue (54,55) and reduced circulating
levels of inflammatory adipocytokines and FFAs, while increasing plasma adiponectin
levels (2). Thus, troglitazone shares many beneficial effects with pioglitazone and
rosiglitazone. However, because of hepatotoxicity troglitazone was removed from the
U.S. market by the FDA in 1997 (56). However, the idiosyncratic liver toxicity observed
with troglitazone does not appear to be a class effect. In a review of the literature,
alanine aminotransferase levels >10 times the upper limit of normal were observed
in 0.68% of diabetic patients treated with troglitazone versus no individuals treated
with pioglitazone or rosiglitazone (57). (See subsequent discussion on nonalcoholic
steatohepatitis [NASH].)
Rosiglitazone shares similar beneficial effects with pioglitazone and troglitazone
on insulin sensitivity, β-cell function, glycemic control, endothelial function, and
adipocyte metabolism (see subsequent discussion). However, because of concerns about
cardiovascular safety rosiglitazone has been severely restricted in the U.S. and has
been removed from the market in Europe and many other countries. In 2007, a meta-analysis
by Nissen and Wolski (58) suggested an increased incidence of cardiovascular events
in diabetic patients treated with rosiglitazone. In 2010, a patient-level analysis
by FDA statisticians of data supplied by GlaxoSmithKline gave hazard ratio (HR) 1.4
for composite MACE end point (cardiovascular death, myocardial infarction [MI], stroke)
and 1.80 for MI (59), leading to removal of rosiglitazone from the U.S. market for
all practical purposes. In a recent literature review, Schernthaner and Chilton found
that rosiglitazone consistently was associated with HR >1.0 for cardiovascular events,
while pioglitazone was associated with HR <1.0 (60).
In subsequent sections, we will focus on the pleotrophic effect of thiazolidinediones,
with emphasis on pioglitazone and rosiglitazone.
Pleotrophic effects of PPARγ agonists
PPARγ agonists exert pleotrophic effects on glucose and lipid metabolism in multiple
tissues and have become an important therapeutic agent for treating T2DM (45,61,62).
Glycemic control.
Thiazolidinediones are potent insulin sensitizers in liver/muscle/adipocytes (14,61–67),
augment/preserve β-cell function (68), and produce durable HbA1c reduction in T2DM.
In eight of eight long-term (>1.5 years), double-blind, or active comparator studies
(Fig. 1), thiazolidinediones caused durable HbA1c reduction (rev. in 61) lasting up
to 5–6 years (69). Their durable effect on glycemic control results from combined
action to both augment β-cell function and enhance insulin sensitivity. In T2DM patients
with starting HbA1c 8.0–8.5%, one can expect a 1.0–1.5% decrease in HbA1c (70–76).
Thiazolidinediones are approved for monotherapy and add-on therapy to all oral hypoglycemic
agents, glucagon-like peptide-1 analogs, and insulin (76).
Figure 1
Thiazolidinediones produce a sustained long-term reduction in HbA1c in eight of eight
double-blind or placebo- or active-comparator controlled studies. (See text for a
more detailed discussion.) Reprinted with permission from DeFronzo (61).
Insulin sensitivity in liver and muscle.
In liver, thiazolidinediones augment insulin sensitivity and inhibit gluconeogenesis,
leading to reduction in fasting plasma glucose concentration (63,64). In muscle, thiazolidinediones
are the only true insulin sensitizers, producing a decline in postprandial glucose
levels (61,66,67). Metformin is a weak insulin sensitizer in muscle, and it has been
difficult to demonstrate a muscle insulin-sensitizing effect in absence of weight
loss (77,78). Thiazolidinedione-mediated improvement in insulin sensitivity in T2DM
is mediated via multiple mechanisms: PPARγ activation, enhanced insulin signaling,
increased glucose transport, enhanced glycogen synthesis, improved mitochondrial function,
and fat mobilization out of muscle/liver, i.e., reversal of lipotoxicity (45,62,79–82).
Recent studies suggest that metabolic effects of thiazolidinediones are mediated by
mitochondrial target of thiazolidinediones, mtot1 and mtot2, which represent the pyruvate
transporter (83,84).
