The DNA-PKcs-primary cilia axis maintains ionizing radiation-induced senescence in tumor cells

Abstract

Senescence is a cellular response closely associated with genotoxic stress and plays a critical role in determining cell fate following irradiation exposure. Primary cilia, which are sensory organelles on the cell surface, detect and transmit diverse signaling cues. However, the relationship between primary cilia and senescence in long-term cell fate decisions after ionizing radiation remains poorly understood. Here, we show that the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) colocalizes with centromeres during various stages of mitosis, whereas during interphase, phosphorylated DNA-PKcs (p-DNA-PKcs) is confined to the nucleus in tumor cells. Following irradiation exposure, primary cilia are formed and persistently maintained at high levels in senescent tumor cells. Inhibition of DNA-PKcs enhances primary cilia formation, whereas combined inhibition with siDNA-PKcs and irradiation reduces cilia generation. Moreover, chloral hydrate-induced primary cilia removal results in senescent cell death and decreases p-DNA-PKcs protein expression. Notably, treatment with the apoptosis inducer ABT263 also leads to increased cell death and decreased incidence of primary cilia. Inhibition of either primary cilia or DNA-PKcs further enhances the radiosensitivity of tumor cells. These findings suggest that DNA-PKcs contributes to primary cilia formation after irradiation and plays a critical role in both the induction and maintenance of cellular senescence.

1. Introduction

Cellular senescence refers to stable growth arrest accompanied by an antiapoptotic state that occurs in response to irreparable stress [1,2]. Research has demonstrated that ionizing radiation (IR) can induce cellular senescence, which is common during tumor radiotherapy [3]. Furthermore, cellular senescence triggered by tumor treatments is closely associated with treatment resistance and poor prognosis. Current evidence indicates that activation of the p16-pRB and p53-p21 signaling pathways, mitochondrial dysfunction [4], and cyclic GMP-AMP synthase stimulation are major mechanisms underlying senescence. However, the specific molecular basis of radiation-induced senescence remains incompletely defined. Previous findings demonstrated that Aurora A acts as a key downstream effector of p21 in radiation-induced senescent tumor cells [5] and that its degradation facilitates the formation of primary cilia [6]. These observations suggest that primary cilia may be critical in mediating the cellular senescence induced by IR.

Primary cilia are hair-like, nonmotile organelles that extend from the cell surface [7] and function as central hubs for sensing physical, chemical, and biological cues from the extracellular environment. By transmitting these signals into the cell, the PC plays a critical role in regulating various cellular processes [8]. Structural or functional defects in primary cilia have been implicated in the development and progression of multiple diseases, including cancer [9]. Numerous studies have demonstrated a strong association between primary cilia dynamics and cellular senescence under physiological conditions. Cilium length increases significantly in aged human fibroblasts and in the kidneys and pancreas of aged mice [10,11]. Similar elongation has also been observed in the hippocampal region of aged rats [12,13]. Additionally, silencing of ciliogenesis-related genes in primary cilia can induce senescence in renal epithelial cells from mouse models of cystic kidney disease [14]. Notably, further investigations have indicated that primary cilia contribute to the senescence of tumor cells. For example, persistent primary cilia have been found to induce senescence in human cervical cancer cells [15], and primary cilia play a key role in etoposide-induced senescence in adrenal cortical tumor cells [16]. Although the involvement of primary cilia in cellular senescence has been partially clarified, the precise regulatory mechanisms remain to be fully defined. In normal cells, recent studies have reported a transient increase in cilia formation following irradiation exposure, followed by a gradual decrease, and this transient cilia formation plays an essential role in initiating cellular senescence postirradiation [17]. However, no studies have reported whether primary cilia exhibit similar functions or expression patterns during IR-induced tumor cellular senescence.