Wnt signalingWnt signaling

http://www.stanford.edu/~rnusse/wntwindow.html

Wnt website

Wnt proteins

• a family of highly conserved secreted signaling molecules that regulate cell-to-cell interactions during embryogenesis

• Wnt genes and Wnt signaling are also implicated in cancer.

• Insights into the mechanisms of Wnt action have emerged from several systems: genetics in Drosophila and C. elegans; biochemistry in cell culture and ectopic gene expression in Xenopus embryos.

• Many Wnt genes in the mouse have been mutated, leading to very specific developmental defects.

• As currently understood, Wnt proteins bind to receptors of the Frizzled family on the cell surface. Through several cytoplasmic relay components, the signal is transduced to beta-catenin, which then enters the nucleus and forms a complex with TCF to activate transcription of Wnt target genes.

 gene Mouse Human Xenopus Chicken Zebrafish

 Wnt-1                         

 Wnt-2                      Wnt-2B/13                      

 Wnt-3                       Wnt-3A                       Wnt-4                          Wnt-4B             

 Wnt-5A                         

 Wnt-5B                    Wnt-6                    Wnt-7A                          Wnt-7B                       Wnt-7C             

 gene Mouse Human Xenopus Chicken Zebrafish

 Wnt-8A                       Wnt-8B                .         Wnt-8C          

 Wnt-9 only isolated from Hagfish (Eptatretus stouti) and Thresher Shark (Alopius vulpinus)

 Wnt-10A                       Wnt-10B                    Wnt-11                          Wnt-12, Wnt-13  

 Wnt-14                   

 Wnt-15                

 Wnt-16               

A model for Wnt/wg signaling

Wnts are secreted from cells Wnts are secreted from cells

albeit rarely in a soluble form.

In the extracellular In the extracellular space space

Several secreted proteins can bind directly to Wnts, to modulate Wnt activity.

In cells not exposed to In cells not exposed to the Wnt signal the Wnt signal

β-catenin levels are kept low through interactions with the protein kinase zw3/GSK-3β, APC and Axin

GSK-3βGSK-3βGSK-3β 는 APC 와 결합하여 beta-catenin과 complex 를 형성하여 beta-catenin 의 양을 조절하여 세포증식 및 분화를 down

regulation 하는 것으로 알려져 있습니다 .

β-catenin is degraded β-catenin is degraded After phosphorylation by GSK-3β, through the

ubiquitin pathway, involving interactions with Slimb/b-TrCP.

The DIX domain in Axin is similar to the NH2 terminus in Dsh, and promotes interactions between Dsh and Axin.

Axin also binds to the phosphatase PP2A, while the B56 subunit of PP2A interacts with APC.

APCAPC APC 는 결장암에서 90% 이상이 돌연변이를

보이는 것으로도 알려져 있다 . APC 의 주요 기능은 베타 - 카테닌의 함량을 저하시키는 것으로 알려져 있는데 , 일단 APC 유전자에 돌연변이가 일어나면 베타 - 카테닌의 함량이 증가해 이 물질이 핵 내부로 침투하는 결과를 빚게 된다 .

Loss of APC in mammalian cells can also lead to a critical loss over Arm control, leading to cell transformation. APC has a specific function in keeping β-catenin out of the nucleus. There are many other proteins binding to APC.

beta-cateninbeta-catenin

β-Catenin plays a dual role in the cell: one in linking the cytoplasmic side of cadherin-mediated cell-cell contacts to the actin cytoskeleton and an additional role in signaling that involves trans activation in complex with transcription factors of the lymphoid enhancing factor (LEF-1) family.

Elevated β-catenin levels in colorectal cancer caused by mutations in β-catenin or by the APC, which regulates β-catenin degradation, result in the binding of β-catenin to LEF-1 and increased transcriptional activation/repression.

In a current model, Wnt signaling initially leads to a complex between Dsh, GBP/Frat1, Axin and Zw3/GSK, which may be the regulatory step in the inactivation of Zw3/GSK.

As a consequence, GSK does not phos- phorylate β-catenin anymore, releasing it from the Axin complex and accumulation. Binding of Axin to the cytoplasmic tail of LRP in a Wnt dependent manner may also play a role in rearranging this complex.

The Wg/Wnt signal The Wg/Wnt signal leadsleads

In the nucleus In the nucleus

In the absence of the Wnt signal,TCF acts as a repressor of Wnt/Wg target genes.

TCF can form a complex with Groucho. The repressing effect of Groucho is mediated by interactions with Histone Deacetylases (HDAC). β-catenin can convert TCF into a transcriptional activator of the same genes that are repressed by TCF alone.

