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Glucose regulates protein catabolism in ras-transformed fibroblasts through a lysosomal-dependent proteolytic pathway
1UR 238, Unité de Nutrition Cellulaire et Moléculaire, Centre de Recherche en Nutrition Humaine, Institut National de la Recherche Agronomique, 63122 St Genès Champanelle, France.
Abstract:
Transformed cells are exposed to heterogeneous microenvironments, including low D-glucose (Glc) concentrations inside tumours. The regulation of protein turnover is commonly impaired in many types of transformed cells, but the role of Glc in this regulation is unknown. In the present study we demonstrate that Glc controls protein turnover in ras-transformed fibroblasts (KBALB). The regulation by Glc of protein breakdown was correlated with modifications in the levels of lysosomal cathepsins B, L and D, while autophagic sequestration and non-lysosomal proteolytic systems (m- and mu-calpains and the zeta-subunit of the proteasome) remained unaffected. Lactacystin, a selective inhibitor of the proteasome, depressed proteolysis, but did not prevent its regulation by Glc. The sole inhibition of the cysteine endopeptidases (cathepsins B and L, and calpains) by E-64d [(2S,3S)-trans-epoxysuccinyl-L-leucylamido-3-methylbutane ethyl ester] was also not sufficient to alter the effect of Glc on proteolysis. The Glc-dependent increase in proteolysis was, however, prevented after optimal inhibition of lysosomal cysteine and aspartic endopeptidases by methylamine. We conclude that, in transformed cells, Glc plays a critical role in the regulation of protein turnover and that the lysosomal proteolytic capacity is mainly responsible for the control of intracellular proteolysis by Glc.
Insights
Glucose (Glc) regulates protein breakdown in cancer cells. Lysosomal proteases, not proteasomes, are key to this Glc-dependent control of protein turnover in transformed cells.
Area of Science:
- Cell Biology
- Biochemistry
- Cancer Research
Background:
- Transformed cells encounter diverse microenvironments, including low glucose (Glc) levels within tumors.
- Impaired protein turnover is common in transformed cells, but the influence of Glc remains unclear.
Purpose of the Study:
- To investigate the role of glucose (Glc) in regulating protein turnover in ras-transformed fibroblasts (KBALB).
- To identify the specific proteolytic systems involved in Glc-mediated protein turnover control.
Main Methods:
- Assessing protein breakdown rates in KBALB cells under varying glucose concentrations.
- Analyzing the impact of specific protease inhibitors (Lactacystin, E-64d, methylamine) on proteolysis.
- Quantifying the levels of lysosomal cathepsins (B, L, D) and other proteases (calpains, proteasome subunits).
Main Results:
- Glucose (Glc) was found to significantly control protein turnover in ras-transformed fibroblasts.
- Glc-dependent regulation of protein breakdown correlated with changes in lysosomal cathepsins B, L, and D levels.
- Autophagic sequestration and non-lysosomal proteolytic systems were unaffected by glucose levels.
- Inhibition of lysosomal cysteine and aspartic endopeptidases by methylamine prevented the Glc-dependent increase in proteolysis.
Conclusions:
- Glucose (Glc) plays a critical role in regulating protein turnover in transformed cells.
- The lysosomal proteolytic system is primarily responsible for glucose-mediated control of intracellular proteolysis.