PLATELET-DERIVED GROWTH FACTOR IN EXPERIMENTAL DIABETIC ATHEROSCLEROSIS
Dr. Markus Lassila and Prof. Mark E. Cooper
Baker Medical Research Institute
Melbourne 8008 VIC
Australia
Diabetes – A Risk Factor for Atherosclerosis
Diabetes is associated with accelerated atherosclerosis, the major factor contributing to increased mortality and morbidity in this population [1,2]. After correction for the other major risk factors, dyslipidemia, hypertension, and obesity, diabetes remains an independent risk factor for atherosclerosis [3,4]. The molecular mechanisms by which diabetes promotes atherosclerosis are not fully delineated but it has been suggested to be multi-factorial. In addition to conventional factors linked to atherosclerosis such as smoking, hypertension, and dyslipidemia, it is likely that other factors may be particularly relevant in the diabetic context, such as activation of the renin-angiotensin system, accumulation of advanced glycation end-products as a result of chronic hyperglycemia and upregulation of certain cytokines and growth factors. A recent study by our group has explored the potential role of recently developed tyrosine kinase inhibitors which interfere with signaling of growth factors such as PDGF [5].
Upregulation of the Platelet-derived Growth Factor Pathway in Diabetes
Platelet-derived growth factor (PDGF) has
at least five different isoforms that bind to two structurally and functionally
related receptors PDGFR-a and PDGFR-ß. Ligand binding induces receptor
dimerization and autophosphorylation, leading to cell growth, proliferation,
chemotaxis, and differentiation [6]. Overactivation of the PDGF system is
suggested to play a role in various vascular proliferative diseases, including
atherosclerosis [7-10].
Considerable evidence suggests that the diabetic state is associated with
vascular upregulation of the various components of the PDGF pathway. PDGF
levels are increased in response to a variety of factors that have been implicated
in diabetic cardiovascular disease, including angiotensin II [11,12], endothelin
[13], inflammatory cytokines [14], and advanced glycation end products [15,16].
In vitro studies have demonstrated that high glucose concentrations per se
increase PDGFR-ß expression in human endothelial cells [17] and monocyte-derived
macrophages [18] as well as in rabbit aortic smooth muscle cells [18]. In
vivo, PDGFR-ß expression is reported to be increased in medial smooth
muscle cells in an animal model of non-insulin dependent diabetes mellitus
[19]. As PDGF has been shown in vitro and in vivo to have atherogenic effects,
it is conceivable that this growth factor could play a pivotal role in the
progression of atherosclerosis, specifically in the context of diabetes. While
this remains unproven in the clinical setting, considerable progress has recently
been made in in vivo studies in experimental diabetes. This sort of investigation
has been greatly facilitated by two specific factors: Firstly, the advent
of a model of diabetes-associated atherosclerosis involving induction of chemical
diabetes in apolipoprotein E knock-out (apo E-KO) mice. This model displays
accelerated atherosclerosis and is considered relevant but not identical to
the human situation [20,21]; and secondly, the development of novel orally
bioavailable antagonists of PDGF receptor phosphorylation, such as imatinib
(STI-571) [22], have made interventional studies possible.
PDGF Antagonism Protects From Experimental Diabetic Atherosclerosis
In a recently published paper in Arteriosclerosis,
Thrombosis and Vascular Biology, we addressed the role of PDGF in the development
of atherosclerosis in apo E-KO mice with long-term diabetes [5]. Our findings
showed that in long-term (20 weeks) diabetic apo E-KO mice, the PDGF pathway
in the aorta, specifically in the atherosclerotic plaques, is upregulated
at the level of the ligand (PDGF-B) and phosphorylation of the PDGFR-ß
receptor. Furthermore, the tyrosine kinase inhibitor, imatinib (STI-571),
which inhibits PDGFR-ß phosphorylation [22], ameliorated the development
of atherosclerosis in these mice as well as reducing putative mediators of
vascular injury in this model including certain cytokines, chemokines, and
adhesion molecules. Interestingly, when imatinib was administered to non-diabetic
apo E-KO mice, no anti-atherosclerotic effect was observed. These results
suggest that PDGF is not only a passive marker of accelerated atherosclerosis
in diabetes but plays an active role in the development of atherosclerosis
and is, therefore, a potential target for preventing atherosclerosis, specifically
in diabetes.
