Date Published: July 14, 2016
Publisher: Public Library of Science
Author(s): Jasmine Sati, Biraja Prasad Mohanty, Mohan Lal Garg, Ashwani Koul, Eve-Isabelle PECHEUR.
Silibinin, a major bioactive flavonolignan in Silybum marianum, has received considerable attention in view of its anticarcinogenic activity. The present study examines its anticancer potential against 7, 12-dimethylbenz(a)anthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate (TPA) induced skin cancer. Male LACA mice were randomly segregated into 4 groups: Control, DMBA/TPA, Silibinin and Silibinin+DMBA/TPA. Tumors in DMBA/TPA and Silibinin+DMBA/TPA groups were histologically graded as squamous cell carcinoma. In the Silibinin+DMBA/TPA group, significant reduction in tumor incidence (23%), tumor volume (64.4%), and tumor burden (84.8%) was observed when compared to the DMBA/TPA group. The underlying protective mechanism of Silibinin action was studied at pre-initiation (2 weeks), post-initiation (10 weeks) and promotion (22 weeks) stages of the skin carcinogenesis. The antioxidant nature of Silibinin was evident at the end of 2 weeks of its treatment. However, towards the end of 10 and 22 weeks, elevated lipid peroxidation (LPO) levels indicate the pro-oxidative nature of Silibinin in the cancerous tissue. TUNEL assay revealed enhanced apoptosis in the Silibinin+DMBA/TPA group with respect to the DMBA/TPA group. Therefore, it may be suggested that raised LPO could be responsible for triggering apoptosis in the Silibinin+DMBA/TPA group. 1H Nuclear Magnetic Resonance (NMR) spectroscopy was used to determine the metabolic profile of the skin /skin tumors. Dimethylamine (DMA), glycerophosphocholine (GPC), glucose, lactic acid, taurine and guanine were identified as the major contributors for separation between the groups from the Principal Component Analysis (PCA) of the metabolite data. Enhanced DMA levels with no alteration in GPC, glucose and lactate levels reflect altered choline metabolism with no marked Warburg effect in skin tumors. However, elevated guanine levels with potent suppression of taurine and glucose levels in the Silibinin+DMBA/TPA group are suggestive of the pro-oxidative nature of Silibinin in regressing tumors. Thus, supporting the theory of augmented LPO levels resulting in increased apoptosis in the skin tumors treated with Silibinin.
Nonmelanoma skin cancer (NMSC) which comprises of squamous cell carcinoma (SCC) and basal cell carcinoma (BCC) is most prevalent among Caucasians.  BCC accounts for 80% while SCC represents about 20% of all diagnosed NMSC cases worldwide.  The Indian subcontinent reports higher prevalence of SCC in comparison to BCC.  Apart from ultraviolet radiation (UVR), the other common causes of NMSC are environmental and occupational exposures to arsenic, polycyclic aromatic hydrocarbons (PAHs), and ionizing radiations.  PAHs originate during incomplete combustion of organic materials, such as wood, petroleum, coal and are known for their toxic effects besides being carcinogenic and mutagenic in nature. 
The chemopreventive potential of topically administered Silibinin on chemically or UV-induced skin tumors has been reported extensively. [18,44–46] Although, this route of administration shows promising effects, but the limitation is that we cannot predict the location of the initiation of skin tumorigenesis. Hence, oral administration, which is convenient and would ultimately reach the systemic circulation may be the most promising route of administration for prophylaxis and was chosen as the mode of drug administration in our study.