Selective Inhibition of Ras, Phosphoinositide 3 Kinase, and Akt Isoforms Increases the Radiosensitivity of Human Carcinoma Cell Lines

Abstract

Ras activation promotes the survival of tumor cells after DNA damage. To reverse this survival advantage, Ras signaling has been targeted for inhibition. Other contributors to Ras-mediated DNA damage survival have been identified using pharmacologic inhibition of signaling, but this approach is limited by the specificity of the inhibitors used and their toxicity. To better define components of Ras signaling that could be inhibited in a clinical setting, RNA interference was used to selectively block expression of specific isoforms of Ras, phosphoinositide 3 (PI3) kinase, and Akt. Inhibition of oncogenic Ras expression decreased both phospho-Akt and phospho-p42/44 mitogenactivated protein (MAP) kinase levels and reduced clonogenic survival. Because pharmacologic inhibition of PI3 kinases and Akt radiosensitized cell lines with active Ras signaling, whereas inhibition of the MAP/extracellular signal-regulated kinase (ERK) kinase/ERK pathway did not, we examined the contribution of PI3 kinases and Akts to radiation survival. Selective inhibition the PI3 kinase P110A + p85B isoforms reduced Akt phosphorylation and radiation survival. Similarly, inhibition of Akt-1 reduced tumor cell radiation survival. Inhibition of Akt-2 or Akt-3 had less effect. Retroviral transduction and overexpression of mouse Akt-1 was shown to rescue cells from inhibition of endogenous human Akt-1 expression. This study shows that Ras signaling to the PI3 kinase-Akt pathway is an important contributor to survival, whether Ras activation results from mutation of ras or overexpression of epidermal growth factor receptor. This study further shows that selective inhibition of the PI3 kinase P110A + p85B isoforms or Akt-1 could be a viable approach to sensitizing many tumor cells to cytotoxic therapies. (Cancer Res 2005; 65(17): 7902-10)

PKB/Akt promotes DSB repair in cancer cells through upregulating Mre11 expression following ionizing radiation

“…For transient transfection, the siRNAs against the sequence 5 0 -AAGGAGGG UUGGCUGCACAAA-3 0 of the human Akt1 mRNA (Kim et al, 2005), the sequence 5 0 -GATGC CATTGAGGAATTAG-3 0 of the human Mre11 mRNA (Pichierri and Rosselli, 2004), and the sequence 5 0 -AAACUG CUAAAUGACGAGGAC-3 0 of the human b-catenin mRNA (Hadjihannas et al, 2006) were synthesized by GenChem Corporation (Shanghai, China). Lef-1 siRNA (h) was obtained from Santa Cruz Biotechnology (sc-35804).…”

“…Inhibition of phosphoinositide 3-kinase/Akt pathway leads to radiosensitization, such as glioblastomas, carcinoma of the breast, colon, bladder, prostate, head and neck and cervix (Gupta et al, 2002;Li et al, 2004;Altomare and Testa, 2005;Kim et al, 2005;Lee et al, 2006;Kao et al, 2007). One of the important reason is that activated Akt promotes survival of cells exposed to IR through inhibition of apoptosis (Brognard et al, 2001).…”

Stimulated PI3K-AKT Signaling Mediated through Ligand or Radiation-Induced EGFR Depends Indirectly, but not Directly, on Constitutive K-Ras Activity

“…In this context, Kim et al (45) reported that Ras signaling to PI3K-AKT is an important contributor to cell survival independent of whether it results from mutation of RAS or overexpression of EGFR. Furthermore, recently, we have reported that targeting of EGFR-PI3K signaling by specific small-molecule inhibitors (i.e., BIBX1382BS and LY294002) impairs repair of radiationinduced DNA double-strand breaks (mainly through the nonhomologous end-joining mechanism), resulting in enhanced radiosensitivity selectively of those human tumor cells that present mutated K-RAS gene (30,31).…”

DNA strand breaks and hypoxia response inhibition mediate the radiosensitisation effect of nitric oxide donors on prostate cancer under varying oxygen conditions

“…Radiation survival curves were plotted after normalizing for the cytotoxicity induced by drug treatment alone. Linearquadratic curve equations were used to fit the SF curves shown in the figures using [12]. Oxygen enhancement ratios (OER) were calculated by dividing the D 10 value under hypoxia by the D 10 value under normoxic conditions.…”