The PI3K/Akt/mTOR pathway is implicated in the premature senescence of primary human endothelial cells exposed to chronic radiation.

The etiology of radiation-induced cardiovascular disease (CVD) after chronic exposure to low doses of ionizing radiation is only marginally understood. We have previously shown that a chronic low-dose rate exposure (4.1 mGy/h) causes human umbilical vein endothelial cells (HUVECs) to prematurely senesce. We now show that a dose rate of 2.4 mGy/h is also able to trigger premature senescence in HUVECs, primarily indicated by a loss of growth potential and the appearance of the senescence-associated markers ß-galactosidase (SA-ß-gal) and p21. In contrast, a lower dose rate of 1.4 mGy/h was not sufficient to inhibit cellular growth or increase SA-ß-gal-staining despite an increased expression of p21. We used reverse phase protein arrays and triplex Isotope Coded Protein Labeling with LC-ESI-MS/MS to study the proteomic changes associated with chronic radiation-induced senescence. Both technologies identified inactivation of the PI3K/Akt/mTOR pathway accompanying premature senescence. In addition, expression of proteins involved in cytoskeletal structure and EIF2 signaling was reduced. Age-related diseases such as CVD have been previously associated with increased endothelial cell senescence. We postulate that a similar endothelial aging may contribute to the increased rate of CVD seen in populations chronically exposed to low-dose-rate radiation

Immunoblot and RPPA analysis of the senescence-associated proteins p21 and members of PI3K/Akt/mTOR pathway.

<p>A representative immunoblot of p21 expression levels at different time points and dose rates is shown (A). The columns represent protein levels of p21 (B) Akt (C), phospho-Akt (D), PI3K (E) and mTOR (F) in control (blue), 1.4 mGy/h (orange) and 2.4 mGy/h (green) irradiated HUVECs. The average ratios of relative protein expression in control and irradiated samples are shown. The protein bands were quantified using TotalLab TL100 software by integration of all the pixel values in the band area after background correction and normalized to the actin expression. The data are represented as ± SEM. Three biological replicates were used in all experiments. (Students t-test; *p<0.05, **p<0.01 and ***p<0.005).</p

List of selected antibodies analyzed using RPPA.

<p>List of selected antibodies analyzed using RPPA.</p

Schematic representation of the ICPL-triplex proteomic approach work flow.

<p>The protein samples from week 10 (control and two irradiated) were reduced and alkylated before labeling with ICPL0, ICPl4 and ICPL6. Samples were mixed and further separated using 1D gel electrophoresis and digested as described in Methods. Samples were analyzed by LC-ESI-MS/MS. Quantification of proteins was performed by Proteome Discoverer software using three biological replicates.</p

Schematic representation of pathways altered by chronic low-dose-rate radiation in HUVECs.

<p>Orange and green colors represent down-regulated proteins detected by RPPA and ICPL triplex proteomic methods, respectively. Blue color indicates unregulated proteins detected by the RPPA technique.</p

Graphical representation of deregulated proteins representing a merge of two most affected functional networks at week 10 (2.4 mGy/h).

<p>Differentially up- or down-regulated proteins are marked in red and green, respectively. Nodal molecules Akt and ERK 1/2 (blue color) were predicted by the IPA software as central transcriptional regulators.</p

Canonical pathways of deregulated proteins.

<p>Canonical pathways of deregulated proteins.</p

Growth curve and senescence associated β-gal of HUVECs exposed to chronic low-dose rates.

<p>A) The growth curve is plotted with cumulative population doublings versus time. Growth curves of control (blue), 1.4 mGy/h (orange) and 2.4 mGy/h (green) irradiated HUVECs are shown. Cumulative population doublings from each week are presented as means ± SEM (n = 3). B) Histograms of positively-stained cells for SA-ß-gal (senescence marker) for control (blue), irradiated by 1.4 mGy/h (orange) and irradiated by 2.4 mGy/h (green). Data are presented as means ± SEM (n = 3). (Students t-test; *p<0.05, **p<0.01 and ***p<0.005).</p