Micropeptide hSPAR regulates glutamine levels and suppresses mammary tumor growth via a TRIM21-P27KIP1-mTOR axis

Area of Science:

• Molecular Biology
• Oncology
• Cell Biology

Background:

• The mechanistic target of rapamycin (mTOR) pathway is crucial for cancer cell growth, responding to amino acid availability.
• CDK inhibitor P27KIP1 was recently identified as a noncanonical inhibitor of mTOR signaling, but its mechanism remained unclear.
• Understanding novel regulators of mTOR signaling is vital for cancer therapy development.

Purpose of the Study:

• To elucidate the mechanism by which P27KIP1 regulates mTOR signaling in breast cancer.
• To investigate the role of human micropeptide hSPAR in controlling P27KIP1 stability and cellular glutamine levels.
• To evaluate the therapeutic potential of hSPAR in preclinical breast cancer models.

Main Methods:

• Investigated the interaction between hSPAR and E3 ligase TRIM21 concerning P27KIP1 degradation.
• Assessed the effect of hSPAR on glutamine transporter SLC38A2 expression and intracellular glutamine levels.
• Utilized cell-based assays and xenograft models to study the impact of hSPAR on breast cancer cell proliferation and tumor growth.

Main Results:

• hSPAR inhibits P27KIP1 degradation by TRIM21, leading to cytoplasmic accumulation of P27KIP1 in breast cancer cells.
• hSPAR suppresses SLC38A2 expression, reducing cellular glutamine levels specifically in cancer cells.
• Glutamine deprivation induced by hSPAR causes P27KIP1 translocation to lysosomes, disrupting the Ragulator complex and inhibiting mTORC1 assembly.
• Administration of hSPAR or its C-terminus (hSPAR-C) significantly reduced breast cancer cell proliferation and tumor growth in vivo.

Conclusions:

• hSPAR acts as an intrinsic regulator of cellular glutamine homeostasis.
• hSPAR functions as a novel tumor suppressor by inhibiting mTORC1 assembly through P27KIP1.
• hSPAR and hSPAR-C demonstrate potential as therapeutic agents for breast cancer treatment.