For insulin to exert its metabolic effects, it must first bind to and activate insulin
receptor by phosphorylating three key tyrosine molecules on β chain (Fig. 2). This
causes insulin receptor substrate (IRS)-1 translocation to plasma membrane, where
it undergoes tyrosine phosphorylation, leading to phosphatidylinositol 3-kinase (PI3
kinase) and Akt activation. This causes glucose transport into cell, activation of
nitric oxide synthase with arterial vasodilation (85–87), and stimulation of multiple
intracellular metabolic processes (45).
Figure 2
Insulin signal transduction in healthy nondiabetic (left panel) and T2DM (right panel)
subjects. Thiazolidinediones improve insulin signaling through the PI-3 kinase pathway,
while inhibiting insulin signaling through the MAP kinase pathway. Reprinted with
permission from DeFronzo (61).
In humans, we demonstrated that insulin-stimulated tyrosine phosphorylation of IRS-1
in muscle is severely impaired in lean T2DM (81,88,89), in obese normal glucose tolerant
(NGT) individuals (89), and in insulin-resistant NGT offspring of two T2DM parents
(90,91) (Fig. 2); similar results have been reported by others (92–95). This insulin-signaling
defect leads to reduced glucose transport, impaired nitric oxide release (explaining
endothelial dysfunction), and multiple defects in intramyocellular glucose metabolism.
In contrast to the defect in IRS-1 activation, the mitogen-activated protein (MAP)
kinase pathway, which can be activated by Shc, is normally responsive to insulin (61,62,88,89)
(Fig. 2). Stimulation of MAP kinase activates multiple intracellular pathways involved
in inflammation, cellular proliferation, and atherogenesis (62,96–98).
The defect in IRS-1 tyrosine phosphorylation impairs glucose transport, and resultant
hyperglycemia stimulates fasting/postprandial insulin secretion. Because MAP kinase
retains normal sensitivity to insulin (62,88,89,94), hyperinsulinemia causes excessive
stimulation of this pathway and activation of multiple intracellular pathways involved
in inflammation and atherogenesis. This provides a pathogenic link that, in part,
can explain the strong association between IR and atherosclerotic cardiovascular disease
in nondiabetic and T2DM individuals (99–102).
Thiazolidinediones are the only antidiabetes drugs that simultaneously augment insulin
signaling through IRS-1 and inhibit MAP kinase pathway (61,77,81), providing a molecular
mechanism to explain results from CHICAGO (104) and Pioglitazone Effect on Regression
of Intravascular Sonographic Coronary Obstruction Prospective Evaluation (PERISCOPE)
(105) studies, in which pioglitazone reduced progression of carotid intima-media thickness
(IMT) and coronary atherosclerosis in T2DM. Consistent with these anatomical studies,
pioglitazone in PROactive (106) decreased (P = 0.027) MACE end point (death, MI, stroke)
by 16%.
Adipocyte insulin sensitivity.
In adipose tissue, thiazolidinediones are potent insulin sensitizers, inhibiting lipolysis
and release of inflammatory cytokines, while increasing adiponectin secretion (67,79,80,107–109).
In T2DM and obese NGT individuals, adipocytes are resistant to insulin’s antilipolytic
effect, resulting in accelerated triglyceride breakdown with release of FFA. Elevated
plasma FFAs enhance FFA flux into cells, leading to accumulation of toxic lipid metabolites
(fatty acyl CoAs, diacylglycerol, ceramides), which inhibit insulin action in muscle/liver
(62,110–112) and impair β-cell function (113). Thus, these lipotoxic molecules antagonize
the core defects that characterize T2DM. By improving insulin sensitivity in adipocytes
and inhibiting lipolysis, thiazolidinediones reduce plasma FFA, leading to enhanced
insulin sensitivity in muscle/liver and improved β-cell function in T2DM.
In T2DM, adipocytes are in a state of chronic inflammation, as evidenced by monocyte
infiltration (114). Inflamed adipocytes release adipocytokines (tumor necrosis factor-α,
resistin, angiotensinogen, plasminogen activator inhibitor 1, interleukin-6, and others),
which cause IR, impair β-cell function, promote inflammation in distant tissues, augment
thrombosis, and accelerate atherogenesis (79,80). Adipocytes from T2DM patients have
reduced ability to secrete adiponectin (81,82), a potent vasodilator and antiatherogenic
molecule. Thiazolidinediones suppress inflammation in adipose tissue, inhibit release
of inflammatory and prothrombotic adipokines, and augment adiponectin secretion.
