Heme oxygenase 1: a novel oncogene in multiple gynecological cancers.

Oxidative stress is caused by the imbalance between oxidation and antioxidant system in our bodies 49 . Common noxious stimuli include hypoxia, inflammatory cytokines, ultraviolet light, heavy metal ions, radiotherapy and so on 24 , 47 . These stimuli changes the body's homeostasis, which triggers the activation of various signaling pathways, leading to disease progression. In normal cells, HO-1 acts as a cytoprotective agent, it can fight against oxidative injuries and regulate inflammatory response 50 , 51 . In ischemic diseases, HO-1 has been be used for the treatment of glucocorticoid-related osteoporosis and osteonecrosis 52 . Noxious stimuli actives upstream signal kinases, promoting the binding of DNA and transcription factors, which results in high expression of HO-1 and providing protection for normal cells 50 , 53 . Nevertheless, the biological effect of HO-1 seems to be tissue-specific. In some cancer cells, HO-1 plays the role of survival factor 29 . Overexpression of HO-1 promotes tumor progression in turn 23 , 54 . It can facilitate angiogenesis and prevent tumor cells from apoptosis, leading to its survival and progression 55 . Researches suggest that heme can induce a stress-inducible protein Sestrin2 (SESN2), which is a protective mechanism to antagonize oxidative stress and colon tumor growth. However, high level of SESN2 can promote tumorigenesis 56 . Another high-risk factor of colorectal carcer is the consumption of red meat. Heme iron is the main component of red meat, which may be the cause of excessive colonic proliferation and carcinogenesis 57 . Antioxidants can stimulate lung cancer metastasis by reducing heme levels and stabilizing the transcription factor BTB and CNC homology 1 (BACH1) 58 . Overexpression of HO-1 induces the expression of Cyclin Dependent Kinase 4 (CDK4) and promotes the occurrence of liver cancer; however, a feedback loop may exist between IL-6 and HO-1, thus HO-1 can be induced as an antitumor gene through the IL-6/JAK/STAT3 pathways 59 , 60 . The application of HO-1 inhibitor zinc protoporphyrin (ZnPPIX) can greatly inhibit the proliferation of pancreatic cancer cells, while HO-1 significantly promotes cell proliferation 61 . Overexpression HO-1 promotes the occurrence of melanoma and plays the role of anti-apoptosis through the B-Raf-ERK signaling pathway 62 . The most common metastatic site of prostate cancer is bone, a research by Anselmino has showed that HO-1 is a pivotal modulator of bone turnover and remodeling because it can promote the growth and invasion of cancer cells both in vivo and in vitro . This may be related to the epithelial-mesenchymal transition (EMT) induction and antioxidant and antiapoptotic effects of the prostate cancer cells 63 , 64 . HO-1 can increase anti-apoptotic ability, which may lead to their uncontrolled cell proliferation and even cause tumorigenesis 24 . Compared with the surrounding normal tissues, therefore, the increased expression of HO-1 can be observed in tumors 11 . Abnormal signaling pathway activation can lead to reduced self-adhesion ability of tumor cells, tumor angiogenesis and changes in microenvironment 65 , preventing the cancer cells from apoptosis and autophagy, and even promoting their proliferation and metastasis 47 . Many studies have shown that HO-1 is a crucial substance for angiogenesis 66 , which can help malignant tumors continue to grow and invasion. Currently, highly-expressed HO-1 has been found in various malignant tumors such as melanoma 2 , thyroid cancer 5 , osteosarcoma 50 , breast cancer 67 , lung cancer 58 , bowel cancer 56 , 57 , 68 , renal cell cancer 69 , hepatoma 59 , prostate cancers 63 , 64 , pancreatic cancer 61 , and so on. However, the formation of tumors is not only related to the cancer cells themselves. In fact, their occurrence, growth and even metastasis are very closely related to the surrounding cells (immune/inflammatory cells, glial cells, fibroblasts, etc.) and extracellular components (cytokines, growth factors, hormones, etc.), namely the so-called tumor microenvironment (TME) 70 , 71 . More and more studies have shown that HO-1 can affect cancer progression through modulating TME 72 . Of note, HO-1 can act as an immunomodulator that inhibits cell maturation, activation and infiltration 73 - 75 . Myeloid-derived suppressor cells (MDSCs) are known to inhibit anti-tumor immunity, HO-1 expression in MDSCs plays a role in the suppression of alloreactive T cells 76 , 77 through promoting the release of many inflammatory factors such as interleukin-10 (IL-10) 78 and tumor necrosis factor-α (TNF-α) 79 , the expression of transforming growth factor β (TGF-β), intercellular adhesion molecule 1, and other fibrogenesis factors increase, activating the NF-κB/Signal Transducer and Activator of Transcription (STAT)3 signaling pathway 80 , 81 , and maintaining the self-renewal ability of cancer stem cells 82 . By regulating the inflammatory response and anti-tumor immunity, these immune/inflammatory cells play significant roles in TME, which can deeply affect cancer progression. Meanwhile, as a key mediator of angiogenesis, vascular endothelial growth factor (VEGF) can form new vasculature around tumors, causing them to grow exponentially 83 . HO-1 also participates in fostering angiogenesis linked to inflammation and tumor by up-regulating the expression of VEGF in macrophages. A study conducted by Gabriel et al. has showed that the ectopic expression of HO-1 can significantly increase the transcriptional activity of VEGF in prostate cancer cells 84 . In addition, VEGF fosters the formation of capillary-like tubular structures in tumor tissues 85 , 86 , and promotes the proliferation and migration of cancer cells. More importantly, the level of HO-1 is closely related to the clinical features and prognosis of tumors. Generally speaking, the higher the expression of HO-1, the lower the tumor differentiation, and the more active the proliferation and metastasis 9 . HO-1 inhibitors can suppress cell proliferation and invasion by increasing intracellular ROS levels and inducing cell cycle arrest 5 . Because HO-1 is so closely related to tumors, some people even proposed that HO-1 can also be used as one of the tumor markers. However, a study on breast cancer showed that HO-1 overexpression can reduce lung metastasis by inhibiting cell EMT and proliferation, suggesting that HO-1 is tissue-specific 87 , which needs to be studied further. HO-1 plays different roles during different stages of tumor formation. Before a tumor is formed, it can remove aging and dead cells, inhibit tumors and protect normal cells. When a tumor is formed, the activation of HO-1 enables tumor cells to gain this anti-apoptotic ability, which leads to the occurrence and proliferation of tumors 53 . In that case, it has a protective effect on tumor cells instead.

