# The nucleocytoplasmic translocation of HINT1 regulates the maturation of cell density

## TL;DR

HINT1 moves between the nucleus and cytoplasm depending on cell density, regulating transcription and actin remodeling to control cell maturation.

## Contribution

HINT1 is identified as a dual-function regulator of cell maturation through nucleocytoplasmic shuttling.

## Key findings

- HINT1 localizes in the nucleus at low density and binds to open chromatin.
- At high density, HINT1 translocates to the cytoplasm and modulates actin remodeling.
- HINT1 is required for full cell confinement and mature monolayer formation.

## Abstract

HINT1’s density-dependent shuttling regulates nuclear transcription at low density and cytoplasmic actin remodeling at high density.

In normal epithelial cells on tissue culture dishes, contact inhibition typically progresses with a reduction in cell size after cell–cell contact. This transition involves actin cytoskeleton reorganization from stress fibers (SFs) to a cortical network, stabilizing cell shape and strengthening connections. However, the regulatory signaling pathways remain unclear. We identified histidine triad nucleotide-binding protein 1 (HINT1), also known as protein kinase C inhibitor 1 (PKCI-1), as a regulator for monolayer maturation. At low density, HINT1 localizes in the nucleus and binds to open chromatin. As density increases, exportin 1 drives HINT1 translocation to the cytoplasm. Forced cytoplasmic localization of HINT1 reduces phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS) at Ser167/170, sites specifically targeted by PKC and involved in regulating SF formation. MARCKS phosphorylation also decreases naturally at high density. Although cells can form a monolayer without HINT1, its presence is required for full cell confinement and mature monolayer formation. Thus, HINT1 plays a dual role: acting as a nuclear transcriptional coregulator at low density and acting as a cytoplasmic SF modulator at high density.