Thiazolidinediones reverse lipotoxicity
The current diabetes epidemic is being driven by the obesity epidemic. Both obesity
and T2DM are characterized by tissue fat overload (Fig. 3). Accumulation of intracellular
toxic lipid metabolites causes IR in muscle/liver by inhibiting insulin signaling,
glycogen synthesis, and glucose oxidation (rev. in 61,62). Fat accumulation in liver
causes nonalcoholic fatty liver disease (NAFLD) and NASH (115), which has become the
leading cause of cirrhosis in Westernized countries. Fat accumulation in β-cells impairs
insulin secretion and promotes apoptosis (113). Fat deposition in arteries promotes
atherogenesis (62), while fat accumulation in visceral depots is associated with coronary
arterial disease (116).
Figure 3
Body fat distribution in T2DM patients and its redistribution with thiazolidinediones
(TZD). (See text for a detailed discussion.) TG, triglyceride. Reprinted with permission
from DeFronzo and colleagues (79).
Thiazolidinediones reverse lipotoxicity by mobilizing fat out of muscle/liver/β-cells/arteries
and relocating fat to subcutaneous adipose depots where it is metabolically “benign”
(62,79,80) (Fig. 3). After binding to PPARγ, thiazolidinediones stimulate subcutaneous
adipocytes to divide and induce multiple genes involved in lipogenesis (117). Newly
formed subcutaneous adipocytes take up FFA, leading to marked reduction in plasma
FFA and decreased FFA flux into liver/muscle/β-cells/arteries. Thiazolidinediones
also increase expression of PPARγ coactivator (PGC-1), the master regulator of mitochondrial
biogenesis (118,119). Increased PGC-1 upregulates multiple mitochondrial oxidative
phosphorylation genes, increasing fat oxidation and decreasing levels of intracellular
toxic lipid metabolites.
Thiazolidinediones and β-cell function
Thiazolidinediones exert potent effects to improve/preserve β-cell function (68) and
demonstrate durability of glycemic control for up to 5–6 years in eight of eight studies
(rev. in 61). This is in contrast to sulfonylureas and metformin, which, after initial
HbA1c decline, are associated with progressive HbA1c rise, resulting from progressive
β-cell failure (120–122).
In addition to studies performed in T2DM, six studies demonstrate that thiazolidinediones
prevent IGT progression to T2DM (123–128). In Diabetes Reduction Assessment with Ramipril
and Rosiglitazone Medication (DREAM), T2DM was reduced by 62% with rosiglitazone (124),
while in Actos Now for the prevention of diabetes (ACT NOW) (127) pioglitazone decreased
IGT conversion to T2DM by 72%. All six studies demonstrated that, in addition to their
insulin-sensitizing effect, thiazolidinediones preserved β-cell function. β-Cells
respond to increased plasma glucose levels with an increase in insulin secretion,
and ΔI/ΔG is modulated by severity of IR (128). The insulin secretion/IR index (ΔI/ΔG
÷ IR) represents the gold standard for β-cell function and should not be equated with
plasma insulin response. In ACT NOW, improvement in insulin secretion/IR index was
the strongest predictor of diabetes prevention in IGT subjects and reversion to NGT.
Similar results have been demonstrated in TRoglitazone In the Prevention Of Diabetes
(TRIPOD) and Pioglitazone In Prevention Of Diabetes (PIPOD) (123,126), in which development
of diabetes in Hispanic women with GDM was decreased by 52 and 62%. In Canadian Normoglycemia
Outcomes Evaluation (CANOE) (128), low-dose rosiglitazone (4 mg/day), combined with
low-dose metformin (1,000 mg/day), reduced IGT conversion to T2DM by 70%. In vivo
and in vitro studies with human/rodent islets demonstrate that thiazolidinediones
exert protective effects on β-cell function (129–131). Studies from our group using
insulin secretion/IR index have shown that thiazolidinediones markedly augment β-cell
function in T2DM patients (68) (Fig. 4).
