Background

In a preclinical melanoma study, treatment targeting SIRT3 did not significantly reduce tumor volume, whereas a compound that inhibits both SIRT1 and SIRT3 significantly reduced tumor volume in two patient-derived xenograft models. The findings require a distinction between the role of the target observed in cells and antitumor effects observed in animals.

Melanoma is a difficult skin cancer to treat because of metastasis and treatment resistance. Targeted therapies and immunotherapies are in use, but the search for new targets to address relapse and resistance continues. Published in the Journal of dermato-oncology, this study evaluated the proliferation-related function of the mitochondrial deacetylase sirtuin 3 (SIRT3) and inhibition strategies. The work ranged from expression analyses in gene-knockout cells to tumor-growth assessment in patient-derived xenograft (PDX) models and genetically engineered mice.

Key findings

The researchers used CRISPR/Cas9 gene editing to knock out SIRT3 in the human melanoma cell lines A375 and G361. Cell growth was significantly reduced in both lines starting 48 hours after seeding, and colony formation, which reflects long-term proliferative capacity, also decreased.

A polymerase chain reaction (PCR) array showed significant expression changes in 18 of 84 cancer-related genes analyzed in A375. A 760-gene NanoString panel also revealed expression changes related to tumor signaling, but the patterns differed between cell lines. Some gene expression changes could also be interpreted as opposing tumor suppression. The results are therefore best viewed as identifying candidate SIRT3-related pathways, rather than establishing direct control of specific metabolic or DNA-repair pathways.

The animal experiments used two PDX models, Mel-PDX-01 and TM01137, and mice with the Braf V600E mutation and Pten deletion. SIRT3-targeting small interfering RNA (siRNA) was mixed with the InvivoJET-PEI delivery agent and administered intraperitoneally at 100 μg per mouse twice weekly. All three models showed a trend toward reduced tumor growth, but the reduction in tumor volume was not statistically significant. Final tumor weight decreased significantly in Mel-PDX-01, so the volume and weight results need to be considered separately. There was also no significant difference in the number of visible lung metastatic lesions in the evaluable models.

By contrast, intraperitoneal administration of the dual SIRT1/SIRT3 inhibitor 4′-bromoresveratrol (4′-BR) at 50 mg/kg twice weekly significantly reduced tumor volume in both PDX models. Final tumor weight decreased significantly in Mel-PDX-01, but not in TM01137 (p=0.1150); the reduction in lung metastatic lesions in TM01137 was also only a trend.

Meaning and outlook

The findings add evidence that SIRT3 is involved in melanoma cell proliferation, but the in vivo effect of SIRT3-targeting treatment was limited under the siRNA dosing conditions used. The researchers propose optimizing delivery efficiency, dose and schedule, and further testing strategies that inhibit other targets, such as SIRT1, alongside SIRT3.

The 4′-BR results alone cannot establish the superiority of dual inhibition or synergy between the two targets. Because siRNA and small molecules differ in delivery and characteristics of action, comparative studies that isolate the effects of the target combination remain necessary. This study did not directly test whether treatment overcame acquired resistance to existing therapies, and it was not a clinical trial evaluating efficacy in patients. The observation that treatment was not associated with significant body-weight loss must also be distinguished from establishing clinical safety.

The methods and results are available in the full-text article and via the article DOI.