FoxO restricts growth and differentiation of cells with elevated TORC1 activity under nutrient restriction

Area of Science:

• Cellular Biology
• Cancer Research
• Developmental Biology

Background:

• TORC1 is a key regulator of cell growth and metabolism, often hyperactivated in cancers.
• Loss of tumor suppressors PTEN or Tsc1/2 leads to TORC1 hyperactivation.
• PTEN-deficient tumors are malignant and nutrient restriction-insensitive, unlike Tsc1/2-deficient ones.

Purpose of the Study:

• Investigate the differential impact of PTEN versus Tsc1/2 loss on tumor behavior, particularly under nutrient restriction.
• Elucidate the molecular mechanisms underlying the distinct malignant potential of PTEN and Tsc1/2 mutant tumors.
• Identify factors contributing to the reduced malignancy observed in Tsc1/2-deficient cancers.

Main Methods:

• Utilized Drosophila models to study epithelial tissue overgrowth.
• Manipulated gene expression, including Rheb and FoxO.
• Assessed TORC1 and PKB activity.
• Analyzed tissue hyperplasia, differentiation, and morphology under varying nutritional conditions.

Main Results:

• Tsc1/2-deficient cells exhibit enhanced growth under nutrient restriction, correlating with high TORC1 activity.
• Activating TORC1 downstream of Tsc1 (via Rheb overexpression) enhances tissue growth.
• Tsc1 mutant cells show reduced PKB activity and increased reliance on FoxO for growth restriction.
• FoxO removal in Tsc1 mutant tissue under nutrient restriction causes severe hyperplasia and defects, confirming FoxO's inhibitory role.

Conclusions:

• FoxO activation is crucial for restricting overgrowth in Tsc1 mutant tissues, especially under nutrient restriction.
• The differential activation status of FoxO likely explains why Tsc1-deficient tumors are less malignant than PTEN-deficient tumors.
• Understanding these pathways offers insights into cancer progression and therapeutic strategies targeting nutrient metabolism.