Loss of function mutations in the NF1 tumor suppressor gene, which encodes the protein neurofibromin, leads to accelerated p21Ras activity and phosphorylation of multiple downstream kinases, including Erk and Akt.
Upon activation, AMPK phosphorylates and alters the activity of multiple downstream kinases and enzymes [ 8, 9].
Even though we did not focus our investigation on the effect of UCN-01 on PDK1 kinase activity, it is conceivable that via inhibition of PDK1, which occupies an apical position in regulating multiple downstream kinases, UCN-01 can simultaneously inhibit multiple parallel targets including PKC, MEK/ERK1/2 and Akt.
Since ASK1 is located upstream of the SEK1/MKK4-JNK/SAPK and MKK3/MKK6-p38 MKK3/MKK6-p38, we examined the activation of the multipathwaysstream protein kinases by Western blot using phospho-specific antibodies (JNK and p38).
Receptor tyrosine kinases (RTKs) activate multiple downstream cytosolic tyrosine kinases following ligand stimulation.
They play key roles in anchorage independence, survival and oxidative stress response by activating multiple downstream pathways including AKT and ERK kinases [ 15, 35].
For example, growth factors (such as NGF acting via the tyrosine kinase receptor TrkA, or GM-CSF acting via tyrosine kinase receptors and JAK-STAT signaling), recruit multiple downstream enzymes, including phospholipase C PLCC), phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK).
The resulting mutant protein, JAK2V617F, is a constitutively activated kinase that activates multiple downstream signaling pathways, such as the signal transducer and activator of transcription, extracellular signal-regulated kinases and Akt signaling pathways, and it transforms hematopoietic cells to cytokine-independent growth.
Src and the related Src family kinases signal through multiple downstream intermediaries including STAT3 (Yu et al, 1995), FAK, and β-catenin (Irby et al, 2005), and Src activation has been implicated in decreased tumour cell adhesion, increased invasiveness, and increased motility (Buettner et al, 2008).
Remarkably, Ras GTPases exert their functions through multiple downstream effectors, such as mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3′-kinase/protein kinase B (PI3K/AM), regulating various cellular processes such as cell proliferation, survival, and differentiation [7].
Binding of FGF and especially VEGF, which is considered a major molecule controlling blood vessel morphogenesis, to their tyrosine kinase receptors activates multiple downstream molecules involved in different signalling pathways that lead to increased vascular permeability, cell migration and proliferation [3].
