Sang-Oh Yoon

The evolutionarily conserved serine-threonine kinase mammalian target of rapamycin (mTOR) plays a critical role in regulating many pathophysiological processes. Functional characterization of the mTOR signaling pathways, however, has been hampered by the paucity of known substrates. We used large-scale quantitative phosphoproteomics experiments to define the signaling networks downstream of mTORC1 and mTORC2. Characterization of one mTORC1 substrate, the growth factor receptor-bound protein 10 (Grb10), showed that mTORC1-mediated phosphorylation stabilized Grb10, leading to feedback inhibition of the phosphatidylinositol 3-kinase (PI3K) and extracellular signal-regulated, mitogen-activated protein kinase (ERK-MAPK) pathways. Grb10 expression is frequently down-regulated in various cancers, and loss of Grb10 and loss of the well-established tumor suppressor phosphatase PTEN appear to be mutually exclusive events, suggesting that Grb10 might be a tumor suppressor regulated by mTORC1.

The p70 ribosomal protein S6 kinase 1 (S6K1) plays a key role in cell growth and proliferation by regulating insulin sensitivity, metabolism, protein synthesis, and cell cycle. Thus, deregulation of S6K contributes to the progression of type 2 diabetes, obesity, aging, and cancer. Considering the biological and clinical importance of S6K1, a complete understanding of its regulation is critical. One of the key motifs in the activation of S6K1 is a turn motif, but its regulation is not well understood. Here we provide evidence for two mechanisms of modulating turn motif phosphorylation and S6K1 activity. First, mammalian target of rapamycin regulates turn motif phosphorylation by inhibiting its dephosphorylation. Second, we unexpectedly found that glycogen synthase kinase (GSK)-3 promotes turn motif phosphorylation. Our studies show that mammalian target of rapamycin and GSK-3 cooperate to control the activity of S6K1, an important regulator of cell proliferation and growth. Our unexpected results provide a clear rationale for the development and use of drugs targeting GSK-3 to treat diseases such as diabetes, cancer, and age-related diseases that are linked to improper regulation of S6K1.

Metastatic cancer cells invade tissue, overcome nutrient stress, and survive transit to distant sites. Many of the mechanisms that support these processes are incompatible with proliferation. This study defines cellular transition states in breast epithelial cells undergoing epithelial-mesenchymal transition (EMT) driven by ERK2 and TGF-β signaling. EMT triggers robust endolysosomal system upregulation and metabolic adaptations that balance proliferative and invasive states. Surprisingly, invasive cells rely on scavenging via lysosomes and macropinocytosis to acquire amino acids, rather than plasma membrane transport, even in nutrient-rich conditions. Macropinocytosis increases intracellular amino acid storage, promoting survival during amino acid deprivation. This metabolic shift depends on c-MYC downregulation, an early EMT event. Reintroducing c-MYC suppresses the metabolic switch, endolysosomal induction, macropinocytosis, and the proliferation-to-migration transition. These findings reveal how cells dynamically balance proliferation and invasion, offering insights into transition states difficult to capture in models of breast cancer metastasis.