Cellular senescence in the response of HR+ breast cancer to radiotherapy and CDK4/6 inhibitors

by Selleckchem | Aug 06 2023

Background: Preclinical evidence from us and others demonstrates that the anticancer effects of cyclin-dependent kinase 4/6 (CDK4/6) inhibitors can be enhanced with focal radiation therapy (RT), but only when RT is delivered prior to (rather than after) CDK4/6 inhibition. Depending on tumor model, cellular senescence (an irreversible proliferative arrest that is associated with the secretion of numerous bioactive factors) has been attributed beneficial or detrimental effects on response to treatment. As both RT and CDK4/6 inhibitors elicit cellular senescence, we hypothesized that a differential accumulation of senescent cells in the tumor microenvironment could explain such an observation, i.e., the inferiority of CDK4/6 inhibition with palbociclib (P) followed by RT (P→RT) as compared to RT followed by palbociclib (RT→P).

Methods: The impact of cellular senescence on the interaction between RT and P was assessed by harnessing female INK-ATTAC mice, which express a dimerizable form of caspase 8 (CASP8) under the promoter of cyclin dependent kinase inhibitor 2A (Cdkn2a, coding for p16Ink4), as host for endogenous mammary tumors induced by the subcutaneous implantation of medroxyprogesterone acetate (MPA, M) pellets combined with the subsequent oral administration of 7,12-dimethylbenz[a]anthracene (DMBA, D). This endogenous mouse model of HR+ mammary carcinogenesis recapitulates key immunobiological aspects of human HR+ breast cancer. Mice bearing M/D-driven tumors were allocated to RT, P or their combination in the optional presence of the CASP8 dimerizer AP20187, and monitored for tumor growth, progression-free survival and overall survival. In parallel, induction of senescence in vitro, in cultured human mammary hormone receptor (HR)+ adenocarcinoma MCF7 cells, triple negative breast carcinoma MDA-MB-231 cells and mouse HR+ mammary carcinoma TS/A cells treated with RT, P or their combination, was determined by colorimetric assessment of senescence-associated β-galactosidase activity after 3 or 7 days of treatment.

Results: In vivo depletion of p16Ink4-expressing (senescent) cells ameliorated the efficacy of P→RT (but not that of RT→P) in the M/D-driven model of HR+ mammary carcinogenesis. Accordingly, P→RT induced higher levels of cellular senescence than R→TP in cultured human and mouse breast cancer cell lines.

Conclusions: Pending validation in other experimental systems, these findings suggest that a program of cellular senescence in malignant cells may explain (at least partially) the inferiority of P→RT versus RT→P in preclinical models of HR+ breast cancer.

Comments:

The provided background describes a study conducted to investigate the differential effects of combining cyclin-dependent kinase 4/6 (CDK4/6) inhibitors and radiation therapy (RT) in the treatment of hormone receptor-positive (HR+) mammary carcinogenesis, a type of breast cancer. The study aimed to understand why the sequence of treatment, specifically RT followed by CDK4/6 inhibition (RT→P), was more effective than CDK4/6 inhibition followed by RT (P→RT) in preclinical models.

The researchers hypothesized that the accumulation of senescent cells in the tumor microenvironment could explain the observed differences in treatment outcomes. Cellular senescence refers to an irreversible growth arrest in cells, accompanied by the release of various bioactive factors. The impact of cellular senescence on the interaction between RT and CDK4/6 inhibitors was assessed using female INK-ATTAC mice, which express a form of caspase 8 (CASP8) that can be activated selectively in p16Ink4-expressing (senescent) cells. These mice were implanted with medroxyprogesterone acetate (MPA) pellets and treated with 7,12-dimethylbenz[a]anthracene (DMBA) to induce HR+ mammary tumors.

The mice bearing M/D-driven tumors were divided into different treatment groups, including RT, CDK4/6 inhibitor palbociclib (P), or a combination of both treatments. The researchers also used a caspase 8 dimerizer (AP20187) to selectively deplete p16Ink4-expressing senescent cells in the mice. Tumor growth, progression-free survival, and overall survival were monitored to assess treatment efficacy. In parallel, in vitro experiments were conducted using cultured human and mouse breast cancer cell lines (MCF7, MDA-MB-231, and TS/A cells) to determine the induction of senescence by RT, CDK4/6 inhibitors, or their combination.

The results showed that depleting p16Ink4-expressing senescent cells in the mice improved the efficacy of the P→RT treatment, but not the RT→P treatment. Furthermore, P→RT induced higher levels of cellular senescence compared to RT→P in the cultured breast cancer cell lines.

In conclusion, the findings from this study suggest that the differential accumulation of senescent cells in malignant cells may, at least partially, explain why RT followed by CDK4/6 inhibition is more effective than the reverse sequence in preclinical models of HR+ breast cancer. However, further validation in other experimental systems is necessary to confirm these findings.