Chemotherapy is a very important adjuvant treatment method after malignant tumor surgeries 89 . Many commonly-used chemotherapy drugs can generate ROS to achieve the purpose of inducing cancer cell apoptosis 65 , 96 , 98 . In normal situations, antioxidants can protect our body from oxidative stress damage; nevertheless, cancer cells may also activate antioxidant signals to fight against the damage caused by chemotherapy drugs, namely, cells develop drug resistance 105 . Cis-platinum (CDDP) is one of the most widely used chemotherapy drugs 8 . Recent reports indicated that its cytotoxic effect may be achieved by ROS-dependent apoptosis or DNA damage 17 , 106 . Other people believe that this may because chemotherapy-generated tumor cell debris hijack tumor-associated macrophages (TAMs). By promoting HO-1 expression and reducing M1-like polarization, tumor cells developed resistance to chemotherapy drugs. What's more, overexpression of HO-1 is often accompanied by an increase in multidrug resistance-related proteins (Mrp), which is an important reason for the difficulty in tumor treatment 67 . HO-1 plays a protective role in tumor cells has been widely recognized 107 . Studies have found that HO-1 inhibitor ZnPPIX can improve sensitivity to chemotherapy of gastric cancer cell 12 , 108 , 109 , other research showed that the inhibition rate of esophageal cancer cells was positively correlated with ZnPPIX concentration 109 , 110 . A study based on the treatment of acute myelogenous leukemia (AML) has found that through non‐covalently modified, lipid‐polymer hybrid nanoparticle loaded with HO1‐inhibitor tin mesoporphyrin (SnMP) can significantly improve the efficacy of daunorubicin and boost immune response 17 , 111 . Furthermore, a strong potential of blocking HO-1 for the treatment of hereditary leiomyomatosis and renal cell carcinoma (HLRCC) has already been verified 69 . All these findings remind us that exploring the possibility of targeting or genetically or pharmacologically inhibiting HO-1 could make immunotherapy more effective 112 , and HO-1 inhibitor may be used as a potential chemotherapeutic sensitizer in the near future 113 .

