Inflammation-induced loss of Pdcd4 is mediated by phosphorylation-dependent degradation

Area of Science:

• Oncology
• Immunology
• Molecular Biology

Background:

• Programmed cell death 4 (Pdcd4) is a tumor suppressor frequently lost in various cancers, correlating with increased tumorigenesis and progression.
• The impact of an inflammatory tumor microenvironment on Pdcd4 protein expression remains largely uncharacterized.

Purpose of the Study:

• To investigate the molecular mechanisms regulating Pdcd4 protein expression in tumor cells within an inflammatory context.
• To determine if inflammatory conditions affect Pdcd4 levels and elucidate the underlying pathways involved.

Main Methods:

• Human U937 monocytes were differentiated and activated to secrete inflammatory cytokines.
• Conditioned medium (CM) from activated macrophages was used to treat various tumor cells, and Pdcd4 protein levels were assessed.
• Indirect co-culture systems and in vivo mouse models (DSS-induced colitis) were employed.
• Mechanisms of Pdcd4 degradation, including proteasomal pathways and PI3K-mTOR signaling, were analyzed.

Main Results:

• Exposure to CM from activated macrophages significantly decreased Pdcd4 protein expression in tumor cells.
• Indirect co-culture with activated macrophages also led to Pdcd4 loss in tumor cells.
• Pdcd4 downregulation was mediated by enhanced proteasomal degradation, reducing its half-life.
• This degradation was dependent on the activation of phosphatidylinositol-3-kinase (PI3K)-mammalian target of rapamycin (mTOR) signaling.
• Pdcd4 protein was also found to be attenuated in vivo in mouse colon tissue during colitis.

Conclusions:

• The inflammatory tumor microenvironment promotes Pdcd4 degradation through a PI3K-mTOR-dependent, proteasome-mediated pathway.
• This represents an indirect, unidirectional effect of macrophages on tumor cells, leading to Pdcd4 loss.
• Stabilizing Pdcd4 protein emerges as a potential therapeutic strategy for targeting inflammation-associated tumors.