title: Differential effect of silibinin on E2F transcription factors and associated biological events in chronically UVB-exposed skin versus tumors in SKH-1 hairless mice.
abstract: UVB radiation-induced DNA damage in skin activates cellular pathways involved in DNA repair, cell cycle regulation, and apoptosis, important events that prevent conversion of damaged skin cells into cancer. We reported recently the efficacy of silibinin against photocarcinogenesis along with altered molecular events in tumors (Cancer Research, 64:6349-56, 2004). The molecular and biological events modulated by silibinin in chronically UVB-irradiated skin leading to cancer prevention, however, are not known. Herein, we describe effect of silibinin on skin 15 and 25 weeks after UVB exposure and compared them with molecular alterations in skin tumors. UVB decreased E2F1 but increased E2F2 and E2F3 protein levels in skin, and these were reversed by silibinin treatment. Silibinin-induced E2F1 was accompanied by an inhibition of apoptosis and decreases in p53 and cyclin-dependent kinase inhibitors. Silibinin-caused decrease in E2F2 and E2F3 was accompanied by reduced levels of cyclin-dependent kinases, cyclins, CDC25C, and mitogen-activated protein kinases and Akt signaling and inhibition of cell proliferation. In tumorigenesis protocols, topical or dietary silibinin significantly inhibited tumor appearance and growth. As opposed to UVB-exposed skin, UVB-induced tumors showed elevated levels of E2F1, but these were reduced in silibinin-treated tumors without any effect on E2F2 and E2F3. Contrary to the inhibition of apoptosis and p53 expression in UVB-exposed skin cells, silibinin increased these variables in tumors. These differential effects of silibinin on E2F1 versus E2F2 and E2F3 and their associated molecular alterations and biological effects in chronic UVB-exposed skin suggest their role in silibinin interference with photocarcinogenesis.
authors.affiliation: Department of Pharmaceutical Sciences, School of Pharmacy, University of Colorado Health Sciences Center, 4200 East Ninth Avenue, Box C238, Denver, 80262, USA.
authors.forename: Mallikarjuna
authors.initials: M
authors.lastname: Gu
authors.forename: Rana P
authors.initials: RP
authors.lastname: Singh
authors.forename: Sivanandhan
authors.initials: S
authors.lastname: Dhanalakshmi
authors.forename: Sarumathi
authors.initials: S
authors.lastname: Mohan
authors.forename: Rajesh
authors.initials: R
authors.lastname: Agarwal
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D002453:Cell Cycle / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects; D050760:Cyclin-Dependent Kinase Inhibitor p27 / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects; D050684:E2F Transcription Factors / Q000187:drug effects* / Q000378:metabolism; D005260:Female; D051379:Mice; D008812:Mice, Hairless; D015398:Signal Transduction / Q000187:drug effects / Q000528:radiation effects; D000077385:Silybin; D012838:Silymarin / Q000494:pharmacology; D012867:Skin / Q000473:pathology / Q000528:radiation effects*; D012878:Skin Neoplasms / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D016159:Tumor Suppressor Protein p53 / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects; D014466:Ultraviolet Rays / Q000009:adverse effects*
chemical_list: C493957:Cdkn1a protein, mouse; D050759:Cyclin-Dependent Kinase Inhibitor p21; D050684:E2F Transcription Factors; D012838:Silymarin; D016159:Tumor Suppressor Protein p53; D050760:Cyclin-Dependent Kinase Inhibitor p27; D000077385:Silybin