As different AP-1 components have reverse functions in cancer development, therapeutic strategies targeting AP-1 should be specifically designed and carefully explored in clinical usage. Yukihiko et al. designed a small molecule c-Fos/AP-1 inhibitor (T-5224) using a three-dimensional pharmacophore modeling based on the crystal structure of the AP-1-DNA complex ( Figure 2A ) (72). It can specifically inhibit AP-1 binding to the DNA, and it has been proven to resolve arthritis in a preclinical model (72). To be noted the daily dose of the T-5224 could be administered up to 150mg/kg in rats and ≥ 750 mg/kg in cynomolgus monkeys for 1 month without observed adverse effects. Evidence showed that T-5224 inhibited matrix metalloproteinase expression in human articular chondrocytes and, hereby, prevented cartilage destruction in an osteoarthritis-induced mouse model (73). Daisuke et al. proved that T-5224 can prevent invasion and migration in head and neck squamous cell carcinoma cells and oral administration of T-5224 inhibit lymph node metastasis in head and neck cancer in an orthotropic tumor implantation mouse model without affecting the tumor growth (74). A daily dose of 150 mg/kg has been confirmed to be safe in rodents, and a lower dose can be used for some inflammatory diseases (74). The efficiency of T-5224 in vivo is about 10 times more effective than in vitro. The high efficiency may be due to its crosstalk with IL-1, IL-6, TNF-α, MMPs, etc. (72, 74). Also, the crosstalk might explain why there are few adverse effects with the AP-1 inhibition.

JNK inhibitor, SP600125 ( Figure 2B ), which can block AP-1 phosphorylation and stop its activation, showed a protective role in atherosclerosis initiation in apolipoprotein E-deficient mice (75). As in vitro and animal studies revealed AP-1 had a critical role in the initiation and progression of vascular dysfunction and atherogenesis (75, 76), hence, Meijer et al. tested whether doxycycline ( Figure 2C ), which had a direct inhibitory effect on JNK1/2 can improve vascular function in a double-blind placebo-controlled cross-over trial (Dutch Trial Registry NTR1389) (77). Results indicated that minimal activation of AP-1 was found in non-progressive and progressive phases of atherosclerosis respectively, and no significant difference was found between progressive and vulnerable lesions. Thus, the clinical trial didn’t confirm AP-1’s role as a therapeutic target for human atherosclerotic (77). It seems the clinical trial had reverse results with the animal studies, in which AP-1 inhibition in hypercholesterolemia mice finally prevent atherosclerosis. However, atherosclerotic in human were in advanced stage, but AP-1 activation occurred much earlier. Also, they found a clear protective effect on vascular inflammation in human abdominal aortic aneurysm (77). The different results might be because AP-1 has a prominent role in aneurysmal disease but has less function in advanced atherosclerotic disease (78).

The MLN944 is a novel cytotoxic drug with anti-tumor activity against human and murine tumor models both in vivo and in vitro ( Figure 2G ) (79). The MLN944 can block c-Jun binding to the AP-1 site hence block AP-1 transcriptional activity. Preclinical studies showed MLN944 delayed tumour growth in the HT29 human colon carcinoma. Further clinical trail has been conducted with MLN944 for treatment with patients with advanced tumours to determine the dose-limiting toxicity. While the lack of correlation between toxicity and pharmacokinetics values made it difficult to continue for phase II clinical trail (71).

The SR11302 inhibit Fra-1/AP-1 binding to TRE site showed significantly suppression in tumor growth and lymph node metastasis of head and neck squamous cell carcinoma (HNSCC) ( Figure 2D ) (80). The SPC839 is a dual inhibitor of AP-1 and NF-κB, showed good inhibitory activity against nitric oxide, TNF-α and FLT3, which is a potential target for acute myeloid leukemia (AML) ( Figure 2E ) (81, 82). The glaucarubinone can block AP-1 promoter result to inhibiting cell growth ( Figure 2F ) (83), but it was not specific to AP-1. Even there are various AP-1 inhibitors used for research, but with low-specificty and unpromising in vivo results, few applied in clinical trails ( Table 1 ).

Immunotherapy or immune checkpoint blockade (ICB) therapy has been proven efficient in metastatic cancers, such as lung cancer, melanoma, and breast cancer (87–89), leading to improved overall survival. Immune checkpoints (e.g., PD-1/PD-L1 and CTLA-4) function in maintaining self-tolerance and restricting immune response. They are frequently exploited by the tumor cells to escape the immune system, which results in immune escape (90). The ICB is beneficial only for a limited fraction of cancer patients. Factors that affect checkpoint expression have been regarded suppressing the ICB efficiency. Among them, AP-1 has been described as an important factor regulating checkpoint expression. Researches revealed c-Fos bound to the promoter region of PD-1 and regulate its expression; Fra-1 mediated PD-L1 expression; c-Jun and JunB can also bound to the enhancer region of PD-L1 (91–93). Thus, AP-1 targeted inhibition can reduce immune checkpoint expression and improve the sensitivity of patients’ response to ICB therapy.

