After a couple of weeks in cell culture and RNAseq land, finally some time for a bit of science reading! However, not much time for writing it all up as I usually do, so here is detailed coverage of two of my recent reads. Enjoy!
Due to some unexpected results, I have had to search the stem cell literature for information on adaptive changes due to selection pressure. Came across one important paper from 2009 in Nature Biotechnology by Werbowetski-Ogilvie et al.: Characterization of human embryonic stem cells with features of neoplastic progression ➡ based on some of these points, it is really important to characterise stem cell models thoroughly, especially if you study a cancer pathway!!
- Variant pluripotent stem cells with neoplastic changes may arise in culture and be mistaken for “better” stem cells because of higher expression of stem cell markers.
- Paper demonstrates that such variant stem cells (2 clones studied) have acquired expression of FGFR1 and IGF1R and have become refractory to differentiation in vitro. In vivo, such cells exhibited a better capacity for teratoma formation, but were not deemed to be malignant because they didn’t metastasise (but mice kept for 6 weeks, so perhaps this could happen eventually?).
Nature just published a very interesting review “Nutrient acquisition strategies of mammalian cells” by Wilhelm Palm and Craigh Thompson. A few key points that I have got out of it:
- Metabolic flexibility in cells achieved by using different nutrient sources: prepared for periods of starvation.
- Proliferating cells: growth factor-induced increases in nutrient uptake required to support biomass accumulation. Proliferating cells need to reorganise metabolism so that nutrients are not oxidised fully to CO2 and ATP, but used in anabolic processes.
- Bulk of ATP production in mammalian cells + generation of non-essential metabolites: glucose, fatty acids, glutamine
- Cancer cells have changes that enable them to survive and grow in poorly vascularised environments by using extracellular nutrients, including low-molecular-mass nutrients and macromolecules as well as cellular debris.
- Class IA PI3K and RAS signalling pathways are central regulators of cellular nutrient acquisition. Activated by various growth factor signals. Such cell-extrinsic regulation of nutrient uptake constitutes a fundamental barrier for cellular transformation. Oncogenic changes that result in hyperactivation of Ras/PI3K signalling ➡ cellular autonomy for both cell cycle entry and nutrient acquisition.
- mTORC1 as a central coordinator of amino acid availability and cell growth. Interplay with AMPK is central to cellular metabolic homeostasis in a variable nutrient environment. Because mTORC1 is also modulated downstream of class IA PI3K, the cell is able to integrate metabolic status and growth factor signals. Activation of AKT/mTORC1 signalling defines a cellular state of high rates of glucose and amino acid uptake through transporters with concomitant suppression of lysosomal catabolism of macromolecules.
- PI3K and Ras pathways increase glucose uptake and glycolysis. Myc increases expression of amino acid transporters. Together, Myc, Ras and PI3K stimulate ribosomal biogenesis and mTORC1-dependent translational initiation.
- Expression of various cell surface receptors that mediate the uptake of certain nutrients (e.g. iron or cholesterol) is ultimately modulated by the very same pathways listed above.
- Macropinocytosis: non-selective endocytic pathway for bulk ingestion of extracellular solutes. Driven by actin-based protrusions. Actin-driven protrusions are immediate downstream responses of growth factor stimulation. Accordingly cells with oncogenic mutation in Ras pathways exhibit high levels of macropinocytosis. It remains to be established if the same is true for cells with oncogenic PI3K mutations.
- Epithelial cells can internalise whole neighboring cells via a non-phagocytic process ➡ process known as entosis ➡ digestion of the internalised cell. This behaviour is enhanced by oncogenic mutations in Ras.
- Autophagy is also important for tumour survival.
- Glutamine is the second most-consumed nutrient in proliferating cells ➡ major carbon source for anaplerotic reactions that replenish TCA cycle intermediates. Glutamine also provides nitrogen for synthesis of amino acids, nucleotides and hexosamines. In vivo, tumours also replenish the TCA cycle from glucose-derived pyruvate. Some cancers also express high levels of branched-chain amino acid transferases ➡ enables them to use leucine, isoleucine and valine as nitrogen sources. These are example metabolic adaptations that give cancer cells a survival advantage.
- Evidence for paracrine signalling based on observations that non-transformed cells often undergo phenotypic changes in the tumour microenvironment. Mechanisms exist that allow tumour cells to acquire their nutrients from neighbouring cells. Tumours can also signal to distant organs.
- More research required to identify how signalling pathways regulate cellular nutrient uptake and what factors determine differences in nutrient usage.
Other bits and pieces to check out:
In Science this week: mTORC1 activity repression by late endosomal phosphatidylinositol 3,4-bisphosphate (Marat et al. http://science.sciencemag.org/content/356/6341/968)
Identification and characterization of a supraclavicular brown adipose tissue in mice (JCI Insight): https://insight.jci.org/articles/view/93166
Feng Zhang and his crew with another CRISPR tool paper in Nature: Engineered Cpf1 variants with altered PAM specificities; https://www.nature.com/nbt/journal/vaop/ncurrent/full/nbt.3900.html
Hepatic Diacylglycerol-Associated Protein Kinase Cε Translocation Links Hepatic Steatosis to Hepatic Insulin Resistance in Humans (Horst et al., Cell Reports): http://www.sciencedirect.com/science/article/pii/S2211124717306770
Phosphorylation of TXNIP by AKT Mediates Acute Influx of Glucose in Response to Insulin (Waldhart et al., Cell Reports): http://www.sciencedirect.com/science/article/pii/S2211124717306836
MYO6 Regulates Spatial Organization of Signaling Endosomes Driving AKT Activation and Actin Dynamics (Masters et al. Cell Reports): http://www.sciencedirect.com/science/article/pii/S2211124717307052
Interesting paracrine signalling mechanism: Intercellular transmission of the unfolded protein response promotes survival and drug resistance in cancer cells (Rodvold et al. Science Signaling) http://stke.sciencemag.org/content/10/482/eaah7177
A Nature Reviews Endocrinology review on why people should study mice at thermoneutrality: https://www.sparrho.com/item/warming-the-mouse-to-model-human-diseases/1385ea1/
An Expanded Genome-Wide Association Study of Type 2 Diabetes in Europeans (Scott et al. Diabetes) http://diabetes.diabetesjournals.org/content/early/2017/05/25/db16-1253