Major Intracellular Signaling Cascades (Cross-Cutting)

Extracellular signals—hormones, growth factors, cytokines, or neurotransmitters—are converted by receptors into a limited set of conserved intracellular cascades. These pathways amplify, integrate, and distribute information to regulate gene expression, metabolism, survival, and proliferation (Pawson & Scott, 2005).

1. MAPK/ERK cascade. The Mitogen-Activated Protein Kinase (MAPK) cascade, specifically the Extracellular signal-Regulated Kinase (ERK) branch, is a three-tiered kinase module (Raf → MEK → ERK) typically activated by Receptor Tyrosine Kinases (RTKs) and G protein-coupled receptors (GPCRs) via the small GTPase Ras. ERK controls proliferation and differentiation (Kolch, 2005). Related MAPKs, JNK (c-Jun N-terminal Kinase) and p38, respond to stress (Kyriakis & Avruch, 2012).

2. PI3K/Akt/mTOR pathway. Phosphoinositide 3-kinase (PI3K) generates the lipid second messenger PIP₃ (Phosphatidylinositol (3,4,5)-trisphosphate), recruiting Akt (also known as Protein Kinase B, PKB). Akt promotes survival, growth, and metabolism. The tumor suppressor PTEN (Phosphatase and Tensin homolog) terminates signaling. Hyperactivation of this pathway is common in cancer (Manning & Toker, 2017).

3. cAMP/PKA pathway. GPCRs coupled to the stimulatory G protein (Gs) activate adenylyl cyclase, producing cyclic AMP (cAMP). cAMP activates Protein Kinase A (PKA), which phosphorylates targets such as the transcription factor CREB (cAMP response element-binding protein), ion channels, and metabolic enzymes, thereby regulating metabolism, memory, and cardiac function (Taylor et al., 2012).

4. IP₃/DAG/PKC cascade. GPCRs coupled to Gq activate phospholipase C-β (PLCβ), generating two second messengers: inositol trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ triggers Ca²⁺ release from intracellular stores, while DAG activates Protein Kinase C (PKC). Ca²⁺ also activates Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and calcineurin (Berridge, 2016).

5. JAK/STAT pathway. Cytokine receptors activate Janus kinases (JAKs), which phosphorylate Signal Transducers and Activators of Transcription (STATs). STAT dimers enter the nucleus to regulate immunity and hematopoiesis (Stark & Darnell, 2012).

6. NF-κB pathway. In resting cells, IκB (Inhibitor of κB) sequesters Nuclear Factor kappa-light-chain-enhancer of activated B cells (NF-κB). The IκB kinase (IKK) complex phosphorylates IκB, targeting it for degradation, freeing NF-κB to induce inflammatory and anti-apoptotic genes (Hayden & Ghosh, 2012).

7. TGF-β/Smad pathway. Transforming Growth Factor-beta (TGF-β) receptors phosphorylate receptor-regulated Smads (R-Smads: Smad2/3 or Smad1/5/8). These R-Smads complex with the common mediator Smad4 and regulate cell cycle arrest and differentiation (Massagué, 2012). Bone Morphogenetic Proteins (BMPs) signal through a similar Smad1/5/8 mechanism.

8. Wnt/β-catenin pathway. Without Wnt (Wingless/Integrated) ligand, β-catenin is degraded. Wnt binding to Frizzled receptors inhibits the destruction complex, allowing β-catenin to accumulate, enter the nucleus, and activate TCF/LEF (T-cell factor/Lymphoid enhancer-binding factor) target genes (Nusse & Clevers, 2017).

These cascades exhibit extensive cross-talk through shared effectors, second messengers, scaffold proteins, and convergent transcription, enabling context-dependent cellular decisions (Jordan et al., 2000).

References

Berridge, M. J. (2016). The inositol trisphosphate/calcium signaling pathway. Physiological Reviews, 96(4), 1261–1296.

Hayden, M. S., & Ghosh, S. (2012). NF-κB: a quarter-century of progress. Genes & Development, 26(3), 203–234.

Jordan, J. D., Landau, E. M., & Iyengar, R. (2000). Signaling networks. Cell, 103(2), 193–200.

Kolch, W. (2005). Coordinating ERK/MAPK signalling. Nature Reviews Molecular Cell Biology, 6(11), 827–837.

Kyriakis, J. M., & Avruch, J. (2012). Mammalian MAPK pathways. Physiological Reviews, 92(2), 689–737.

Manning, B. D., & Toker, A. (2017). AKT/PKB signaling. Cell, 169(3), 381–405.

Massagué, J. (2012). TGFβ signalling in context. Nature Reviews Molecular Cell Biology, 13(10), 616–630.

Nusse, R., & Clevers, H. (2017). Wnt/β-catenin signaling. Cell, 169(6), 985–999.

Pawson, T., & Scott, J. D. (2005). Protein phosphorylation in signaling. Trends in Biochemical Sciences, 30(6), 286–290.

Stark, G. R., & Darnell, J. E. (2012). The JAK-STAT pathway at twenty. Immunity, 36(4), 503–514.

Taylor, S. S., et al. (2012). Assembly of allosteric switches: lessons from PKA. Nature Reviews Molecular Cell Biology, 13(10), 646–658.