However, during tumorigenesis there is often constitutive activation of PTKs due to activating mutations or aberrant growth factor or cytokine signaling.
EGFR inhibitors, MEK inhibitors, and BRAF inhibitors can control the growth of cancer cells by abating extra pathway activity, so effects of these drugs partially depend on their ability to control aberrant activity of the pathway due to activating oncogene mutations.
In a family of HBM cases due to activating low-density lipoprotein receptor-related protein 5 (LRP5) gene mutations, which enhance osteoblast activity, radiographs have shown widened long bones and cortices [2].
Reporter activity was enhanced by exogenous SHh ligand (p<0.005) and inhibited by the ligand-neutralizing monoclonal antibody 5E1 (p<0.0001), suggesting that Hh signaling was ligand dependent and not likely due to activating mutations within pathway components.
This pathway is often dysregulated in human cancers, frequently due to activating mutations of the KRAS, NRAS, or BRAF genes.
Notably, the SF-1 mRNA up-regulation was neither due to NR5A1 gene amplification nor due to activating mutations in the promoter area.
Neonatal diabetes due to activating mutations in the ATP-sensitive potassium channel genes KCNJ11 and ABCC8 responds to oral sulfonylureas instead of insulin (1).
The reduction of the duration of immobility in the swim test may be due to activating catecholaminergic mechanism in the brain [ 55, 56].
This mechanism would also be expected to operate in tumour cells in which Akt is hyperactivated due to activating mutations in subunits of PI3K, or mutation or overexpression of upstream receptors [ 26].
Hyperactivation of Akt due to activating mutations, gene amplification or enhanced signaling from receptors seems to be genetically selected during tumorigenesis and was found in many human cancers including carcinomas, glioblastoma multiforme and various hematological malignancies (reviewed in [ 47]).