In previous findings in the same mouse strain, in apo E-KO mice fed a normal
rodent diet, an antibody for either receptor subtype PDGFR-? or PDGFR-? administered
for 12 weeks had no significant effect on the development of early fatty streak
lesions. However, in apo E-KO mice fed with a high-fat and high-cholesterol
diet from six weeks of age, the antibody for PDGFR-? (but not for PDGFR-?)
administered for 12 weeks significantly reduced the aortic atherosclerotic
lesion size and smooth muscle infiltration into the intima [10]. Furthermore,
a recent study in Science explored the effect of imatinib in LDL-receptor
knock-out/smooth muscle specific LDL-receptor related protein (LRP1) gene
inactivated mice [23]. These mice developed accelerated atherosclerosis which
was attenuated by imatinib [23] while no such effect was demonstrated in mice
with only gene inactivation for the LDL receptor. Therefore, it seems that
the PDGF pathway plays a major role in the development of complex atherosclerotic
lesions not only in diabetes but also in the context of other exacerbating
factors such as hyperlipidemia.
Our study did not directly address the mechanisms which led to upregulation
of the PDGF pathway but it is likely that various stimuli relevant to the
diabetic milieu were involved. Not only glucose but angiotensin II, advanced
glycation end-products and other growth factors could have been implicated.
Indeed, other therapeutic approaches to reduce diabetes associated atherosclerosis
including targeting of these stimuli have been demonstrated to be anti-atherosclerotic
in diabetic apo E-KO mice. This includes inhibition of advanced glycation
either using soluble RAGE [24] or inhibition of vascular accumulation advanced
glycation endproducts [25] as well as agents which block the renin-angiotensin
system such as ACE inhibitor and angiotensin II antagonists [20,26].
In summary, our recent study provides a different approach for combating atherosclerosis
by targeting certain molecular mediators of vascular injury using novel, often
highly specific, orally active agents, which modulate critical signaling pathways
which are pivotal in inducing or promoting the development of atherosclerotic
lesions.
References
1.
Laakso M. Hyperglycemia and cardiovascular disease in type 2 diabetes. Diabetes
1999;48:937-42.
2. Garcia MJ, McNamara PM, Gordon T, Kannel WB. Morbidity and mortality in
diabetics in the Framingham population. Sixteen year follow-up study. Diabetes
1974;23:105-11.
3. Uusitupa MI, Niskanen LK, Siitonen O, Voutilainen E, Pyorala K. 5-year
incidence of atherosclerotic vascular disease in relation to general risk
factors, insulin level, and abnormalities in lipoprotein composition in non-insulin-dependent
diabetic and nondiabetic subjects. Circulation 1990;82:27-36.
4. Manson JE, Colditz GA, Stampfer MJ, et al. A prospective study of maturity-onset
diabetes mellitus and risk of coronary heart disease and stroke in women.
Arch Intern Med 1991;151:1141-47.
5. Lassila M, Allen TJ, Cao Z, et al. Imatinib attenuates diabetes-associated
atherosclerosis. Arterioscler Thromb Vasc Biol 2004 Feb 26 [Epub ahead of
print].
6. Heldin CH, Westermark B. Mechanism of action and in vivo role of platelet-derived
growth factor. Physiol Rev 1999;79:1283-316.
7. Billett MA, Adbeish IS, Alrokayan SA, Bennett AJ, Marenah CB, White DA.
Increased expression of genes for platelet-derived growth factor in circulating
mononuclear cells of hypercholesterolemic patients. Arterioscler Thromb Vasc
Biol 1996;16:399-406.
8. Lamb DJ, Avades TY, Ferns GA. Endogenous neutralizing antibodies against
platelet-derived growth factor-aa inhibit atherogenesis in the cholesterol-fed
rabbit. Arterioscler Thromb Vasc Biol 2001;21:997-1003.
9. Rutherford C, Martin W, Carrier M, Anggard EE, Ferns GA. Endogenously elicited
antibodies to platelet derived growth factor-BB and platelet cytosolic protein
inhibit aortic lesion development in the cholesterol-fed rabbit. Int J Exp
Pathol 1997;78:21-32.
