Native and oxidized low density lipoproteins oppositely modulate the effects of insulin-like growth factor I on VSMC

Changes in the local expression and signaling activity of the insulin-like growth factor-I (IGF-I) axis regulate growth and survival of plaque-derived vascular smooth muscle cells (VSMC) and influence plaque fate. Recent evidence suggests that accumulation of low density lipoproteins (LDL) in VSMC during the progression of atherogenesis is linked to local changes in IGF-I signaling. We investigated the effects of LDL on the biological actions and downstream signaling pathways mediated by this growth factor in A10 VSMC.

We first characterized the effects of LDL on the proliferative and anti-apoptotic actions of IGF-I in A10 VSMC. Native LDL were mitogenic and synergistically enhanced DNA synthesis induced by IGF-I from 4-, 9- up to 7.8-fold, while having no effect on its anti-apoptotic actions. In contrast, oxidized LDL, at oxidation levels that did not modify these actions by themselves, significantly reduced the mitogenic and survival effects of IGF-I by 40% and 60%, respectively. These observations correlated with opposite changes exerted by native and oxidized LDL on the insulin receptor substrate-1 (IRS)-associated PI3 kinase/Akt response to IGF-I. The extracellular signal-regulated kinase (ERK) signaling response was not affected.

Our study demonstrates a previously unidentified modulation of the actions of IGF-I on A10 VSMC by LDL, dependent on their extent of oxidative modification. Our findings suggest that the differential modulation of the PI3 kinase/Akt response to IGF-I play a pivotal role.