Alternative splicing variants of human Fbx4 disturb cyclin D1 proteolysis in human cancer.
Fbx4 is a specific substrate recognition component of SCF ubiquitin ligases that catalyzes the ubiquitination and subsequent degradation of cyclin D1 and Trx1. Two isoforms of human Fbx4 protein, the full length Fbx4α and the C-terminal truncated Fbx4β have been identified, but their functions remain elusive. In this study, we demonstrated that the mRNA level of Fbx4 was significantly lower in hepatocellular carcinoma tissues than that in the corresponding non-tumor tissues. More importantly, we identified three novel splicing variants of Fbx4: Fbx4γ (missing 168-245 nt of exon1), Fbx4δ (missing exon6) and a N-terminal reading frame shift variant (missing exon2). Using cloning sequencing and RT-PCR, we demonstrated these novel splice variants are much more abundant in human cancer tissues and cell lines than that in normal tissues. When expressed in Sk-Hep1 and NIH3T3 cell lines, Fbx4β, Fbx4γ and Fbx4δ could promote cell proliferation and migration in vitro. Concordantly, these isoforms could disrupt cyclin D1 degradation and therefore increase cyclin D1 expression. Moreover, unlike the full-length isoform Fbx4α that mainly exists in cytoplasm, Fbx4β, Fbx4γ, and Fbx4δ locate in both cytoplasm and nucleus. Since cyclin D1 degradation takes place in cytoplasm, the nuclear distribution of these Fbx4 isoforms may not be involved in the down-regulation of cytoplasmic cyclin D1. These results define the impact of alternative splicing on Fbx4 function, and suggest that the attenuated cyclin D1 degradation by these novel Fbx4 isoforms provides a new insight for aberrant cyclin D1 expression in human cancers.
Department of Microbiology & Infectious Disease Center, School of Basic Medical Sciences, Peking University Health Science Center, Beijing 100191, China.
Xiufeng
X
Chu
Department of Microbiology & Infectious Disease Center, School of Basic Medical Sciences, Peking University Health Science Center, Beijing 100191, China.
Ting
T
Zhang
Department of Microbiology & Infectious Disease Center, School of Basic Medical Sciences, Peking University Health Science Center, Beijing 100191, China.
Jie
J
Wang
Department of Microbiology & Infectious Disease Center, School of Basic Medical Sciences, Peking University Health Science Center, Beijing 100191, China.
Meng
M
Li
Department of Microbiology & Infectious Disease Center, School of Basic Medical Sciences, Peking University Health Science Center, Beijing 100191, China.
Xiaolei
X
Zhang
Department of Microbiology & Infectious Disease Center, School of Basic Medical Sciences, Peking University Health Science Center, Beijing 100191, China.
Jing
J
Tu
College of Pharmacy, Harbin Medical University-Daqing, Daqing, Heilongjiang 163319, China.
Shiqin
S
Sun
Department of Microbiology & Infectious Disease Center, School of Basic Medical Sciences, Peking University Health Science Center, Beijing 100191, China. Electronic address: xm_chen6176@bjmu.edu.cn.
Xiangmei
X
Chen
Department of Microbiology & Infectious Disease Center, School of Basic Medical Sciences, Peking University Health Science Center, Beijing 100191, China.
Fengmin
F
Lu
D017398:Alternative Splicing; D000818:Animals; D006528:Carcinoma, Hepatocellular / Q000737:chemistry / Q000378:metabolism*; D045744:Cell Line, Tumor; D002467:Cell Nucleus / Q000378:metabolism; D019938:Cyclin D1 / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D015536:Down-Regulation; D044783:F-Box Proteins / Q000235:genetics* / Q000502:physiology*; D006801:Humans; D008099:Liver / Q000378:metabolism; D008113:Liver Neoplasms / Q000737:chemistry / Q000378:metabolism*; D051379:Mice; D041681:NIH 3T3 Cells; D020033:Protein Isoforms / Q000502:physiology; D059748:Proteolysis
D044783:F-Box Proteins; C508146:FBXO4 protein, human; D020033:Protein Isoforms; D019938:Cyclin D1
Alternative splicing; Cancer; Cyclin D1; Fbx4