Domains and binding Domains and binding sites on TCFsites on TCF

Amadillo/b-catenin Groucho

CBP

HMG box

K25

TCF

Among the target Among the target genesgenes

Of this pathway are the c-myc gene, is activited in colon cancer by loss of APC.

Another target of β-catenin is Cyclin D.

βBD HMG B box

βBD A B HMG HA

Protein stability

N-TA1 781C-TALEF-1 / TCF

α- catenin

APCcadherin

CTA

•TCF (T cell factor)

•LEF-1 (Lymphoid enhancer factor)

•β-catenin

High mobility groupB-catenin binding site

Context-dependent transactivation domains

Gene

TCF1

Species/GenBank

Mouse Tcf-1 also called Tcf-7

mutant phenotype

Thymocyte Differentiation additional neural tubes defects

in the formation of the placenta and in the

development of limb buds in double mutant with Lef-1

Mammary tumors, accelerated by loss of Min/APC

Gene Mutant Gene Mutant PhenotypePhenotype

Gene

TCF3

Species/GenBank

Mouse Tcf-3 also called Tcf-7

mutant phenotype

Headless gene (hdl) in Zebrafish, essential in head

formation

Gene

TCF4

Species/GenBank

Mouse Tcf-4 also called Tcf7

mutant phenotype

Absence of epithelial stem cells in small intestine

Gene

LEF1

Species/GenBank

Mouse Lef-1

mutant phenotype

Several organs, epithelial-mesenchymal

interactions additional neural tube defects in the formation

of the placenta andin the development of

limb buds in double mutant with TCF-1 Defects in pro-B Cell proliferation and survival

This is a list of target This is a list of target genes of Wnt/β-catenin genes of Wnt/β-catenin

signalingsignaling gene Organism Up/down Ref.Direct/indirect

C-myc Human colon cancer up He 1998yes

Cyclin D Human colon cancer up Tetsu 1999 yes

Tcf-1 Human colon cancer up Roose 1999 yes

PPARdelta Human colon cancer up He TC 1999 yes

c-jun Human colon cancer up  Mann B, 1999 yes

fra-1 Human colon cancer upyes

uPAR Human colon cancer upyes

MMP-7 Human colon cancer upyes

CD44 Human colon cancer up Wielenga 1999 yes

 Mann B, 1999

 Mann B, 1999

Crawford 1999

Cyclin D1Cyclin D1

사이클린 D1 은 세포 생장을 조절하는 주요 조절 인자 가운데 하나이기 때문에 사이클린 D1 단백질에 이상이 생기게 되면 세포 분열 (cell proliferation) 이 급 격 히 증 가 하 게 되 는 결 과 를 빚 는 다 . 이 로 인 해 세 포 조 직 이 비정상적으로 생장하게 되고 결국에는 종양 (tumour) 이 형성되는 것이다 .

C-mycC-myc

사람의 c-myc 유전자는 nuclear oncogene 으로 8 번 염 색 체 에 위 치 하 며 , 3 개 의 exon 으로 구성되어 있다 . 여기서 exon 1 은 조절역할을 하며 exon 2, 3 이 합쳐져 단백질을 만들게 된다 . c-myc 은 대 장 암 , 자 궁 경 부 암 , 폐 암 , 소세포폐암 , 유방암 , 위암 , 생식기암 , 결장암 , 위선암 , 전골수세포 백혈병 , 섬유 아세포증 , 상피세포증 , 골수세포증 등 발암유전자중에서도 가장 많은 종류의 암에 관련이 있다 .

또한 염색체 전좌에 의해 c-myc 유전자의 활성화가 Burkitt's 임파종 (BL) 에서도 발견되고 있다 (translocation 때 mutation). 이것은 암세포 형성의 직접적인 원인 보다는 ras 등 다른 발암유전자등과 함께 작용하여 세포를 변형시키는 조력자의 역할을 하는 것으로 알려져 있는데 , 보통 증폭이 빠르고 악성 빈도가 높은 암에서 나타난다 . 그러므로 c-myc 발암유전자의 활성으로서 또 다른 발암유전자의 발현을 예측할수 도 있다 . 암 유발 유전자인 c-myc 또한 세포사를 촉진시키는 것으로 알려졌다

암에서의 양상추론암에서의 양상추론β-catenin 이 암에서는 E-cadherin 과

떨어진다 .β-catenin 은 GSK-3β 에 의해서 인산화

되지않는다 .APC 돌연변이 β-catenin, LEF/TCFs 발현증가β-catenin 자체의 mutationWnt pathway activation