Figure 4
Thiazolidinediones enhance β-cell function (insulin secretion/IR index) in new-onset,
drug-naïve T2DM patients and in long-standing, sulfonylurea-treated T2DM individuals
(69). *P < 0.01.
Improved β-cell function with thiazolidinediones results from 1) stimulatory effect
of PPARγ to increase GLUT2, glucokinase (132), and Pdx (133) in β-cells; 2) reduced
intracellular levels of toxic lipid metabolites (129,132,134,135); 3) muscle/liver
insulin-sensitizing effect of thiazolidinediones, which reduce insulin and, therefore,
amylin secretion (amylin degradation products are toxic to β-cells [136,137]; the
ability of thiazolidinediones to protect human islets from amylin toxicity is mediated
via PI3 kinase–dependent pathway [138]); and 4) studies in β-cell insulin receptor
knockout (BIRKO) mice suggest that defective insulin signaling through IRS-1/PI3 kinase
impairs insulin secretion (139) and that thiazolidinediones correct this insulin signaling
defect (129), resulting in enhanced insulin secretion.
Summary
Thiazolidinediones improve multiple defects (IR in liver/muscle/adipocytes and β-cell
dysfunction) that comprise the Ominous Octet (61) (Fig. 5), cause durable HbA1c reduction,
and can be used as monotherapy or in combination with any other antidiabetes agent.
Pioglitazone and rosiglitazone similarly reduce HbA1c, improve insulin sensitivity
in muscle/liver/adipocytes, and enhance β-cell function.
Figure 5
Pioglitazone corrects four of the eight pathophysiologic components of the Ominous
Octet. Modified with permission from DeFronzo (61). TZD, thiazolidinediones.
THIAZOLIDINEDIONES AND IR SYNDROME
IR (metabolic) syndrome represents a cluster of metabolic and cardiovascular disorders,
each of which represents a major cardiovascular risk factor (62). A common thread
linking all IR syndrome components is the basic molecular etiology of IR (61,62,81,88,89),
which not only promotes inflammation and atherogenesis but also aggravates other components
of the syndrome. Pioglitazone and rosiglitazone ameliorate the molecular defect in
insulin signaling, enhance muscle/hepatic/adipocyte insulin sensitivity, correct hyperinsulinemia,
improve glucose tolerance and endothelial dysfunction, reduce blood pressure, decrease
plasma FFA levels, increase HDL cholesterol, transform small dense LDL particles into
larger less atherogenic ones, shift body fat from visceral to subcutaneous depots,
mobilize fat out of muscle/liver, reduce plasminogen activator inhibitor 1/tumor necrosis
factor-α levels, and increase plasma adiponectin (rev. in 62). Rosiglitazone produces
metabolic effects similar to those of pioglitazone with two notable exceptions: rosiglitazone
increases both plasma LDL cholesterol and triglycerides (140). Concerns about cardiovascular
safety (58) have led to removal of rosiglitazone from U.S. (56) and European markets.
Pioglitazone reduces cardiovascular events
Pioglitazone is the only antidiabetes medication shown, in a large prospective placebo-controlled
outcome study, to reduce cardiovascular events. In PROactive, 5,238 T2DM patients
with prior cardiovascular event or multiple CVD risk factors were randomized to pioglitazone
or placebo plus standard of care for all cardiovascular risk factors (106). Compared
with placebo, pioglitazone reduced the second principal MACE end point (cardiovascular
mortality, MI, stroke) by 16% (P < 0.02) (Fig. 6A
). Cardiovascular benefit most likely resulted from combined improvements in dyslipidemia
(increased HDL cholesterol), endothelial dysfunction, blood pressure, HbA1c, other
inflammatory markers that were not measured, and direct effect on arterial wall to
inhibit atherogenesis (141). In a subgroup of 2,445 patients with previous MI, pioglitazone
reduced (HR 0.72, P = 0.04) likelihood of subsequent MI by 16% (142) (Fig. 6C
). In 984 patients with previous stroke, pioglitazone caused 47% reduction (HR 0.53,
P = 0.008) in recurrent stroke (3,143) (Fig. 6D
).
Figure 6
A: Kaplan-Meier plot of time to MACE end point (mortality, MI, stroke) in T2DM patients
treated with pioglitazone (PIO) or placebo (Plc) in PROactive. Redrawn with permission
from Dormandy et al. (106). B: Pioglitazone reduces recurrent MI in diabetic patients
with a previous MI in PROactive. Redrawn with permission from Erdmann et al. (142).
