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Potential role of insulin receptor isoforms and IGF receptors in plaque instability of human and experimental atherosclerosis
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Clinical complications associated with atherosclerotic plaques arise
from luminal obstruction due to plaque growth or destabilization leading
to rupture. We previously demonstrated that overexpression of insulin
receptor isoform A (IRA) and insulin-like growth factor-I receptor
(IGF-IR) confers a proliferative and migratory advantage to vascular
smooth muscle cells (VSMCs) promoting plaque growth in early stages of
atherosclerosis. However, the role of insulin receptor (IR) isoforms,
IGF-IR or insulin-like growth factor-II receptor (IGF-IIR) in VSMCs
apoptosis during advanced atherosclerosis remains unclear.--
Results
We observed a significant decrease of IRA/IRB ratio in human complicated plaques as compared to non-complicated regions. Moreover, complicated plaques showed a reduced IGF-IR expression, an increased IGF-IIR expression, and lower levels of α-SMA indicating a loss of VSMCs. In experimental atherosclerosis, we found a significant decrease of IRA with an increased IRB expression in aorta from 24-week-old BATIRKO; ApoE−/− mice. Furthermore, atherosclerotic plaques from BATIRKO; ApoE−/− mice had less VSMCs content and higher number of apoptotic cells. In vitro experiments showed that IGF-IR inhibition by picropodophyllin induced apoptosis in VSMCs. Apoptosis induced by thapsigargin was lower in IR−/− VSMCs expressing higher IGF-IR levels as compared to IRLoxP+/+ VSMCs. Finally, IRB VSMCs are more prone to thapsigargin-induced apoptosis than IRA or IRLoxP+/+ VSMCs.Conclusions
In advanced human atherosclerosis, a reduction of IRA/IRB ratio, decreased IGF-IR expression, or increased IGF-IIR may contribute to VSMCs apoptosis, promoting plaque instability and increasing the risk of plaque rupture and its clinical consequences.
Background
Atherosclerosis
is a chronic disease affecting large arteries that involves the
formation of plaques containing vascular and inflammatory cells, lipids
and extracellular matrix [1].
Its clinical complications arise from luminal obstruction due to plaque
growth leading to vessel stenosis, and/or formation of unstable plaques
that acutely rupture leading to an occlusive thrombus formation [2].
Vascular smooth muscle cells (VSMCs) play a main role in this process
as they contribute to plaque growth in early stages, but favor plaque
stability in advanced stages of atherogenesis [3].
The insulin and insulin-like growth factors (IGFs) signaling is mediated by hormone interaction with the insulin receptor (IR) and the IGF-I receptor (IGF-IR), which are members of subclass II tyrosine kinase receptor super-family [4, 5]. In mammals, alternative splicing of the IR gene gives rise to two isoforms: IRA and IRB [6]. Indeed, IRB has an additional 12-amino acid sequence encoded by the exon 11. Although both isoforms have similar affinity for insulin, IRA exhibits a higher affinity for IGFs, especially for IGF-II [7]. Thus, IRB is preferentially associated with metabolic and differentiating signals, whereas IRA mainly favors cell growth, proliferation and survival [8]. In addition to IR and IGF-IR, IGF-II binds IGF-II receptor (IGF-IIR) with high affinity. IGF-IIR is a type I transmembrane glycoprotein that also have high affinity for mannose-6-phosphate, and can therefore bind lysosomal enzymes and other growth factors and cytokines [9]. It plays a well-documented role in the intracellular transport of lysosomal enzymes and in clearance of IGF-II from the circulation. However, although IGF-IIR contains neither tyrosine kinase activity nor an autophosphorylation site, it does link to G-proteins providing a mechanism for signal transduction that may be involved in cell behavior regulation [10, 11].
In early atherosclerotic lesions, IGFs contribute to plaque growth by promoting VSMCs proliferation and migration [12]. In this regard, we previously demonstrated that overexpression of IGF-IR or IRA isoform during early atherosclerosis confers a proliferative and migratory advantage to VSMCs favoring atherosclerotic progression [13, 14]. In advanced stages, the imbalance between cell death and survival may substantially affect the cellularity and integrity of atherosclerotic lesions contributing to plaque instability. Unstable plaques that are prone to rupture have a thin fibrous cap with a decreased number of VSMCs and a dense infiltration of inflammatory cells [15, 16], as well as an increased apoptosis of VSMCs and macrophages [17]. IGF-I, through IGF-IR, has been reported to prevent atherosclerotic plaque instability by its mitogenic and antiapoptotic effects on VSMCs [18,19,20]. However, the role of IR isoforms or IGF-IIR in VSMCs apoptosis and thereby in plaque instability remains unknown. In the present study, we analyzed the expression of IR isoforms, IGF-IR and IGF-IIR, as well as VSMCs content in human carotid atherosclerotic plaques and in experimental models of atherosclerosis. Finally, we assessed the contribution of IR isoforms and IGF-IR to the apoptosis of murine aortic VSMCs lines.
