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Modulation of the transport of a lysosomal enzyme by PDGF
E M Prence1, J M Dong, G G Sahagian
1Department of Physiology, School of Medicine, Tufts University, Boston, Massachusetts 02111.
Abstract:
The major excreted protein (MEP) of transformed mouse fibroblasts is the lysosomal protease, cathepsin L. MEP is also secreted by untransformed mouse cells in response to growth factors and tumor promoters, and is thought to play a role in cell growth and transformation. To determine the relationship between MEP synthesis and MEP secretion, we have examined these events in PDGF-treated NIH 3T3 cells. PDGF enhanced MEP synthesis and caused the diversion of MEP from the lysosomal delivery pathway to a secretory pathway. These two effects were found to be regulated independently at various times after growth factor addition. Short PDGF treatments (0.5 or 1 h) resulted in quantitative secretion of MEP although synthesis was near the control level. High levels of both synthesis and secretion occurred between 2 and 14 h of PDGF treatment. Between 18 and 30 h, the amount of secreted MEP returned to the low control level even though synthesis remained elevated. The secretion was specific for MEP; other lysosomal enzymes were not found in the media from PDGF-treated cells. PDGF-induced secretion of MEP was inhibited 84% by cycloheximide, suggesting that protein synthesis is required to elicit this effect. PDGF also caused a time-dependent increase in mannose 6-phosphate (Man-6-P) receptor-mediated endocytosis. These data support a model in which PDGF alters the distribution of Man-6-P receptors such that the Golgi concentration of receptors becomes limiting, thereby causing the selective secretion of the low affinity ligand, MEP.
Insights
Platelet-derived growth factor (PDGF) regulates the secretion of major excreted protein (MEP), a lysosomal protease. PDGF independently controls MEP synthesis and secretion, impacting cell growth and transformation.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- The major excreted protein (MEP) is a lysosomal protease, cathepsin L, secreted by transformed and untransformed mouse cells.
- MEP is implicated in cell growth, transformation, and secretion in response to growth factors and tumor promoters.
Purpose of the Study:
- To investigate the relationship between MEP synthesis and secretion in NIH 3T3 cells treated with platelet-derived growth factor (PDGF).
- To elucidate the regulatory mechanisms controlling MEP diversion from the lysosomal to the secretory pathway.
Main Methods:
- NIH 3T3 cells were treated with PDGF to analyze MEP synthesis and secretion over time.
- Investigated the effect of cycloheximide on PDGF-induced MEP secretion.
- Assessed changes in mannose 6-phosphate (Man-6-P) receptor-mediated endocytosis.
Main Results:
- PDGF treatment independently enhanced MEP synthesis and secretion, diverting it to a secretory pathway.
- MEP secretion occurred rapidly upon PDGF addition, preceding elevated synthesis; both peaked between 2-14 hours.
- PDGF-induced MEP secretion was protein synthesis-dependent and specifically affected MEP, not other lysosomal enzymes.
Conclusions:
- PDGF differentially regulates MEP synthesis and secretion, with independent temporal control.
- PDGF alters Man-6-P receptor distribution, leading to selective MEP secretion due to limited receptor availability.
- These findings provide insights into the role of MEP in cell growth and transformation.