Mini-GRID radiotherapy reduces senescence markers in glioma cell lines

Researchers have demonstrated that spatially fractionated mini-GRID radiotherapy significantly reduces features of radiation-induced cellular senescence in two glioma cell lines. This approach maintained an antiproliferative effect similar to conventional radiotherapy, according to a study published in Volume 18 of Aging.

Study authors and affiliations

The investigation was led by equal-contributing first authors M. Isabel Acuña and Miguel Ángel Prados, both affiliated with the University of Santiago de Compostela. Corresponding authors Manuel Collado and Yolanda Prezado also hold affiliations with the same university.

Collado is additionally associated with the National Centre for Biotechnology (CNB-CSIC). Prezado holds a secondary affiliation with the Oportunius Program of the Galician Agency of Innovation (GAIN), Xunta de Galicia.

Radiation mechanisms and senescence

Radiotherapy remains a cornerstone of cancer treatment but can push surviving cells into senescence, a state of persistent cell-cycle arrest. These senescent cells may develop a senescence-associated secretory phenotype (SASP), releasing inflammatory and other signaling molecules that affect surrounding tissues and the tumor microenvironment.

While senescence can restrict tumor-cell proliferation, persistent therapy-induced senescence may contribute to inflammation, treatment resistance, and other adverse effects. Spatially fractionated radiotherapy (SFRT) differs from conventional methods by delivering radiation in a deliberately non-uniform pattern, creating high-dose "peaks" separated by lower-dose "valleys."

Experimental comparison results

The team investigated whether mini-GRID, a form of SFRT, produces different senescence responses compared to conventional uniform irradiation. The study compared these approaches in F98 and RG2 rat glioma cells, immortalized rat astrocytes, and primary mouse embryonic fibroblasts.

Cells received single radiation doses ranging from 5 to 20 Gy and were evaluated seven days later using morphological, biochemical, and molecular measures of senescence. At 20 Gy, conventional and mini-GRID radiotherapy produced comparable reductions in glioma cell numbers.

However, their effects on senescence differed substantially. Conventional radiotherapy increased cell size and senescence-associated β-galactosidase activity, while these changes were significantly attenuated following mini-GRID irradiation.

Molecular markers and gene induction

Molecular results reinforced this difference. Conventional radiotherapy increased senescence and persistent DNA-damage markers, including p53, p21, p16, and γH2AX. Mini-GRID irradiation reduced their accumulation, with levels approaching those of non-irradiated controls.

It also markedly reduced the induction of SASP-related genes. Il1a, Il6, and Serpine1 were strongly induced by conventional irradiation but remained closer to baseline after mini-GRID treatment. Cxcl1 showed a more cell-line-specific response.

Dose distribution hypothesis

The researchers propose that this difference may arise from mini-GRID's spatial dose distribution. High-dose peaks can produce lethal damage in directly exposed cells, while lower-dose valleys may allow surviving cells to repair damage without fully activating a stable senescence program.

This mechanism remains a proposed explanation and requires further investigation. In non-tumor cells, the pattern differed. In immortalized astrocytes and mouse embryonic fibroblasts, radiation induced senescence in a dose-dependent manner, but no significant differences were detected between conventional and mini-GRID irradiation at matched doses.

Under the experimental conditions tested, mini-GRID therefore attenuated senescence in glioma cells without increasing senescence relative to conventional radiation in the non-tumor models.

Clinical implications and limitations

"These observations indicate that spatially fractionated mini-GRID RT can alter the qualitative nature of radiation-induced stress responses in tumor cells without exacerbating senescence in healthy tissues," the authors stated.

The findings could have important implications because therapy-induced senescence acts as a double-edged sword. While it prevents damaged cells from continuing to divide and contributes to antitumor immune responses, persistent senescent cells and their SASP can promote inflammation and alter the tumor microenvironment.

Preserving radiation-induced tumor-cell reduction while limiting persistent senescence could represent a useful biological advantage. However, the study is preclinical. Experiments used a limited number of rodent cell models grown in two-dimensional culture, with single radiation doses, one mini-GRID configuration, and measurements at a single time point.

The study did not evaluate long-term SASP dynamics or interactions with immune and other components of the tumor microenvironment.