Breast cancer is the second leading cause of malignant death among women. A 
crucial feature of metastatic cancers is their propensity to lose adhesion to 
the underlying basement membrane as they transition to a motile phenotype and 
invade surrounding tissue. Attachment to the extracellular matrix is mediated by 
a complex of adhesion proteins, including integrins, signaling molecules, actin 
and actin-binding proteins, and scaffolding proteins. Focal adhesion kinase 
(FAK) is pivotal for the organization of focal contacts and maturation into 
focal adhesions, and disruption of this process is a hallmark of early cancer 
invasive potential. Our recent work has revealed that myoferlin (MYOF) mediates 
breast tumor cell motility and invasive phenotype. In this study we demonstrate 
that noninvasive breast cancer cell lines exhibit increased cell-substrate 
adhesion and that silencing of MYOF using RNAi in the highly invasive human 
breast cancer cell line MDA-MB-231 also enhances cell-substrate adhesion. In 
addition, we detected elevated tyrosine phosphorylation of FAK (FAK(Y397)) and 
paxillin (PAX(Y118)), markers of focal adhesion protein activation. Morphometric 
analysis of PAX expression revealed that RNAi-mediated depletion of MYOF 
resulted in larger, more elongated focal adhesions, in contrast to cells 
transduced with a control virus (MDA-231(LVC) cells), which exhibited smaller 
focal contacts. Finally, MYOF silencing in MDA-MB-231 cells exhibited a more 
elaborate ventral cytoskeletal structure near focal adhesions, typified by 
pronounced actin stress fibers. These data support the hypothesis that MYOF 
regulates cell adhesions and cell-substrate adhesion strength and may account 
for the high degree of motility in invasive breast cancer cells.