C: Pioglitazone reduces recurrent stroke in diabetic patients with a previous stroke
or PROactive. Redrawn with permission from Wilcox et al. (143). D: Meta-analysis of
all published studies (excluding PROactive) in which the effect of pioglitazone versus
placebo or active comparator on cardiovascular events is examined. Redrawn with permission
from Lincoff et al. (145).
The composite primary end point (mortality, nonfatal MI, silent MI, stroke, acute
coronary syndrome, coronary artery bypass grafting/percutaneous coronary intervention,
leg amputation, leg revascularization) did not reach significance (HR 0.90, P = 0.09)
because of increased number of leg revascularization procedures in the pioglitazone
group. Leg revascularization is not a MACE end point and typically is excluded from
cardiovascular intervention trials, i.e., with statins, because the major risk factors
for peripheral vascular disease are gravity (i.e., subject’s height) and smoking,
which are not influenced by antidiabetes therapy. Subsequent PROactive analyses confirmed
that pioglitazone has no beneficial effect on peripheral vascular disease (144). Consistent
with PROactive, a meta-analysis of all pioglitazone studies published (excluding PROactive)
and reported to the FDA demonstrated a 25% decrease in cardiovascular events (145)
(Fig. 6B
), and a recent review recommended that pioglitazone should be considered in diabetic
patients with cardiovascular disease (146).
Two additional studies demonstrated that pioglitazone slows anatomical progression
of atherosclerotic cardiovascular disease. In PERISCOPE (105), T2DM patients with
established coronary artery disease were randomized to pioglitazone or glimepiride
for 1.5 years. In the glimeperide-treated group, percent atheroma volume progressed,
while percent atheroma volume regressed in the pioglitazone-treated group. In CHICAGO,
pioglitazone halted progression of carotid IMT, whereas carotid IMT progressed in
the glimepiride-treated group (P = 0.008) (104). Results of these two anatomical trials
(104,105), when viewed in concert with cardiovascular outcome trials (106,145), strongly
suggest that pioglitazone provides cardiovascular protection, especially in individuals
with established cardiovascular disease.
The different effects of pioglitazone and rosiglitazone on cardiovascular outcomes
remains unexplained. One obvious explanation is rise in plasma LDL cholesterol and
triglyceride observed with rosiglitazone (140). Another explanation involves differential
regulation of gene expression by rosiglitazone and pioglitazone. In muscle (147) and
adipocytes (148), multiple genes are differentially stimulated or inhibited by the
two thiazolidinediones, and the function of these genes is largely unknown.
Thiazolidinediones prevent T2DM in high-risk individuals
Six large prospective, randomized, double-blind, placebo-controlled studies (TRIPOD
[126], PIPOD [123], DPP [125], DREAM [124], CANOE [128], and ACT NOW [127]) have provided
conclusive evidence that thiazolidinediones dramatically reduce by 52–72% conversion
of prediabetes (IGT and/or IFG) to T2DM. In ACT NOW, IGT conversion to T2DM was reduced
by 72% and carotid IMT progression was diminished by >50% versus placebo (127). Increased
β-cell function (insulin secretion/IR index) was the strongest predictor of diabetes
prevention. In ACT NOW and other prevention trials reductions in HbA1c, blood pressure,
triglycerides, inflammatory cytokines, and rise in HDL cholesterol also have been
observed (127).
THIAZOLIDINEDIONES AND NASH
In T2DM hepatic fat accumulation, NAFLD is common and represents a precursor for NASH.
NASH is associated with hepatic/muscle IR (115) and accelerated atheogenesis (148).
Several large, placebo-controlled studies have demonstrated that pioglitazone mobilizes
fat from liver, reduces hepatic injury, and causes histologic improvement in inflammation/fibrosis
in NASH (149–151). Pioglitazone also reduces liver fat and improves IR in lipodystrophic
patients (152). Studies examining effect of rosiglitazone in NASH have shown an initial
beneficial effect on liver histologic parameters with no benefit from prolonged continuous
treatment (153).
