CXCR2 Receptor Blocks Radiation Sensitization in Hard-to-Treat Head and Neck Cancer

Locally advanced HPV-negative HNSCC is treated with cumulative radiation doses of 60–80 Gy, yet local relapse occurs in a large proportion of patients within three years. A central biological driver of this resistance is therapy-induced senescence: irradiation creates persistent DNA damage that locks cells in a growth-arrested but metabolically active state, prompting secretion of a complex mixture of cytokines, chemokines, and growth factors (the SASP) that can reinforce tumor survival and regeneration. Prior work from this group showed that senescence and SASP levels correlate with radioresistance in HNSCC and that the NF-κB-driven CXCR2 chemokine axis has prognostic significance.

To exploit the dependency of senescent cells on anti-apoptotic Bcl-2 family proteins, the investigators combined the BH3-mimetic ABT-263 (navitoclax) with photon irradiation in the HPV-negative cell lines Cal33 and UPCI:SCC040—both known to be highly radioresistant and senescence-prone. Drug cytotoxicity was first titrated (1 µM for Cal33, 5 µM for UPCI:SCC040), then combined treatments were evaluated via viability (alamarBlue), apoptosis (Annexin V/PI flow cytometry), senescence (SA-β-gal/C12FDG), clonogenic survival (linear-quadratic fitting), secreted SASP proteins (ELISA for IL-1α, IL-1β, IL-8, CXCL1), receptor expression (qRT-PCR and Western blot), and residual DNA double-strand breaks (γH2AX/53BP1 co-localization foci at 24 h).

In Cal33 cells, ABT-263 combined with radiation increased apoptosis (via Bax activation and Bcl-xL inhibition), reduced irradiation-induced senescence, and significantly lowered clonogenic survival in a synergistic manner. In UPCI:SCC040 cells, the combination again reduced viability, yet clonogenic survival was not significantly altered. The mechanistic difference mapped to CXCR2: Cal33 cells showed strong downregulation of CXCR2 following ABT-263 plus radiation, whereas UPCI:SCC040 cells paradoxically upregulated CXCR2. When CXCR2 was transiently knocked down by siRNA in UPCI:SCC040 cells before the combined treatment, radiosensitization was restored. Notably, γH2AX/53BP1 co-localization foci at 24 h post-irradiation were not significantly elevated by any combination, indicating that the sensitizing effect operates through survival-signaling pathways rather than increased DNA damage or impaired DNA repair.

These findings propose a model in which ABT-263 eliminates SASP-producing senescent cells and amplifies apoptosis, but its radiosensitizing benefit can be neutralized by CXCR2-mediated pro-survival chemokine signaling. CXCR2 and its ligands (including IL-8/CXCL8 and CXCL1) are established NF-κB targets whose paracrine and autocrine activity supports cell survival and proliferation. UPCI:SCC040 cells appear to mount a compensatory CXCR2 upregulation that bypasses the apoptotic pressure imposed by ABT-263, thereby preserving clonogenic capacity despite reduced overall viability.

Clinically, these results suggest that CXCR2 expression profiling could serve as a predictive biomarker for ABT-263 radiosensitization efficacy in HPV-negative HNSCC. Patients whose tumors fail to downregulate CXCR2 in response to treatment may require co-administration of a CXCR2 antagonist to achieve meaningful radiosensitization. The study also reinforces the broader concept that targeting senescence—either by eliminating senescent cells (senolytics) or suppressing the SASP (senomorphics)—represents a rational therapeutic strategy to improve durable tumor control in this difficult-to-treat cancer subgroup.