High-Molecular-Weight Hyaluronan Is a Hippo Pathway Ligand Directing Cell Density-Dependent Growth Inhibition via PAR1b

High-molecular-weight hyaluronan, a major component of the extracellular matrix, is anti-oncogenic, whereas low-molecular-weight hyaluronan is pro-oncogenic, though the mechanisms underlying the size-dependent opposite bioactivities of hyaluronan remain uncertain. We show here that treatment with high-molecular-weight hyaluronan stimulates tumor-suppressive Hippo signaling in breast epithelial cells. Mechanistically, clustering of the CD44 extracellular domain by high-molecular-weight hyaluronan leads to recruitment of the polarity-regulating kinase PAR1b by the CD44 intracellular domain, which results in disruption of the Hippo signaling-inhibitory PAR1b-MST complex. Once liberated from PAR1b, MST activates Hippo signaling. Conversely, low-molecular-weight hyaluronan, which is produced by hyaluronidase-mediated degradation of high-molecular-weight hyaluronan, inhibits Hippo signaling by competing with high-molecular-weight hyaluronan for CD44 binding. Triple-negative breast cancers with higher hyaluronidase-2 expression show poorer prognosis than those with lower hyaluronidase-2 expression. Consistently, decreased hyaluronidase-2 is associated with reduced tumorigenicity in a tumor xenograft model. Hence, perturbation of high-molecular-weight hyaluronan-mediated Hippo signaling activation contributes to cancer aggressiveness. [Display omitted] •High-molecular-weight hyaluronan stimulates Hippo signaling by clustering CD44•CD44 clustering activates MST by disrupting the Hippo inhibitory PAR1b-MST complex•Low-molecular-weight hyaluronan impedes CD44 clustering to subvert Hippo signaling•Loss of Hippo signaling activation by hyaluronan confers cancer aggressiveness High-molecular-weight (HMW) hyaluronan, a major extracellular matrix component, is anti-oncogenic, whereas low-molecular-weight (LMW) hyaluronan is pro-oncogenic. Ooki et al. uncover the cellular basis of these effects. HMW hyaluronan-mediated membrane CD44 clustering disrupts the Hippo signaling-inhibitory PAR1b-MST complex to stimulate signaling, whereas LMW hyaluronan counteracts this by competitively inhibiting CD44 clustering.