SIX1 induces lymphangiogenesis and metastasis via upregulation of VEGF-C in mouse models of breast cancer

Anassociationbetweenlymphnodemetastasisandpoorprognosisinbreastcancerwasobserveddecadesago.� However,�themechanismsbywhichtumorcellsinfiltratethelymphaticsystemarenotcompletelyunderstood.� Recently,�ithasbeenproposedthatthelymphaticsystemhasanactiveroleinmetastaticdisseminationand� thattumor-secretedgrowthfactorsstimulatelymphangiogenesis.�WethereforeinvestigatedwhetherSIX1,�a� homeodomain-containingtranscriptionfactorpreviouslyassociatedinbreastcancerwithlymphnodeposi- tivity,�wasinvolvedinlymphangiogenesisandlymphaticmetastasis.�Inamodelinwhichhumanbreastcancer� cellswereinjectedintoimmune-compromisedmice,�wefoundthatSIX1�expressionpromotedperitumoral� andintratumorallymphangiogenesis,�lymphaticinvasion,�anddistantmetastasisofbreastcancercells.�SIX1� inducedtranscriptionoftheprolymphangiogenicfactorVEGF-C,�andthiswasrequiredforlymphangiogen- esisandlymphaticmetastasis.�Usingamousemammarycarcinomamodel,�wefoundthatVEGF-Cwasnot� sufficienttomediateallthemetastaticeffectsofSIX1,�indicatingthatSIX1�actsthroughadditional,�VEGF-C-� independentpathways.�Finally,�weverifiedtheclinicalsignificanceofthisprometastaticSIX1/VEGF-Caxis� bydemonstratingcoexpressionofSIX1�andVEGF-Cinhumanbreastcancer.�Thesedatadefineacriticalrole� forSIX1�inlymphaticdisseminationofbreastcancercells,�providingadirectmechanisticexplanationforhow� VEGF-Cexpressionisupregulatedinbreastcancer,�resultinginlymphangiogenesisandmetastasis.