# All‐Trans Retinoic Acid Suppresses Hepatocellular Carcinoma Progression via the CSTB/CYTB Axis

## TL;DR

All-trans retinoic acid (ATRA) slows liver cancer growth by reducing the CSTB/CYTB pathway, which affects mitochondrial function.

## Contribution

This study reveals a new mechanism by which ATRA inhibits hepatocellular carcinoma via the CSTB/CYTB axis.

## Key findings

- ATRA reduces CSTB expression, inhibiting HCC cell proliferation and migration.
- CSTB targets CYTB, a mitochondrial protein, to impair tumor cell energy production.
- ATRA treatment suppresses tumor growth in animal models by disrupting mitochondrial function.

## Abstract

Cystatin B (CSTB) is highly expressed in hepatocellular carcinoma (HCC) tissues and serum, indicating its potential as an early diagnostic biomarker. Given the known tumour‐suppressive effects of all‐trans retinoic acid (ATRA) in solid tumours, this study investigated whether ATRA inhibits HCC progression by modulating CSTB expression. Bioinformatics analyses of databases revealed the elevated CSTB expression in HCC, correlating with poor patient prognosis. These findings were validated in human HCC tissues and HepG2 cells. Through in vitro and in vivo functional assays, ATRA treatment was shown to significantly inhibit HCC cell proliferation, migration, and invasion, concomitantly with reduced CSTB expression. Proteomic sequencing identified cytochrome b (CYTB), a core component of mitochondrial respiratory chain complex III, as a downstream target of CSTB. Further experiments demonstrated that ATRA decreases mitochondrial membrane potential, complex III activity, and cellular ATP levels, these above effects were partially reversed upon CSTB overexpression. In vivo, ATRA administration effectively suppressed subcutaneous tumour growth. Collectively, these results indicated that ATRA exerts anti‐tumour activity in HCC by targeting the CSTB/CYTB axis, thereby impairing mitochondrial function and inhibiting tumour progression.