THIAZOLIDINEDIONES AND KIDNEY
Diabetic rodents develop renal insufficiency and histologic lesions analogous to those
in man, and thiazolidinediones reduce mesangial matrix (hallmark lesion of diabetic
nephropathy) volume, decrease urinary protein excretion, and prevent renal failure
(154,155). PPARγ is expressed diffusely throughout kidney, and PPARγ agonists inhibit
mesangial cell proliferation and reduce mRNA expression of matrix proteins (collagen,
fibronectin) and transforming growth factor-β, which has been implicated in glomerular
injury (156). In diabetic humans, pioglitazone (157) and rosiglitazone (158) reduce
albuminuria, although long-term studies examining effect of thiazolidinediones on
GFR have not been performed. Beneficial effect of thiazolidinediones to reduce albuminuria
cannot be explained by improved glycemic control and is closely correlated with improved
insulin sensitivity (159).
Diabetic individuals with renal insufficiency are at increased risk for cardiovascular
disease/mortality (159). In PROactive, pioglitazone significantly reduced MACE end
point in patients with and without reduced GFR (160). Thiazolidinediones also reduced
all-cause mortality in hemodialysis-treated patients (161).
SAFETY
Benefits of pioglitazone on glycemic control and prevention of cardiovascular disease
are well established. However, physicians must be cognizant of potential side effects
to maximize benefit and minimize risk. The majority of pioglitazone’s beneficial effects
on glucose metabolism, insulin sensitivity, insulin secretion, and cardiovascular
risk factors are observed with a dose of 30 mg/day (70,162). At this dose, side effects
are mild and manageable. Increasing dose to 45 mg/day provides little more efficacy
and substantially increases risk of side effects (70). Therefore, we recommend a starting
dose of 7.5–15 mg/day, tritiated to 30 mg/day (163–165). Combined pioglitazone/metformin
therapy (166,167) is particularly effective in reducing HbA1c, does not cause hypoglycemia,
and minimizes side effects. Moreover, both pioglitazone (106,145) and metformin (121)
reduce cardiovascular events, although the number (n = 344) of subjects in the metformin
arm of the UK Prospective Diabetes Study (UKPDS) was small and would not satisfy current
standards for a cardiovascular intervention study.
Fat weight gain
On average, pioglitazone-treated subjects gain ∼2–3 kg of fat weight after 1 year
(70,76,106,168), which results from PPARγ stimulation of hunger centers in hypothalamus
(169). Simultaneously, PPARγ activation redistributes fat from visceral to subcutaneous
depots (55,79,170), mobilizes fat out of muscle/liver/β-cells (79,80,149,150,171),
inhibits lipolysis/reduces plasma FFA (79,80,109), and stimulates PGC-1/other mitochondrial
genes involved in lipid oxidation (118). The net result is a metabolically more favorable
fat distribution from visceral to subcutaneous depots where it is metabolically benign
(79,80) and depletion of toxic lipid metabolites in muscle/liver/β-cells (62). Of
note, the greater the weight gain, the greater the improvements in β-cell function
and insulin sensitivity and the greater the reduction in HbA1c (68,170,172). On a
short-term basis, i.e., up to 3 years (106), no adverse effects of thiazolidinedione-associated
weight gain have been observed. Long-term effects, if any, of thiazolidinedione-associated
weight gain remain unknown. Weight gain, if excessive, should be managed with reinforcement
of dietary advice and exercise, reduction in pioglitazone dose, or use of pharmacologic
agents approved for weight loss.
Bone fractures
T2DM patients treated with thiazolidinediones have increased risk of fracture (173–176),
which primarily occurs in distal long bones of upper (forearm, hand, wrist) and lower
(foot, ankle, fibula, tibia) limbs and is related to trauma. Excess fracture risk
is 0.8 fractures per 100 patient-years (1.9 in pioglitazone treated vs. 1.1 in comparator
treated) (173–176). This represents a small but significant risk. Since increased
fracture risk primarily occurs in postmenopausal females and not in premenopausal
women or men, pioglitazone should be used with caution in postmenopausal women or
not at all.
Fluid retention and congestive heart failure
Thiazolidinediones may cause fluid retention, which can exacerbate heart failure in
diabetic patients who do not uncommonly have underlying diastolic dysfunction (106).
