Silibinin Protects against Photocarcinogenesis via Modulation of Cell Cycle Regulators, Mitogen-Activated Protein Kinases, and Akt Signaling

Abstract

Here, we assessed the protective effect of silibinin on UVB-induced skin carcinogenesis in SKH-1 hairless mice. Topical application of silibinin before or immediately after UVB exposure or its dietary feeding resulted in a strong protection against photocarcinogenesis, in terms of tumor multiplicity (60 -66%; P < 0.001), tumor volume per mouse (93-97%; P < 0.001) and tumor volume per tumor (80 -91%; P < 0.001). Silibinin also moderately inhibited tumor incidence (5-15%; P < 0.01) and delayed tumor latency period (up to 4 weeks; P < 0.01-0.001). To investigate in vivo molecular mechanisms of silibinin efficacy, tumors and uninvolved skin from tumor-bearing mice were examined immunohistochemically for proliferation, p53, apoptosis, and activated caspase-3. Silibinin treatment showed a strong decrease (P < 0.001) in proliferating cell nuclear antigenpositive cells and an increase in p53-positive (P < 0.005-0.001), terminal deoxynucleotidyltransferase-mediated nick end labeling-positive (P < 0.005-0.001), and cleaved caspase-3-positive cells (P < 0.001). Western blot analysis of normal skin and tumor lysates showed that silibinin decreases the levels of cyclin-dependent kinase 2 and cyclin-dependent kinase 4 and associated cyclins A, E, and D1, together with an up-regulation of Cip1/p21, Kip1/p27, and p53. Silibinin also showed a strong phosphorylation of extracellular signal-regulated protein kinase 1/2, stress-activated protein kinase/c-JUN NH 2 -terminal kinase 1/2, and p38 mitogen-activated protein kinases but inhibited Akt phosphorylation and decreased survivin levels with an increase in cleaved caspase-3. Together, these results show a strong preventive efficacy of silibinin against photocarcinogenesis, which involves the inhibition of DNA synthesis, cell proliferation, and cell cycle progression and an induction of apoptosis. Furthermore, these results also identify in vivo molecular mechanisms of silibinin efficacy against photocarcinogenesis.

Silymarin inhibits UV radiation-induced immunosuppression through augmentation of interleukin-12 in mice

“…It also has been shown that silibinin (a major constituent of silymarin) inhibits UVBinduced skin photodamage, including the inhibition of photocarcinogenesis in mice whether it is applied topically before or after UVB irradiation or given in the diet (33). As UVB-induced immunosuppression has been implicated in the development of photocarcinogenesis, we examined the efficacy of silymarin on UVB-induced immunosuppression using local and systemic models of contact hypersensitivity in C3H/HeN mice.…”

Differential effect of silibinin on E2F transcription factors and associated biological events in chronically UVB-exposed skin versus tumors in SKH-1 hairless mice

“…The UVB light source consisted of four FS40T12-UVB sunlamps equipped with a UVB Spectra 305 Dosimeter (Daavlin Co., Bryan, OH). This emits 80% radiation within 280 to 340 nm, with a peak at 314 nm as monitored with SEL 240 photodetector, 103 filter, and 1008 diffuser attached to IL1400A Research Radiometer (11).…”

“…Silibinin prevents tumorigenesis, inhibits cell growth via a cell cycle arrest, and induces differentiation or apoptosis in many human epithelial cancer cell types, including skin, prostate, breast, cervical, bladder, colon, and lung (4). We reported recently the efficacy of silibinin against photocarcinogenesis together with alterations in molecular events related to cell cycle progression and mitogenic and apoptotic pathways within tumors (11). During acute UVB exposure, silibinin inhibits the activation of mitogen-activated protein kinases (MAPK) and Akt, induces p53 and Cip1/p21 expression, and suppresses DNA damage in mouse skin (12,13).…”

Silibinin Up-regulates DNA-Protein Kinase-dependent p53 Activation to Enhance UVB-induced Apoptosis in Mouse Epithelial JB6 Cells

“…We also found that silymarin protects against chemical carcinogenesis in SENCAR mouse skin by modulating mitogen-activated protein kinases (MAPKs) and inducing apoptosis (3). In a more recent study, we observed a strong protective effect of silibinin against UVB-induced skin carcinogenesis in SKH-1 mice where topical applications before or after UVB or its dietary feeding were effective in inhibiting tumor multiplicity as well as tumor volume (7). Also, we have reported the dual effects of silibinin in protecting from or enhancing apoptosis in response to UVB-induced moderate/excessive damage in spontaneously immortalized human keratinocyte HaCaT cells (5).…”

“…In recent years, chemoprevention is considered to be a promising strategy for the control of this malignancy, where several studies have shown that various phytochemicals from both nutritive and non-nutritive sources protect against UVB-induced skin cancer (2)(3)(4)(5)(6). In this regard, silymarin (a polyphenolic flavonoid antioxidant from milk thistle) and silibinin (the bioactive component in silymarin) have also shown strong potential in preventing UV-induced skin damages and photocarcinogenesis (2,(5)(6)(7). We have demonstrated the chemopreventive efficacy of silymarin/silibinin in the SKH-1 mouse skin model, where these agents showed strong protection against UVBinduced tumor initiation, tumor promotion, and complete carcinogenesis (2).…”