# Inhibitory effects of β-galactoside α2,6-sialyltransferase 1 on the Hippo pathway in breast cancer cells

## TL;DR

This study shows that ST6GAL1, an enzyme involved in glycosylation, inhibits the Hippo pathway in breast cancer cells, affecting tumor growth and metastasis.

## Contribution

The study identifies ST6GAL1 as a novel upstream negative regulator of the Hippo pathway through α2,6-sialylation.

## Key findings

- ST6GAL1 depletion increases YAP phosphorylation and activity in breast cancer cells.
- α2,6-sialylation modulates cell surface receptor signaling and YAP dephosphorylation.
- ST6GAL1 affects integrin β1–epidermal growth factor receptor/LPAR4 complex formation.

## Abstract

The Hippo signaling pathway is crucial in various pathological functions, such as tumor development. Yes-associated protein (YAP), a well-known downstream effector of the Hippo pathway, has been intensively studied; emerging evidence suggests that multiple cell membrane receptors can regulate the Hippo pathway. However, the mechanistic roles of these upstream pathways remain largely unknown. Here, we identified the β-galactoside α2,6-sialyltransferase 1 (ST6GAL1) catalyzed α2,6-sialylation as a pivotal upstream modulator of Hippo pathway by a glycosyltransferases overexpression sublibrary screening. Depletion of ST6GAL1 results in increased phosphorylation of large tumor suppressor kinase 1 and YAP, which induces YAP's nuclear localization, transcriptional activity, and multiple biological functions in breast cancer cells, including cell adhesion, spreading, growth, migration, and metastasis. These phenotypes were majorly due to the altered signal transduction of cell surface receptors, as deletion of ST6GAL1 exhibited attenuated G protein–coupled receptor, epidermal growth factor receptor, and integrins response and suppression of dephosphorylation of YAP. Mechanistically, these representative membrane receptors are α2,6-sialylated proteins, and their α2,6-sialylation could be inhibited by β-galactoside α2,3-sialyltransferase 4 via substrate competition. In addition, the α2,6-sialylation is essential for integrin β1–epidermal growth factor receptor/LPAR4 complex formation. Altogether, our findings demonstrate ST6GAL1 is an upstream negative regulator of the Hippo pathway in breast cancer cells, providing a new insight into the regulation between N-glycosylation and Hippo signaling.