CHOP and AP-1 cooperatively mediate PUMA expression during lipoapoptosis

Area of Science:

• Molecular Biology
• Cell Biology
• Hepatology

Background:

• Saturated free fatty acids (FFA) induce hepatocyte apoptosis via endoplasmic reticulum (ER) stress.
• This lipoapoptosis involves upregulation of the BH3-only protein PUMA (p53 upregulated modulator of apoptosis).
• Both activator protein (AP)-1 and CAAT/enhancer binding homologous protein (CHOP) are implicated in PUMA induction.

Purpose of the Study:

• To investigate the cooperative roles of CHOP and AP-1 in mediating PUMA induction during ER stress-induced hepatocyte apoptosis.
• To elucidate the molecular mechanisms by which CHOP and AP-1 regulate PUMA expression in response to saturated FFA.

Main Methods:

• Utilized short-hairpin RNA (shRNA) to knockdown CHOP expression in Huh-7 cells.
• Assessed apoptosis, PUMA mRNA and protein levels, and Bax activation following palmitate treatment.
• Performed co-immunoprecipitation and chromatin immunoprecipitation assays to study protein interactions and DNA binding.

Main Results:

• CHOP knockdown attenuated palmitate-induced apoptosis and reduced PUMA induction and Bax activation.
• CHOP directly interacts with c-Jun (an AP-1 component) upon palmitate treatment.
• A CHOP:phosphorylated c-Jun complex binds to the AP-1 consensus site on the PUMA promoter, indicating a novel regulatory mechanism.

Conclusions:

• CHOP and AP-1 act cooperatively to mediate maximal PUMA induction during hepatocyte lipoapoptosis.
• This study reveals a novel mechanism of transcriptional regulation involving CHOP-AP-1 complex formation in ER stress-induced apoptosis.