Activation of the AKT/cyclin D1/Cdk4 survival signaling pathway in radioresistant cancer stem cells

Abstract

Radioresistance, which is a major cause of failure of radiotherapy (RT), is proposed as one of the intrinsic characteristics of cancer stem cells (CSCs) whose unique DNA damage response (DDR), efficient DNA repair and resistance to apoptosis are thought to confer the phenotype. We have isolated surviving CSCs by exposure to long-term fractionated radiation for 82 days from HepG2 and A172 cells (82FR-31NR cells). 82FR-31NR cells exhibited CSC properties, such as high expression of CSC marker CD133 and the ABC transporters (MDR1 and BCRP1), and high tumorigenic potential after transplantation into nude mice. The advantage of our isolated CSCs is that they can proliferate in as the same growth medium as that of parental cells without loss of CSC properties. Therefore, we can analyze DDR of non-stem cells and CSCs without any influences caused by different culture conditions. 82FR-31NR cells showed efficient DNA repair of radiation-induced DNA damage and radioresistance with activation of the AKT/cyclin D1 survival signaling pathway. In contrast, DNA damage persisted for a long time after irradiation in parental cells compared with isolated CSCs. Persisted DNA damage induced apoptosis in parental cells without activation of the AKT/cyclin D1 pathway. Therefore, inhibition of the AKT/cyclin D1 pathway by an AKT inhibitor, API-2, or cyclin D1 siRNA resulted in a loss of efficient DNA repair and radiosensitization of 82FR-31NR cells. Furthermore, knockdown of Cdk4 by its siRNA or a Cdk4 inhibitor was sufficient to suppress radioresistance of CSCs. In this study, we present a newly discovered DDR regarding the AKT/cyclin D1/Cdk4 pathway in response to radiation in CSCs. Combination of fractionated RT and reagents targeting the AKT/cyclin D1/Cdk4 pathway to eradicate CSCs would be effective therapeutic modality.

MicroRNA-145 sensitizes cervical cancer cells to low-dose irradiation by downregulating OCT4 expression

“…Cyclin D1 is also a key mediator that contributes to reduce cancer cell radiosensitivity via an established mechanism that facilitates G1-S cell cycle transition to improve cell self-renew and proliferation after irradiation (29)(30)(31). In the present study, cyclin D1 downregulation was observed in cells exposed to irradiation and cells transfected with miR-145 mimics was associated with considerable reduction of cell migration rate in wound healing assay.…”

AKT-mediated enhanced aerobic glycolysis causes acquired radioresistance by human tumor cells

“…We previously reported that AKT regulated acquired radioresistance by tumor cells after long-term FR exposure [2] and the radioresistance of cancer stem cells [14]. Thus, the AKT pathway is a key to overcoming tumor radioresistance during RT.…”

Ultrasound-Stimulated Microbubbles Enhance Radiosensitization of Nasopharyngeal Carcinoma

“…Following radiation-induced damage of cells in tumors, many survival pathways are activated to protect cells from death. These pathways not only directly promote cell survival by increasing proliferation, invasion, and anti-apoptosis, but also contribute to improve the microenvironment, such as through angiogenesis [28][29][30][31]. Similarly, NPC can acquire resistance to radiation through antiapoptotic pathways by increasing DNA damage repair and through facilitating angiogenesis by enhancing ANG II.…”