Chemopreventive Effect And Related Mechanisms Of Lycopene On Cutaneous Carcinoma

Background and PurposeUntil now,cutaneous carcinoma remains the most common human malignancy(par-ticularly for the white population)owing to increasing urbanization,rising life expec-tancy and lifestyle changes.Millions of new cases are being diagnosed worldwide annually.Even if effective treatment modalities,including chemotherapy,radiotherapy,immunotherapy,photodynamic therapy and selective inhibitors,have been developed successfully,the burden of cutaneous carcinoma is still chronically high3.Alternatively,as one of the most promising approaches that suppress,reverse or block carcinogenesis in the initiation,promotion or progression phase,chemoprevention has been widely used to decrease the rates of cutaneous carcinoma incidence and related death.Lycopene is derived from tomato,the world's most popular fruit.It is a carotenoid compound and is an acyclic isomer of beta-carotene,which has been demonstrated to protect the skin cancer carcinogens induced skin damage.Thus,the present study focused on lcyopene-evoked defense mechanism to combat the chemically induced skin carcinogenesis,and which might clue the molecular mechanisms that mediate the action of lycoepene.Research Contents and ResultsFor the first time,we herein found in a DMBA-initiated,TPA-promoted mouse model of cutaneous tumorigenesis that lycopene obviously decreased cancer incidence rate and multiplicity,delayed the latency and kept the benign nature of papillomas only in the promotion phase through a kind of grouping unlike most previous studies.Additionally,most currently available preclinical studies concerning natural compounds were performed by using fully transformed cancer cells in vitro.In this study,we also verified that lycopene prevented the carcinogenesis of pre-malignant JB6 P+ cells in vitro by inhibiting TPA-stimulated malignant transformation.As suggested by the significant reduction of chemically induced tumorigenesis of cutaneous tissues and cells,lycopene may be an eligible cancer chemopreventive agent.Therefore,the mechanisms by which lycopene exerted antitumor effects in vitro and in vivo should be further explored.By using a comprehensive strategy,we found that lycopene prevented mouse cuta-neous papillomas in the promotion phase may mainly through regulating intracellular autophagy and redox state.As the tumor promoter for the model we used,TPA evidently speeded up the malignant transformation of preneoplastic cells by causing redox imbalance,which was significantly reversed by lycopene pretreatment both in vitro and in vivo.Furthermore,the antioxidant defense enzyme system was also enhanced by lycopene,suggesting that lycopene exerted preventive effect on cutaneous carcinoma by keeping intracellular redox homeostasis.The transcriptional activation of gene-encoding cytoprotective or antioxidant proteins was dominantly regulated by the redox-sensitive transcription factor Nrf2.Lycopene speeded up the transcription and translocation of Nrf2,and induced the expression of downstream gene.Interestingly,this effect can be exponentially enhanced with TPA present,suggesting only a primary balance maintained ability of lycopene against in-tracellular circumstance undermined by carcinogen rather than a normal one,and this action model of drugs may lead a relative lower adverse reaction.In addition,Nrf2 was crucial for lycopene to maintain cellular redox homeostasis and to relieve tu-morigenesis in both gene knock-down cells and transgenic mice.Lycopene pretreatment induced Nrf2 protein accumulation,but did not change its mRNA level.Nrf2 is a stress-responsive transcription factor that is constantly de-graded by the Keapl-Cul3 E3 ubiquitin ligase complex through polyubiquitination34.Upon stress challenges,Nrf2 escapes polyubiquitination by this complex and accu-mulates in the cell nucleus,inducing the transcription of stress-responsive genes.Nrf2 activation by chemopreventive agents has been explained by two plausible mecha-nisms.One is Keap1 inhibition,and the other is Nrf2 phosphorylation by kinases such as Akt and MAPKs.To clarify the mechanisms underlying lycopene-induced Nrf2 activation,we first focused on some upstream kinases well-documented to participate in the activation of Nrf2 signaling.Lycopene induced the phosphorylation of p3 8 MAPK and ERK,but blocking these two kinases did not affect nuclear Nrf2 localiza-tion,implying that the second mechanism may not be applicable to this study.On this basis,we studied the plausibility of the first mechanism.It is well-established that endogenous Keapl level plays an important role in controlling Nrf2 activity,and that knock-down of Keap1 gene can boost Nrf2 activity and alleviate oxidative stress and steatosis induced by fasting.Lycopene herein induced Keap1 reduction at the pro-tein level,because its mRNA level remained constant.To the best of our knowledge,microbial and cellular components are eliminated mainly by autophagy and ubiquiti-nation to maintain homeostasis.In this study,autophagy mediated the inhibitory ef-fects of lycopene on Keap1 protein.First of all,MG132,a proteasome inhibitor,did not work,but 3-MA and CQ,as specific inhibitors for autophagy,abolished bar-baloin-induced reduction of Keapl protein.Secondly,lycopene treatment increased LC3 and LC3-II cleavage,synchronize to the keap1 degradation.We herein demonstrated that pretreatment with lycopene induced autophagy by rais-ing p62 protein level and enhancing the binding of p62 to Keap1.In the absence of p62,lycopene did not induce Keapl degradation.Similarly,knock-down of p62 reversed the inhibitory effects of lycopene on the malignant transformation of JB6 P+ cells induced by TPA,indicating that lycopene prevented cutaneous carcinoma through inducing the Nrf2 pathway activation 'by p62-triggered autophagic Keap1 degradation.Conclusion and SignificanceIn summary,topical lycopene application inhibited TPA-induced intracel-lular redox imbalance and mouse cutaneous papillomas in the promotion phase by accelerating nuclear localization of Nrf2,which may be mediated by up-regulating p62 protein level,facilitating Keap1 degradation in an autophagy-lysosomal pathway.Based on the results of this study,lycopene or tomato-related skin care products in combination with sun screen protection might be helpful in reducing skin cancer in humans.