The most important component of red blood cell is hemoglobin, which is composed of globin and heme 1 . Heme is a large complex, containing iron and protoporphyrin IX 2 - 4 . It is a cellular oxidant, participating in the formation of oxidative free radicals and leading to oxidative injury 5 . Noxious stimulation leads to the increase of heme oxygenase 1 (HO-1) for antagonizing these oxidative stresses and protecting our bodies 6 . Therefore, the hypothesis that HO-1 may be used as a targeted gene in tumor treatment has attracted more and more attentions. It is one of the most widely distributed antioxidative enzymes in the body and is the rate-limiting enzyme of heme metabolism 7 . Among all these malignant tumours, gynecological cancers is a specific type of fatal disease which only happens to women, and can seriously threaten the lives and health of women around the world. In recent years, many studies have confirmed that HO-1 is highly-expressed in a variety of gynecological tumors, such as ovarian cancer, cervical cancer and endometrial cancer. The elevated level of HO-1 and the deviation of its dynamic trend from the baseline may be a signal of disease alert 8 - 11 . Studies have also shown that as a novel oncogene, HO-1 is closely related to tumor proliferation and metastasis, and may become a potential marker for predicting the prognosis of gynecological tumors 9 . Furthermore, HO-1 itself is also expected to become a target for tumor treatment. HO-1 inhibitors such as zinc protoporphyrin (ZnPP) have obtained certain efficacy in clinical work 12 . Therefore, this review summarizes the expression, regulation, roles and treatment values of HO-1 in gynecological tumors.

Different HO-1 inducers activate different protein phosphorylation-dependent signaling pathways, then activate various transcription factors. MAPK is one of the most important signal kinases in HO-1 transcription, other signal kinases such as phosphatidylinositol 3-kinase (PI3K), tyrosine kinases and many protein kinases (PK) also participate in this process 24 . The latest study suggests that the regulation of its enzymatic activity depends heavily upon the expression of transcriptional level 88 . Nuclear factor E2-related factor 2 (Nrf2) is a key transcription factor involving in maintaining cell redox homeostasis 89 , and HO-1 is one of the most important regulatory products. It is a bZIP transcription factor 10 . Under resting conditions, Nrf2 binds to kelch-like ECH-related protein 1 (keap1) and form a Keap1-Nrf2 complex 90 . The complex will be degraded by ubiquitous proteasome and exist in the cytoplasm in an inactive state. However, oxidative stress induces the modification of cysteine ​​residues in Keap1, causing Nrf2 to dissociate from the complex and increase the translocation of nucleus 10 . Within the nucleus, it binds to the antioxidant-responsive element (ARE) in target gene promoters and form the Nrf2-ARE signaling pathway 90 , activating the transcription of its downstream target genes, such as HO-1 and NADPH quinone dehydrogenase 1 (NQO1), protecting cells from oxidative damage and participating in maintaining redox homeostasis 24 , 42 , 56 . The PI3K/protein kinase B (PKB, also known as AKT) signaling is one of the most critical pathways in regulating cell growth, proliferation and apoptosis. Studies have found that Nrf2 is significantly elevated in tumor cells. Inactivating PI3K/AKT pathway can significantly reduce the level of Nrf2, inhibiting tumor cell proliferation, inducing cell apoptosis, and improving the sensitivity of tumor cells to treatment 91 . In addition, MAPK signaling pathways can also regulate the activity of Nrf2. Nuclear factor κB (NF-κB) and Bach1 also play a key role in the occurrence and development of cancer 92 , and is considered to be a target for the cancer therapy. Under resting conditions, IκB binds to NF-κB and Bach1 binds to Maf recognition element (MARE), they form a new complex respectively, preventing NF-κB and Bach1 translocation from the cytoplasm to the nucleus 93 . Once oxidative stress stimulates the complex, NF- κB dissociates from IκB, Bach1 dissociates from MARE. Both of them can activate the transcription of HO-1 18 , 43 .

Heme/HO system is one of the most important anti-oxidant mechanisms in our bodies. As a potential novel oncogene, HO-1 has received increasing levels of attention in recent years. On the one hand, HO-1 is highly expressed in a variety of gynecological malignancies, so the deviation of its dynamic trend from baseline could be used as a signal of disease alert and a predictor for the occurrence of tumors. On the other hand, for oncology patients with clinical manifestations and imaging evidence, high-level of HO-1 can also assist diagnosis. HO-1 level has certain relevance with prognosis and can be used as a potential indicator. Through inhibiting HO-1 directly or indirectly, HO-1 inhibitors can promote ROS-dependent autophagy and apoptosis. At present, HO-1 inhibitors have been used in clinical work and achieved certain efficacy. However, the exact mechanism of HO-1 in gynecological tumors is still unclear. For one thing, HO-1 was proposed as a novel oncogene in gynecological malignancies not long ago, thus it has not yet been fully studied. For another, HO-1 seems to be tissue-specific. In normal tissues, it plays the role of anti-inflammation and anti-apoptosis, which indicates us to seek for non-stress HO-1 inducers for body protection. However, in cancer cells, HO-1 facilitates angiogenesis and tumor metastasis in turn. The contradiction makes the research on HO-1 very difficultly. Anyway, regulating the expression of HO-1 may be a potential target of clinical treatment for patients with gynecological malignancies, although further studies are still needed to be done. Clarifing the exact mechanism of HO-1 in gyncological cancers may pave a new way for preventing the onset or progression of gyncological cancers.