CD44 Attenuates Metastasis During Breast Cancer Progression

Abstract

Progression to metastatic disease is the leading cause of deaths resulting from breast cancer. Understanding the mechanisms underlying a cell's ability to move away from its site of origin and populate a distant site is important for the future development of therapies. The interactions between a tumor cell and the microenvironment can modulate a cell's ability to invade through tissues and access distant organs. In this study we present evidence indicating the differential modulation of invasive and proliferative phenotypes by hyaluronan present in the cellular microenvironment.We establish the role of CD44, the primary receptor for hyaluronan, in breast cancer progression and metastasis through the use of transgenic mouse models of breast cancer. While no differences were seen in the onset of primary breast tumors, mice expressing CD44 had a reduced rate of pulmonary metastasis compared to mice that lacked CD44. This establishes an anti-invasive role for CD44 in breast tumor progression. We also identify a decreased population of alveolar macrophages in CD44 negative mice that could affect metastatic breast cancer cell colonization of the lungs.We then focused our study in vitro, where we assessed the invasive properties of breast cancer cells as they move through three dimensional (3D) matrices containing or lacking hyaluronan. We show that in 3D type I collagen gels, breast cancer cells invade more readily in the absence of hyaluronan compared to when hyaluronan (HA) is embedded within the gel. HA mediated inhibition of invasion is dependent on CD44 binding as demonstrated through the use of a CD44 functional blocking antibody.We also show that HA promotes differential phenotypes of breast cancer cell. HA promotes filopodia formation and invasion when soluble in the cell microenvironment. Alternatively, matrix-embedded HA inhibits invasion and promotes migration through the formation of lamellipodia. The differential HA invasive and proliferative phenotypes are mediated by differential activation of ERK or γPAK. Activation of γPAK is mediated by CD44 while ERK activation by HA occurs by CD44 independent mechanisms.We also demonstrate an inhibition of MMP9 mediated invasion by HA when embedded within a type IV collagen matrix, but not a type I collagen matrix. This differential activity indicates that it is not only the immobilization of HA in a matrix that determines its activity, but also the context in which it is present within the matrix.These data underscore the importance of studying matrix components in an environment that closely resembles in vivo conditions. HA is a prime example as it has the capability of both promoting and inhibiting invasion depending on how it is presented to a cell. Differential HA activity also underlies the importance of understanding extracelluar matrix degradation and the release of matrix components as these can adversely affect disease progression.