When used as monotherapy, edema occurs in 3–5% of individuals and is dose related
(177). Edema most commonly occurs when thiazolidinediones are used with sulfonylureas
and especially with insulin (177–180). Fluid retention occurs secondary to peripheral
vasodilation (181) and stimulation of ENac (epithelial sodium) channel in collecting
duct (182). Sodium retention responds well to distally acting diuretics, spironolactone
or triamterene (183). Pedal edema identifies individuals at risk to develop congestive
heart failure (CHF) and who should be treated with a diuretic or reduction in pioglitazone
dose. In PROactive, incidence of CHF was 6%. However, cases were not adjudicated,
and mortality and cardiovascular events tended to be decreased in pioglitazone-treated
individuals who developed CHF (106,184). These results suggest that after excess fluid
has been diuresed, the cardioprotective effect of pioglitazone becomes evident. Lastly,
pioglitazone has no negative impact on cardiac function (185) and improves endothelial
dysfunction (186).
THIAZOLIDINEDIONES AND CANCER
In PROactive (106), incidence of malignancy was similar in pioglitazone (3.7%) and
placebo (3.8%) groups. However, two imbalances were noted. There were more cases of
bladder cancer in pioglitazone (n = 16) versus placebo (n = 6) groups (P = 0.069).
Prior to unblinding, external experts adjudicated that 11 cases could not plausibly
be related to treatment. Of the remaining nine case subjects, six were treated with
pioglitazone and three with placebo (P = 0.309). The other imbalance was related to
breast cancer; there were fewer breast cancers in the pioglitazone versus placebo
group (3 vs. 11, P = 0.034). Thus, the nonsignificant increase in bladder cancer was
numerically offset by the statistically significant decrease in breast cancer.
In 2003, the FDA requested that a safety study be conducted to assess whether pioglitazone
increased bladder cancer risk. After 4 years of a 10-year longitudinal cohort study
of 193,099 patients (187), ever use of pioglitazone was not associated with increased
bladder cancer risk (HR 1.2 [95% CI 0.9–1.5]). However, in patients receiving pioglitazone
for ≥24 months, there was slight increased bladder cancer risk (1.4 [1.03–2.0]); 95%
of cancers were detected at an early in situ stage, and authors acknowledged that
this could have been attributed to the fact that pioglitazone-treated patients underwent
greater surveillance for bladder cancer. Bladder cancer risk increased from 7/10,000
patient-treatment years (no pioglitazone) to 10/10,000 (with pioglitazone)—an increase
of 3 cases per 10,000 patient-treatment years. Overall, there was no increase in total
cancers in pioglitazone-treated patients (187,188), and risk of some cancers (colon,
kidney/renal pelvis, breast) was decreased (188). In a recent 8-year analysis of the
same study population, HR for bladder cancer was 0.98 (95% CI 0.81–1.18) (189). If
pioglitazone actually increased bladder cancer risk, one would have expected HR to
increase—not decrease—after 8 years. These results argue against a putative role for
pioglitazone in development of bladder cancer. Further, overall incidence of malignancy
has been reported not to increase (106) or decrease in certain cancer types (breast
and liver) in pioglitazone-treated patients (188,190–192). Lastly, any increased bladder
cancer risk must be viewed in the context of protection against all-cause death, MI,
and stroke, i.e., MACE end point in PROactive. It has been estimated that treatment
of 10,000 patients with pioglitazone would avoid 210 MIs, stroke, or deaths over 3
years (193) compared with a potential increase of three cases of bladder cancer per
10,000 patients over the same period. Moreover, even this increase of 3/10,000 disappeared
after 8 years (189).
Based upon the body of evidence reviewed above (not including 8-year follow-up data
reported by Lewis), the FDA recommended that pioglitazone not be used in patients
with active bladder cancer or prior bladder cancer history. We recommend that any
hematuria be evaluated to exclude bladder cancer before starting pioglitazone.
BENEFIT-RISK ANALYSIS
As reviewed in preceding sections, the benefit-to-risk ratio for pioglitazone is very
favorable. Importantly, if physicians are aware of potential risks associated with
thiazolidinediones and if the pioglitazone dose does not exceed 30 mg/day, side effects
can be reduced even further (Table 1).
Table 1
Benefits and risks associated with thiazolidinedione therapy