Entry into the cell cycle occurs only when sufficient growth has occurred. In budding yeast, the cyclin Cln3 is thought to initiate cell cycle entry by inactivating a transcriptional repressor called Whi5. Growth-dependent changes in the concentrations of Cln3 or Whi5 have been proposed to link cell cycle entry to cell growth. However, there are conflicting reports regarding the behavior and roles of Cln3 and Whi5. Here, we found no evidence that changes in the concentration of Whi5 play a major role in controlling cell cycle entry. Rather, the data suggest that cell growth triggers cell cycle entry by driving an increase in the concentration of Cln3. We further found that accumulation of Cln3 is dependent upon homologs of mammalian SGK kinases that control cell growth and size. Together, the data are consistent with models in which Cln3 is a crucial link between cell growth and the cell cycle.

Entry into the cell cycle occurs only when sufficient growth has occurred. In budding yeast, the cyclin Cln3 initiates cell cycle entry in late G1 phase by inactivating Whi5, a repressor that blocks transcription of genes that drive cell cycle entry. Growth-dependent changes in the concentrations of Cln3 and/or Whi5 have been proposed to link cell cycle entry to cell growth. However, there are conflicting reports regarding the behavior and roles of Cln3 and Whi5 during G1 phase, and little is known about the molecular mechanisms that link changes in their concentrations to cell growth. Here, we analyzed levels of Cln3 and Whi5 as a function of growth in G1 phase. We found no evidence that changes in the concentration of Whi5 play a major role in controlling cell cycle entry. Rather, the data are consistent with models in which a growth-dependent increase in the concentration of Cln3 drives cell cycle entry. We further found that Cln3 levels in G1 phase are strongly modulated by nutrient availability and by homologs of mammalian SGK kinases that are required for normal control of cell growth and size.

Severe defects in cell size are a nearly universal feature of cancer cells. However, the underlying causes are unknown. A previous study suggested that a hyperactive mutant of yeast Ras (ras2G19V) that is analogous to the human Ras oncogene causes cell size defects, which could provide clues to how oncogenes influence cell size. However, the mechanisms by whichras2G19Vinfluences cell size are unknown. Here, we found thatras2G19Vinhibits a critical step in cell cycle entry, in which an early G1 phase cyclin induces transcription of late G1 phase cyclins. Thus,ras2G19Vdrives overexpression of the early G1 phase cyclin Cln3, yet Cln3 fails to induce normal transcription of late G1 phase cyclins, leading to delayed cell cycle entry and increased cell size.ras2G19Vinfluences transcription of late G1 cyclins via a poorly understood step in which Cln3 inactivates the Whi5 transcriptional repressor. Previous studies found that Ras relays signals via protein kinase A (PKA) in yeast; however,ras2G19Vappears to influence G1 phase cyclin expression via novel PKA-independent signaling mechanisms. Together, the data define new mechanisms by which hyperactive Ras influences cell cycle entry and cell size in yeast. Expression of G1 phase cyclins is also strongly influenced by mammalian Ras via mechanisms that remain unclear. Therefore, further analysis of PKA-independent Ras signaling in yeast could lead to discovery of conserved mechanisms by which Ras family members control expression of G1 phase cyclins.

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Severe defects in cell growth and size are a nearly universal feature of cancer cells and have provided a basis for cancer pathology for over 100 years; however, the underlying mechanisms remain largely unknown. Recent work in budding yeast has shown that a highly conserved signaling network surrounding Tor complex 2 (TORC2) controls cell growth and size. A key component of the network is the yeast homolog of serum and glucocorticoid-regulated kinase (SGK), which controls production of ceramide signaling lipids that control cell growth and size. Here, we present evidence that a similar TORC2 network controls cell growth and size in mammalian cells. In addition, we show that expression of oncogenic drivers in NIH3T3 cells causes defects in cell size, as well as defects in TORC2-dependent control of cell size. Cancer cells also show severe defects in TORC2-dependent control of cell size. Our working hypothesis is that the TORC2 network is downstream of multiple oncogenes, and that aberrant TORC2 signaling is a major cause of the nearly universal size defects observed in cancer cells. A better understanding of the mechanisms that control cell growth and size in normal cells, and how they go awry in cancer, could identify new targets for drugs to improve cancer therapies. Citation Format: Jerry T. DeWitt, Douglas Kellogg. TORC2 dependent control of cell growth and size in cancer cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1432.