Besides, AP-1 was described to regulate the immune system during cancer development (94). T cell anergy states the unresponsive status of T cells, and T cell exhaustion refers to the states of CD8⁺ T cells, which respond poorly during chronic infection or cancer (95, 96). Evidence suggested that AP-1 cooperated with NFAT regulated gene expression after immune response, hence lack of AP-1 led to blockage of T cell activation and eventually resulted in T cell energy (97, 98). In addition, exhausted cells exhibit low expression of AP-1 factors (Fos, FosB, and JunB) (99). JunB plays a vital role in T cell differentiation and proliferation in multi-ways: it is responsible for IL-2Rα expression in cooperation with other AP-1 components; it contributes to IL-2 production in conventional T cells; forms heterodimer with BATF to regulate the expression of Th-17-related factors (100). JunB can also bind directly to the promoter region of IL-4, therefore promoting its expression during the differentiation of T helper 2 (Th2) cells (45).

c-Jun can enhance T cell functional capacity and promotes its anti-tumor potency. T cell exhaustion has been regarded as a cause of Chimeric antigen receptor (CAR)-T cell dysfunction, so editing exhaustion resistant CAR-T cells can enhance its anti-tumor activity and improve clinical outcomes (101, 102). c-Jun overexpression can exhibit CAR-T cell exhaustion resistance, improve expansion potential, reduce terminal differentiation, and improve its anti-tumor potency in in vivo models (103). c-Jun overexpression reduced the T cell exhaustion associated genes and increased memory genes (103) Lynn. et al. described c-Jun enhanced CAR-T cell anti-tumor activity in a Nalm6-GD2⁺ leukemia model and dramatically increased T cell expansion, preventing 143B osteosarcoma tumor growth in vivo (103).

Therefore, AP-1 can mediate ICB expression to block immune response, and on the other hand, AP-1 can drive immune system activation. Different AP-1 components in different cells may play a reverse effect in regulating the immune response. Specific AP-1 members in target tissues should be further explored to enhance ICB.

Poly(ADP-ribose)polymerase 1 (PARP1) can transfer ADP-ribose units from nicotinamide adenine dinucleotide (NAD⁺) to target proteins, such as histones, DNA polymerase, DNA ligases, and itself (104, 105). This process is called poly(ADP-ribosyl)ation (PARylation). PARP1 functions in single-strand DNA break (SSB) repair through base excision repair (BER) (106). When the SSB repair is blocked by PARP1 inhibitor, SSB can accumulate to double-strand break (DSB), which can be repaired by BRCA-mediated homologous recombination (107). This is the basis of PARP1 inhibitor target therapy for BRCA-mutated breast cancer and ovarian cancer.

PARP1 inhibitor has been applied in the clinic to treat BRCA1/2 mutated breast and ovarian cancer. Our group applied multi-omics approach revealed PARP1 can PARylate Fra-1 and repress its expression in TNBC cells; PARP1 inhibitors can increase Fra-1 expression, and the increased Fra-1 results in the resistance to PARP1 inhibitors (92). Evidence has also shown that PARP1 PARylates c-Jun/c-Fos and promotes AP-1 phosphorylation and DNA-binding activity (32, 108, 109). As PARP1 inhibition can increase Fra-1 expression without affecting c-Jun expression (92), AP-1 (Fra-1/c-Jun) inhibition combined with PARP1 inhibition therapy may serve as a new joint therapy for breast and ovarian cancer patients. As AP-1 is overexpressed in TNBC, the new joint therapy may be applied in TNBC patients.

Cyclin-dependent kinases 4 and 6 (CDK4/6) mediate the cellular cell cycle by transitioning the G1 to S phase. The CDK4/6 inhibitors, as a result of this, induce G1 cell cycle arrest in tumor cells (110). They are prescribed routinely in clinical to treat estrogen receptor-positive breast cancer. Trials that apply it against other cancer types (e.g., Human epidermal growth factor receptor 2 positive breast cancer, triple-negative breast cancer) are ongoing (111). Watt et al. revealed that CDK4/6 inhibitor increased AP-1 components (e.g., c-Jun, JunB, Fra-2) expression, driving its transcriptional activity (112). Therefore, AP-1 may mediate the resistance of the CDK4/6 inhibitor therapy. Thus, AP-1 blockage may sensitize the CDK4/6 inhibitor therapy and improve patients’ outcomes.

The histone modulation, including acetylation, and methylation, alters the structural interaction between the histone proteins and DNA and modulates the DNA transcription and protein expression (113). Histone acetylation is controlled by histone acetyltransferases and deacetylases, the aberrant histone acetylation is associated with many tumors (114). The histone deacetylase (HDAC) inhibitors have been used in clinical for various cancers, including cutaneous T cell lymphoma (113), breast (115), non-Hodgkin’s lymphoma, and mantle cell lymphoma (114, 116). Yuan et al. revealed that HDAC inhibition could promote c-Fos expression without affecting c-Jun expression. The increased c-Fos activated AP-1 formation and mediated the resistance of HDAC targeted therapy (117). Thus, the c-Fos/AP-1 may be the side effect of HDAC targeted therapy, and c-Fos/AP-1 inhibition may improve the efficiency of HDAC targeted therapy.

The AP-1 and NF-κB dual inhibitor SP100030 is one of the first reported small molecules to inhibit gene expression induced by stimuli (118). It can recover muscle weight by increasing MyoD gene expression in the cachectic tumor-bearing rat (119). Suto et al. constructed an adenovirus-expressing TAM67, a dominant-negative mutant of c-Jun (120). It has no transactivation domain and can inhibit the endogenous AP-1. The TAM67 has been shown to reduce the tumor volume in the xenograft mice model, indicating that TAM67 may be a new cancer treatment strategy.