- SUOMENNOSTA:
- Insuliinin (Ins) ja insuliininkaltaisten kasvutekijöiden (IGFs) signalointi välittyy hormoni-interaktioilla ja reseptoreina toimivat insuliinireseptori (IR) ja IGF-I reseptori (IGF-IR), jotka ovat Tyrosiinikinaasireseptorien superperheen II-alaluokan jäseniä. Imettäväisissä alternatiivisella pleissauksellaa insuliinireseptorigeenistä tulee kaksi isoformia IR A ja IR B. IRB omaa 12 aminohapon lisäsekvenssin, jota koodaa exoni 11. Vaikka molemmat isoformit omaavat saman affiniteetin insuliinia kohtaan, niin IRA ilmentää suurempaa affiniteettia insuliininkaltaisia kasvutekijöitä ( IGFs) kohtaan, erityisesti IGF-II-muotoa kohtaan. Täten voidaan todeta,että IRB ensisijassa on assosioitunut metabolisiin ja differentioiviin signalointeihin, kun taas IRA pääasiassa suosii solukasvua, proliferaatiota ja elossapysymistä. Lisäksi IGF-II-muoto voi sitoutua omaan IGF-II-reseptoriin (IGF-IIR) suurella affiniteetilla.
- IGF-IIR on tyypiltään transmembraaninen glykoproteiini, jolla on suuri affiniteetti myös mannoosi-6-fosfaattiin, joten se voi sitoutua myös lysosomaalisiin entsyymeihin ja muihin kasvutekijöihin ja sytokiineihin. Sillä onkin hyvin dolumentoitu tehtävä solunsisäisessä lysosomaalisten entsyymien kuljetuksessa ja IGF-II:n puhdistamisessa verenkierrosta. Mutta IGF-IIR ei omaa tyrosiinikinaasiaktiivisuutta eikä se autofosforyloidukaan, vaan linkkiytyy G-proteiinivälitteisesti signaalin johtumisverkostoon ja täten voi osallistua solukäytöksen säätelyyn.
The insulin and insulin-like growth factors (IGFs) signaling is mediated by hormone interaction with the insulin receptor (IR) and the IGF-I receptor (IGF-IR), which are members of subclass II tyrosine kinase receptor super-family [4, 5]. In mammals, alternative splicing of the IR gene gives rise to two isoforms: IRA and IRB [6]. Indeed, IRB has an additional 12-amino acid sequence encoded by the exon 11. Although both isoforms have similar affinity for insulin, IRA exhibits a higher affinity for IGFs, especially for IGF-II [7]. Thus, IRB is preferentially associated with metabolic and differentiating signals, whereas IRA mainly favors cell growth, proliferation and survival [8]. In addition to IR and IGF-IR, IGF-II binds IGF-II receptor (IGF-IIR) with high affinity. IGF-IIR is a type I transmembrane glycoprotein that also have high affinity for mannose-6-phosphate, and can therefore bind lysosomal enzymes and other growth factors and cytokines [9]. It plays a well-documented role in the intracellular transport of lysosomal enzymes and in clearance of IGF-II from the circulation. However, although IGF-IIR contains neither tyrosine kinase activity nor an autophosphorylation site, it does link to G-proteins providing a mechanism for signal transduction that may be involved in cell behavior regulation [10, 11].
In early atherosclerotic lesions, IGFs contribute to plaque growth by promoting VSMCs proliferation and migration [12]. In this regard, we previously demonstrated that overexpression of IGF-IR or IRA isoform during early atherosclerosis confers a proliferative and migratory advantage to VSMCs favoring atherosclerotic progression [13, 14]. In advanced stages, the imbalance between cell death and survival may substantially affect the cellularity and integrity of atherosclerotic lesions contributing to plaque instability. Unstable plaques that are prone to rupture have a thin fibrous cap with a decreased number of VSMCs and a dense infiltration of inflammatory cells [15, 16], as well as an increased apoptosis of VSMCs and macrophages [17]. IGF-I, through IGF-IR, has been reported to prevent atherosclerotic plaque instability by its mitogenic and antiapoptotic effects on VSMCs [18,19,20]. However, the role of IR isoforms or IGF-IIR in VSMCs apoptosis and thereby in plaque instability remains unknown. In the present study, we analyzed the expression of IR isoforms, IGF-IR and IGF-IIR, as well as VSMCs content in human carotid atherosclerotic plaques and in experimental models of atherosclerosis. Finally, we assessed the contribution of IR isoforms and IGF-IR to the apoptosis of murine aortic VSMCs lines.
- SUOMENNOSTA TÄMÄN TUTKIMUKSEN TULOKSISTA.Komplisoiduissa ateroskleroottisissa plakeissa on havaittu, että IRA/IRB-suhde on laskenut ia IGF-IR on laskenut, IGF-IIR on lisääntynyt ja VSMC- vaskulaarisia sileitä lihassoluja on kadonnut.
Results
