Imiquimod-Induced TLR7 Signaling Enhances Repair of DNA Damage Induced by Ultraviolet Light in Bone Marrow-Derived Cells

Abstract

Imiquimod is a TLR7/8 agonist that has anti-cancer therapeutic efficacy in the treatment of pre-cancerous skin lesions and certain non-melanoma skin cancers. To test our hypothesis that imiquimod enhances DNA repair as a mechanism for its anticancer activity, the nucleotide excision repair genes were studied in bone marrow derived cells. Imiquimod enhanced the expression of XPA and other DNA repair genes (qPCR analysis), and resulted in an increased nuclear localization of the DNA repair enzyme XPA. This was dependent on MyD88, as bone marrow derived cells from MyD88−/− mice did not increase XPA gene expression, and did not enhance the survival of MyD88−/− derived bone marrow derived cells after UVB exposure as was observed in bone marrow derived cells from MyD88+/+ mice. Imiquimod also enhanced DNA repair of UVL irradiated gene expression constructs, and accelerated the resolution of cyclobutane pyrimidine dimers (CBPD) after UVL exposures in P388 and XS52. Lastly, topical treatment of mouse skin with 5% imiquimod cream prior to UVL irradiation resulted in a decrease in the number of cyclobutane pyridimine dimer positive, APC that were found in local lymph nodes 24 hours after UVL irradiation in both wild type and IL-12 gene targeted mice. In total, these data support the idea that TLR7 agonists such as imiquimod enhance DNA repair in bone marrow derived cells. This property is likely to be an important mechanism for its anti-cancer effects, because it protects cutaneous APC from the deleterious effects of UVL.

Inhibition of type I interferon signalling prevents TLR ligand-mediated proteinuria

“…We confirmed constitutive expression of TLR1 through TLR6 and TLR8 ( Figure 1B) on primary podocyte cultures. Treatment of podocytes with the TLR7 ligand imidazoquinoline compound [16] or the TLR9 ligand type A CpG oligonucleotide [17] caused a significant up-regulation of TRL7 or TRL9 mRNA expression, respectively (p < 0.05; Figure 1C).…”

Ultraviolet Radiation Signaling through TLR4/MyD88 Constrains DNA Repair and Plays a Role in Cutaneous Immunosuppression

“…Another type of DNA damage-causing radiation, gamma irradiation, results in double-strand breaks that cannot be repaired using a complementary template; it is associated with mutagenesis and chromosome abnormalities that often lead to cell death. Repair of both UV and gamma irradiation damage was shown to be altered in the presence of endosomal TLR agonists (12,13), implying that both the NER and double-strand breaks repair complex may be affected by the status of TLR activation. In this study, we focus on DNA damage caused by UVR in the form of CBPDs, which requires NER machinery to be excised and repaired.…”

Human in vitro skin organ culture as a model system for evaluating DNA repair

“…Following UV irradiation, imiquimod-treated specimens show upregulation of NER genes, and increased CBPD resolution in a MyD88-dependent manner [11]. Toll-like receptors (TLRs) are a family of pattern recognition receptors that alter the immune response to pathogen associated molecular patterns, which are structurally conserved “danger” signals such as microbial glycolipids and viral ssRNA.…”

“…The primary goal of this study was to create a viable ex vivo model that closely emulates the physical and biochemical properties of whole human skin, which can be easily adapted to evaluate various photoprotective drugs. Given recent publications highlighting its role in preventing UV-induced immunosuppression as well as its known use as an anticancer treatment, we utilized our human skin organ culture to study the effects of imiquimod in the context of post-UV DNA repair [11,16]. Additionally, we studied 3 patients treated with 5% imiquimod cream for actinic keratoses over a course of 2 weeks.…”