title: Proteasome mediated degradation of Id-1 is associated with TNFalpha-induced apoptosis in prostate cancer cells.
abstract: Overexpression of the helix-loop-helix protein Id-1 has been reported in over 20 types of cancer. While a number of factors have been demonstrated to regulate Id-1 gene transcription, little is known about the mechanisms responsible for its degradation. In this study, we have demonstrated that Id-1 protein stability was regulated by TNFalpha in prostate cancer cells. We found that exposure of prostate cancer cell lines, DU145 and PC-3, to TNFalpha resulted in a rapid and significant downregulation of the Id-1 protein level. The fact that neither the Id-1 promoter activity nor the Id-1 mRNA level was affected by the TNFalpha treatment suggested that the decrease in Id-1 protein was not due to the suppression of gene transcription. In addition, the half-life of the Id-1 protein was decreased in both cell lines in the presence of TNFalpha, and the addition of an ubiquitin/proteasome inhibitor (MG-132) prior to the TNFalpha treatment completely blocked the effect of the TNFalpha-induced Id-1 protein degradation. Furthermore, introduction of a Flag-tag sequence into the N-terminus region of the Id-1 protein, which has been shown to stabilize the protein, was able to protect the Id-1 protein from TNFalpha-induced degradation. These results suggest that TNFalpha downregulated Id-1 through activation of the ubiquitin/proteasome degradation pathway in prostate cancer cells. Interestingly, in both DU145 and PC-3 cells, the decrease of Id-1 protein was associated with the activation of apoptotic pathway, as evidenced by the increased expression of cleaved PARP and caspase 3. In addition, TNFalpha failed to downregulate Id-1 in a sub-line of LNCaP cells that was resistant to TNFalpha-induced apoptosis. These results further suggest that the downregulation of Id-1 may facilitate TNFalpha-induced apoptosis in prostate cancer cells. In conclusion, our findings indicate that Id-1 protein may be regulated by TNFalpha through the ubiquitin/proteasome degradation pathway and the stability of the Id-1 protein appears to correlate with the sensitivity of TNFalpha-induced apoptosis.
authors.affiliation: Cancer Biology Group, Department of Anatomy, Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Hong Kong, SAR, China.
authors.forename: Ming-Tat
authors.initials: MT
authors.lastname: Ling
authors.forename: Wai-Kei
authors.initials: WK
authors.lastname: Kwok
authors.forename: Maggie K
authors.initials: MK
authors.lastname: Fung
authors.forename: Wang
authors.initials: W
authors.lastname: Xianghong
authors.forename: Yong-Chuan
authors.initials: YC
authors.lastname: Wong
mesh_terms: D017209:Apoptosis; D045744:Cell Line, Tumor; D015536:Down-Regulation; D006207:Half-Life; D006801:Humans; D051798:Inhibitor of Differentiation Protein 1 / Q000096:biosynthesis / Q000378:metabolism*; D008297:Male; D011471:Prostatic Neoplasms / Q000235:genetics* / Q000473:pathology*; D046988:Proteasome Endopeptidase Complex; D012333:RNA, Messenger / Q000032:analysis / Q000096:biosynthesis; D014409:Tumor Necrosis Factor-alpha / Q000502:physiology*; D025801:Ubiquitin
chemical_list: C497453:ID1 protein, human; D051798:Inhibitor of Differentiation Protein 1; D012333:RNA, Messenger; D014409:Tumor Necrosis Factor-alpha; D025801:Ubiquitin; D046988:Proteasome Endopeptidase Complex