Protein-tyrosine kinases (PTKs) are important regulators of intracellular signal-transduction pathways mediating development and multicellular communication in metazoans. Their activity is normally tightly controlled and regulated. Perturbation of PTK signalling by mutations and other genetic alterations results in deregulated kinase activity and malignant transformation. The lipid kinase phosphoinositide 3-OH kinase (PI(3)K) and some of its downstream targets, such as the protein-serine/threonine kinases Akt and p70 S6 kinase (p70S6K), are crucial effectors in oncogenic PTK signalling. This review emphasizes how oncogenic conversion of protein kinases results from perturbation of the normal autoinhibitory constraints on kinase activity and provides an update on our knowledge about the role of deregulated PI(3)K/Akt and mammalian target of rapamycin/p70S6K signalling in human malignancies.

Upregulated epidermal growth factor (EGF) receptor (EGFR) expression and EGFR-induced signaling havebeen correlated with progression to invasion and metastasis in a wide variety of carcinomas, but the mechanism behind this is not well understood. We show here that, in various human carcinoma cells that overexpress EGFR, EGF treatment induced rapid tyrosine dephosphorylation of focal adhesion kinase (FAK) associated with downregulation of its kinase activity. The downregulation of FAK activity was both required and sufficient for EGF-induced refractile morphological changes, detachment of cells from the extracellular matrix, and increased tumor cell motility, invasion, and metastasis. Tumor cells with downregulated FAK activity became less adherent to the extracellular matrix. However, once cells started reattaching, FAK activity was restored by activated integrin signaling. Moreover, this process of readhesion and spreading could not be abrogated by further EGF stimulation. Interruption of transforming growth factor alpha-EGFR autocrine regulation with an EGFR tyrosine kinase inhibitor led to a substantial increase in FAK tyrosine phosphorylation and inhibition of tumor cell invasion in vitro. Consistent with this, FAK tyrosine phosphorylation was reduced in cells from tumors growing in transplanted, athymic, nude mice, which have an intact autocrine regulation of the EGFR. We suggest that the dynamic regulation of FAK activity, initiated by EGF-induced downregulation of FAK leading to cell detachment and increased motility and invasion, followed by integrin-dependent reactivation during readhesion, plays a role in EGF-associated tumor invasion and metastasis.The invasive and metastatic stage of cancer progression correlates with poor clinical prognosis and represents the most formidable barrier to successful treatment. Cell motility and invasiveness are defining characteristics of tumors, which enable tumor cells to migrate into adjacent tissues or through limiting basement membranes and extracellular matrices. Invasive tumor cells are characterized by dysregulated cell motility in response to extracellular signals from growth factors and cytokines. In addition to roles in organ morphogenesis, maintenance, and repair, epidermal growth factor (EGF)-induced signaling has often been associated with tumor invasion and metastasis (75). EGF receptor (EGFR) overexpression has been found in many human tumors, including lung, colon, breast, prostate, brain, head and neck, thyroid, ovarian, and bladder, gliomas, and renal carcinoma (4,20,39,40,63,71), and has been correlated with an advanced tumor stage and a poor clinical prognosis. In addition, EGFR overexpression in tumor cells is often accompanied by production of transforming growth factor alpha (TGF-␣) or other EGF family ligands (73), and autocrine regulation through EGFR by such ligands has also been implicated in tumor progression. It has been reported that EGF promotes tumor cell motility and invasion (58,62,66). However, the basis for initiation and maintenanc...

The c-kit-encoded receptor protein tyrosine kinase for stem cell factor (Kit/SCF-R) is essential for the development of cells within the hematopoietic, melanogenic and gametogenic lineages [1]. SCF stimulation induces activation of phosphatidylinositol (PI) 3-kinase, which is required for SCF-induced mitogenesis and cell survival [2-4], and for activation of the serine/threonine protein kinase Akt [5-7]. Using Kit/SCF-R mutants, we found that, in response to SCF, Akt became activated and mediated phosphorylation of Bad, a pro-apoptotic molecule, in a PI-3-kinasedependent manner. Phosphorylation of Bad was restricted to Ser112 and Ser136 in vivo, but only the Akt phosphorylation site Ser136 was essential for SCFpromoted cell survival. Furthermore, Bad and Akt interacted and colocalized in intact cells. A Kit/SCF-R gain-of-function mutant that has increased mitogenic and PI 3-kinase activation potential, due to the absence of the two protein kinase C negative feedback phosphorylation sites [8,9], enhanced both Akt activation and Bad phosphorylation and also resulted in increased cell survival. Such a mechanism may account for how deregulated PI 3-kinase activity and naturally occurring gain-of-function point mutants of Kit/SCF-R lead to cellular transformation and fatal malignancies in man [10-12].

The c-kit-encoded transmembrane tyrosine kinase receptor for stem cell factor (Kit/SCF-R) is required for normal haematopoiesis, melanogenesis and gametogenesis. However, the roles of individual Kit/SCF-R-induced signalling pathways in the control of developmental processes in the intact animal are completely unknown. To examine the function of SCF-induced phosphatidylinositol (PI) 3'-kinase activation in vivo, we employed the Cre-loxP system to mutate the codon for Tyr719, the PI 3'-kinase binding site in Kit/SCF-R, to Phe in the genome of mice by homologous recombination. Homozygous (Y719F/Y719F) mutant mice are viable. The mutation completely disrupted PI 3'-kinase binding to Kit/SCF-R and reduced SCF-induced PI 3'-kinase-dependent activation of Akt by 90%. The mutation induced a gender- and tissue-specific defect. Although there are no haematopoietic or pigmentation defects in homozygous mutant mice, males are sterile due to a block in spermatogenesis, with initially decreased proliferation and subsequent extensive apoptosis occurring at the spermatogonial stem-cell level. In contrast, female homozygotes are fully fertile. This is the first report so far demonstrating the role of an individual signalling pathway downstream of Kit/SCF-R in the intact animal. It provides the first in vivo model for male sterility caused by a discrete signalling pathway defect affecting early germ cells.

The proto‐oncogene c‐kit is allelic with the murine white spotting (W) locus and encodes a transmembrane protein tyrosine kinase that is structurally related to the receptors for platelet‐derived growth factor (PDGF) and colony‐stimulating factor‐1 (CSF‐1). Recently the ligand for the c‐kit product, stem cell factor (SCF), was identified in both transmembrane and soluble forms. In order to examine the mechanism for receptor activation by SCF and biological properties of the activated c‐kit product, we transfected the wild‐type human c‐kit cDNA into porcine aortic endothelial cells. We found that the receptor was down‐regulated and transmitted a mitogenic signal in response to stimulation with soluble SCF. We also demonstrate that SCF induces dimerization of the c‐kit product in intact cells, and that dimerization of the receptor is correlated with activation of its kinase. Activation of the c‐kit product by SCF was found to induce circular actin reorganization indistinguishable from that mediated by the PDGF beta‐receptor in response to PDGF‐BB. Furthermore, soluble SCF was a potent chemotactic agent for cells expressing the c‐kit product, a property which might be of importance during embryonic development.