Cancer has become one of the world's major health problems, posing a serious threat to human life and causing widespread concern. Radiotherapy has become a fundamental component in the management of cancer [1,2]. RT has been demonstrated that this treatment can induce DNA double-strand breaks, DNA damage, and subsequently promote cell apoptosis and tumor necrosis [[3], [4], [5]]. Meanwhile, radiotherapy-induced DNA double-strand breaks can promote DNA damage repair (DDR) by upregulating homologous recombination-associated DNA proteins (e.g. RAD51 and RAD51 associated protein 1 (RAD51AP1)), which reduces cellular sensitivity to RT and impairs its efficacy [5,6]. Furthermore, radiotherapy has been demonstrated to induce significant immunogenic cell death effects (ICD) [7,8]. The release of damage-associated molecular patterns (DAMPs) such as calcium reticulin (CRT), high mobility group protein B1 (HMGB1), and adenosine triphosphate (ATP) is triggered by ICD [9,10], triggering an anti-tumor immune response [11,12]. However, it is important to note that ICD resulted in the release of large amounts of ATP, instead inhibiting the activity of CD8+ T cells while upregulating programmed death ligand 1 (PD-L1), producing an immunosuppressive effect. The upregulation of PD-L1 has been shown to inhibit T cell activity, reduce T cell infiltration, and further promote immune escape [13,14]. Therefore, endeavours to remodel the radiotherapy-induced immunosuppressive microenvironment have emerged as a potentially efficacious approach to augment the effectiveness of radiotherapy treatment.

In recent years, targeted degradation of Bromodomain-containing protein 4 (BRD4) has emerged as a highly efficacious cancer treatment modality [15]. A substantial body of evidence indicates that the inhibition of BRD4 may result in the suppression of BRD4-based DNA repair mechanisms, which play pivotal roles in cell cycle, proliferation and apoptosis [16,17]. BRD4 is highly expressed in a variety of tumors and has been shown to promote the transcriptional activity of the RAD51AP1 gene, which in turn enhances DNA repair. The inhibition of BRD4 in tumor cells has been shown to reduce the expression level of RAD51AP1, thus decreasing DNA repair [18,19]. On the other hand, inhibition of BRD4 may also increase the expression of the DNA damage marker γH2AX and enhance the sensitivity of tumor cells to radiotherapy-induced DNA damage treatment [20]. In addition, recent studies suggest that the ability of BRD4 to modulate the tumor immune microenvironment may have the potential to have wide application as a therapeutic agent in the field of immunotherapy [[21], [22], [23], [24], [25], [26], [27]]. Inhibition of BRD4 has been demonstrated to suppress interferon (IFN)-γ-induced increases in the programmed death ligand 1 (PD-L1) and to overcome PD-L1-induced immune escape [28,29]. A significant number of BRD4 inhibitors have demonstrated encouraging outcomes in clinical trials [[30], [31], [32]]. However, the use of BRD4 inhibitors may result in aberrant feedback accumulation of BRD4, which attenuates its antiproliferative and pro-apoptotic effects [33].

In contrast to BRD4 inhibitors, PROteolysis TArgeting Chimeras (PROTAC) induce the degradation of the protein of interest (POI) through the ubiquitin-proteasome system (UPS) in a cyclic catalytic manner, rather than acting by inhibiting BRD4 [[34], [35], [36], [37], [38]]. In comparison with traditional BRD4 small molecule inhibitors, PROTACs exhibit significant advantages [38,39], including lower dosage requirements and higher specificity [40,41], effective overcoming of drug resistance [42,43], and the ability to target targets that are traditionally difficult to drug [44,45]. Consequently, PROTAC technology has emerged as a promising cancer treatment strategy. However, the poor water solubility, high molecular weight (more than 800 Da) and poor targeting ability of PROTAC molecules, resulting in low bioavailability, as well as the fact that intravenous administration of PROTAC drugs leads to POI degradation in non-tumor tissues, limit their therapeutic and application potential [[46], [47], [48]]. Therefore, there is an urgent need to develop innovative delivery strategies to overcome the challenges of PROTAC application in tumor tissues.

Herein, we developed an injectable nanoparticle hydrogel (PLM@Gel) in combination with radiotherapy to enhance anti-tumor effects in cancer and to remodel the immunosuppressive microenvironment. Firstly, liposomal nanoparticles (PL) encapsulating PROTAC (PRO) targeting BRD4 were synthesized, and then fused to cancer cell membranes with targeting ability to obtain cancer cell membrane-modified nanoparticles (PLM). Subsequently, the PLM was loaded into a hydrogel precursor solution to form a injectable hydrogel nanocomposite (Scheme 1A). PLM@Gel and RT co-treatment promoted DNA damage, reduced DNA repair, and promoted cancer cell apoptosis. Concurrently, RT instigated a pronounced ICD effect, resulting in the release of substantial quantities of DAMPs. At the same time, PLM@Gel remodeled the radiotherapy-induced immunosuppressive microenvironment, promoting the maturation of DCs and enhancing the infiltration of tumor-specific T cells (Scheme 1B). Our findings extend the application of PROTAC technology to the improvement of radiotherapy-induced immunosuppression, with considerable potential for biomedical translation and clinical application.