10. Sano H, Sudo T, Yokode M, et al. Functional blockade of platelet-derived
growth factor receptor-beta but not of receptor-alpha prevents vascular smooth
muscle cell accumulation in fibrous cap lesions in apolipoprotein E-deficient
mice. Circulation 2001;103:2955-60.
11. Deguchi J, Makuuchi M, Nakaoka T, Collins T, Takuwa Y. Angiotensin II
stimulates platelet-derived growth factor-B chain expression in newborn rat
vascular smooth muscle cells and neointimal cells through Ras, extracellular
signal-regulated protein kinase, and c- Jun N-terminal protein kinase mechanisms.
Circ Res 1999;85:565-74.
12. Johnson RJ, Alpers CE, Yoshimura A, et al. Renal injury from angiotensin
II-mediated hypertension. Hypertension 1992;19:464-74.
13. Jaffer FE, Knauss TC, Poptic E, Abboud HE. Endothelin stimulates PDGF
secretion in cultured human mesangial cells. Kidney Int 1990;38:1193-98.
14. Silver BJ, Jaffer FE, Abboud HE. Platelet-derived growth factor synthesis
in mesangial cells: induction by multiple peptide mitogens. Proc Natl Acad
Sci U S A 1989;86:1056-60.
15. Doi T, Vlassara H, Kirstein M, Yamada Y, Striker GE, Striker LJ. Receptor-specific
increase in extracellular matrix production in mouse mesangial cells by advanced
glycosylation end products is mediated via platelet-derived growth factor.
Proc Natl Acad Sci U S A 1992;89:2873-77.
16. Throckmorton DC, Brogden AP, Min B, Rasmussen H, Kashgarian M. PDGF and
TGF-beta mediate collagen production by mesangial cells exposed to advanced
glycosylation end products. Kidney Int 1995;48:111-17.
17. Okuda Y, Adrogue HJ, Nakajima T, et al. Increased production of PDGF by
angiotensin and high glucose in human vascular endothelium. Life Sci 1996;59:1455-61.
18. Inaba T, Ishibashi S, Gotoda T, et al. Enhanced expression of platelet-derived
growth factor-beta receptor by high glucose. Involvement of platelet-derived
growth factor in diabetic angiopathy. Diabetes 1996;45:507-12.
19. Tamura K, Kanzaki T, Tashiro J, et al. Increased atherogenesis in Otsuka
Long-Evans Tokushima fatty rats before the onset of diabetes mellitus: association
with overexpression of PDGF beta-receptors in aortic smooth muscle cells.
Atherosclerosis 2000;149:351-58.
20. Candido R, Jandeleit-Dahm KA, Cao Z, et al. Prevention of accelerated
atherosclerosis by angiotensin-converting enzyme inhibition in diabetic apolipoprotein
E-deficient mice. Circulation 2002;106:246-53.
21. Park L, Raman KG, Lee KJ, et al. Suppression of accelerated diabetic atherosclerosis
by the soluble receptor for advanced glycation endproducts. Nat Med 1998;4:1025-31.
22. Buchdunger E, O'Reilly T, Wood J. Pharmacology of imatinib (STI571). Eur
J Cancer 2002;38(Suppl 5):S28-36.
23. Boucher P, Gotthardt M, Li WP, Anderson RG, Herz J. LRP: role in vascular
wall integrity and protection from atherosclerosis. Science 2003;300:329-32.
24. Bucciarelli LG, Wendt T, Qu W, et al. RAGE blockade stabilizes established
atherosclerosis in diabetic apolipoprotein E-null mice. Circulation 2002;106:2827-35.
25. Forbes J, Yee LTL, Thallas V, et al. Advanced glycation end product interventions
reduce diabetes accelerated atherosclerosis. Diabetes 2004; in press.
26. Candido R, Allen TJ, Lassila M, et al. Irbesartan but not amlodipine suppresses
diabetes-associated atherosclerosis. Circulation 2004;109:1536-42.
Please address correspondence to:
Prof. Mark Cooper
Diabetes Complications Group
Baker Medical Research Institute
PO Box 6492
Melbourne 8008 VIC
Australia
Tel: 613 8532 1362
Fax: 613 8532 1480
E-mail: mark.cooper@baker.edu.au