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

Carcinogenesis. 2011 Oct;32(10):1427-33. doi: 10.1093/carcin/bgr131. Epub 2011 Jul 18.

Abstract

The tumor suppressor programmed cell death 4 (Pdcd4) is lost in various tumor tissues. Loss of Pdcd4 has been associated with increased tumorigenic potential and tumor progression. While various mechanisms of Pdcd4 regulation have been described, the effect of an inflammatory tumor microenvironment on Pdcd4 protein expression has not been characterized so far. In the present study, we aimed to elucidate the molecular mechanisms of Pdcd4 protein regulation in tumor cells under inflammatory conditions. 12-O-tetradecanoylphorbol 13-acetate-induced differentiation of human U937 monocytes increased the expression and secretion of inflammatory cytokines such as tumor necrosis factor α, interleukin (IL)-6 and IL-8. Exposure to conditioned medium (CM) of these activated macrophages markedly decreased Pdcd4 protein expression in various tumor cells. Similarly, indirect coculture with such activated U937 monocyte-derived macrophages resulted in the loss of Pdcd4 protein in tumor cells. Decreased Pdcd4 protein levels were attributable to enhanced proteasomal degradation, diminishing Pdcd4 protein half-life. Proteasomal degradation required activation of phosphatidylinositol-3-kinase (PI3K)-mammalian target of rapamycin (mTOR) signaling. Since macrophage-CM sufficed to induce Pdcd4 degradation, Pdcd4 downregulation was determined to be an indirect unidirectional effect of the macrophages on the tumor cells. Pdcd4 protein expression was also attenuated in vivo in mouse colon tissue in response to dextran sodium sulfate-induced colitis. In summary, we characterized PI3K-mTOR-dependent proteasome-mediated Pdcd4 degradation in tumor cells in the inflammatory tumor microenvironment. Consequently, stabilization of Pdcd4 protein could provide a promising novel avenue for therapeutics targeting inflammation-associated tumors.

Publication types

  • Research Support, N.I.H., Intramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Apoptosis Regulatory Proteins / genetics
  • Apoptosis Regulatory Proteins / metabolism*
  • Blotting, Western
  • Breast Neoplasms / genetics
  • Breast Neoplasms / metabolism*
  • Breast Neoplasms / pathology*
  • Carcinogens / pharmacology
  • Cell Differentiation / drug effects
  • Culture Media, Conditioned / pharmacology
  • Female
  • Genes, Tumor Suppressor*
  • Humans
  • Inflammation / etiology*
  • Inflammation / metabolism
  • Inflammation / pathology
  • Interleukin-6 / metabolism
  • Interleukin-8 / metabolism
  • Macrophages / cytology
  • Macrophages / drug effects
  • Macrophages / metabolism
  • Mice
  • Mice, Inbred C57BL
  • Monocytes / cytology
  • Monocytes / drug effects
  • Monocytes / metabolism
  • Phosphatidylinositol 3-Kinases / genetics
  • Phosphatidylinositol 3-Kinases / metabolism
  • Phosphorylation
  • Proteasome Endopeptidase Complex / metabolism*
  • RNA, Messenger / genetics
  • RNA, Small Interfering / genetics
  • RNA-Binding Proteins / genetics
  • RNA-Binding Proteins / metabolism*
  • Reverse Transcriptase Polymerase Chain Reaction
  • Signal Transduction
  • TOR Serine-Threonine Kinases / genetics
  • TOR Serine-Threonine Kinases / metabolism
  • Tetradecanoylphorbol Acetate / pharmacology
  • Tumor Microenvironment
  • Tumor Necrosis Factor-alpha / metabolism
  • U937 Cells

Substances

  • Apoptosis Regulatory Proteins
  • Carcinogens
  • Culture Media, Conditioned
  • Interleukin-6
  • Interleukin-8
  • PDCD4 protein, human
  • Pdcd4 protein, mouse
  • RNA, Messenger
  • RNA, Small Interfering
  • RNA-Binding Proteins
  • Tumor Necrosis Factor-alpha
  • Phosphatidylinositol 3-Kinases
  • TOR Serine-Threonine Kinases
  • Proteasome Endopeptidase Complex
  • Tetradecanoylphorbol Acetate