The plasticity of oncogene addiction: implications for targeted therapies directed to receptor tyrosine kinases.

A common mutation of the epidermal growth factor receptor (EGFR) in glioblastoma multiforme (GBM) is an extracellular truncation known as the de2-7 EGFR (or EGFRvIII). Hepatocyte growth factor (HGF) is the ligand for the receptor tyrosine kinase (RTK) c-Met, and this signaling axis is often active in GBM. The expression of the HGF/c-Met axis or de2-7 EGFR independently enhances GBM growth and invasiveness, particularly through the phosphatidylinositol-3 kinase/pAkt pathway. Using RTK arrays, we show that expression of de2-7 EGFR in U87MG GBM cells leads to the coactivation of several RTKs, including platelet-derived growth factor receptor beta and c-Met. A neutralizing antibody to HGF (AMG102) did not inhibit de2-7 EGFR-mediated activation of c-Met, demonstrating that it is ligand-independent. Therapy for parental U87MG xenografts with AMG 102 resulted in significant inhibition of tumor growth, whereas U87MG.Delta 2-7 xenografts were profoundly resistant. Treatment of U87MG.Delta 2-7 xenografts with panitumumab, an anti-EGFR antibody, only partially inhibited tumor growth as xenografts rapidly reverted to the HGF/c-Met signaling pathway. Cotreatment with panitumumab and AMG 102 prevented this escape leading to significant tumor inhibition through an apoptotic mechanism, consistent with the induction of oncogenic shock. This observation provides a rationale for using panitumumab and AMG 102 in combination for the treatment of GBM patients. These results illustrate that GBM cells can rapidly change the RTK driving their oncogene addiction if the alternate RTK signals through the same downstream pathway. Consequently, inhibition of a dominant oncogene by targeted therapy can alter the hierarchy of RTKs resulting in rapid therapeutic resistance.

Cell cycle-dependent regulation of SFK, JAK1 and STAT3 signalling by the protein tyrosine phosphatase TCPTP.

Janus-activated kinases (JAKs) and Src family kinases (SFKs) and their common substrate signal transducer and activator of transcription (STAT)-3 are frequently hyperactivated in human cancer contributing to the proliferative drive by promoting G(1)/S and G(2)/M progression. Previous studies have established that the protein tyrosine phosphatase TCPTP can dephosphorylate and inactivate the SFK and JAK protein tyrosine kinases (PTKs) to attenuate cytokine signalling in vivo. In this study we determined whether TCPTP regulates SFK and JAK signalling during the cell cycle. We used primary mouse embryonic fibroblasts (MEFs) isolated from TCPTP(-/-) versus +/+ mice, immortalised TCPTP(-/-) MEFs versus those reconstituted with physiological levels of TCPTP and HeLa cells in which TCPTP protein levels had been suppressed by RNA interference, to establish TCPTP as a negative regulator of SFK, JAK1 and STAT3 signalling during the cell cycle. We found that the progression of TCPTP-deficient MEFs after the G(1) restriction point into S-phase was enhanced. We used RNA interference and pharmacological inhibitors to demonstrate that elevated SFK and downstream phosphatidylinositol 3-kinase signalling but not JAK1 or STAT3 signalling were required for the enhanced G(1)/S transition. These results identify TCPTP as a negative regulator of the cell cycle.

DNA replication stalling attenuates tyrosine kinase signaling to suppress S phase progression.

Here we report that T cell protein tyrosine phosphatase (TCPTP)-dependent and -independent pathways attenuate the JAK and Src protein tyrosine kinases (PTKs) and STAT3 phosphorylation to suppress cyclin D1 expression and S phase progression in response to DNA replication stress. Cells that lack TCPTP fail to suppress JAK1, Src, and STAT3, allowing for sustained cyclin D1 levels and progression through S phase despite continued replication stress. Cells that bypass the checkpoint undergo aberrant mitoses with lagging chromosomes that stain for the DNA damage marker gamma H2AX. Therefore, inactivating JAK, Src, and STAT3 signaling pathways in response to DNA replication stress may be essential for the suppression of S phase progression and the maintenance of genomic stability.

Outer membrane protein 25-a mitochondrial anchor and inhibitor of stress-activated protein kinase-3.

Stress-activated protein kinase-3 (SAPK3) is unique amongst the mitogen-activated protein kinase (MAPK) family with its C-terminal 5 amino acids directing interaction with the PDZ domain-containing substrates alpha1-Syntrophin and SAP90/PSD95. Here, we identify three additional PDZ domain-containing binding partners, Lin-7C, Scribble, and outer membrane protein 25 (OMP25). This latter protein is localised together with SAPK3 at the mitochondria but it is not a SAPK3 substrate. Instead, OMP25 inhibits SAPK3 activity towards PDZ domain-containing substrates such as alpha1-Syntrophin and substrates without PDZ domains such as the mitochondrial protein Sab. This is a new mechanism for the regulation of SAPK3 and suggests that its intracellular activity should not be solely assessed by its phosphorylation status.

Counting on mitogen-activated protein kinases--ERKs 3, 4, 5, 6, 7 and 8.

Signal transduction pathways in eukaryotic cells integrate diverse extracellular signals, and regulate complex biological responses such as growth, differentiation and death. One group of proline-directed Ser/Thr protein kinases, the mitogen-activated protein kinases (MAPKs), plays a central role in these signalling pathways. Much attention has focused in recent years on three subfamilies of MAPKs, the extracellular signal regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs) and the p38 MAPKs. However, the ERK family is broader than the ERK1 and ERK2 proteins that have been the subject of most studies in this area. Here we overview the work on ERKs 3 to 8, emphasising where possible their biological activities as well as distinctive biochemical properties. It is clear from these studies that these additional ERKs show similarities to ERK1 and ERK2, but with some interesting differences that challenge the paradigm of the archetypical ERK1/2 MAPK pathway.

Phosphorylation of the mitochondrial protein Sab by stress-activated protein kinase 3.

Mitogen-activated protein kinases (MAPKs) transduce extracellular signals into responses such as growth, differentiation, and death through their phosphorylation of specific substrate proteins. Early studies showed the consensus sequence (Pro/X)-X-(Ser/Thr)-Pro to be phosphorylated by MAPKs. Docking domains such as the "kinase interaction motif" (KIM) also appear to be crucial for efficient substrate phosphorylation. Here, we show that stress-activated protein kinase-3 (SAPK3), a p38 MAPK subfamily member, localizes to the mitochondria. Activated SAPK3 phosphorylates the mitochondrial protein Sab, an in vitro substrate of c-Jun N-terminal kinase (JNK). Sab phosphorylation by SAPK3 was dependent on the most N-terminal KIM (KIM1) of Sab and occurred primarily on Ser321. This appeared to be dependent on the position of Ser321 within Sab and the sequence immediately surrounding it. Our results suggest that SAPK3 and JNK may share a common target at the mitochondria and provide new insights into the substrate recognition by SAPK3.

Activation of signal transducer and activator of transcription (STAT) pathways in failing human hearts.

OBJECTIVES: The signal transduction pathways mediating the progression to failure have been intensively studied in a variety of in vitro and in vivo animal models. Recently, acute activation of the Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) has been observed in the heart, but whether this is sustained in ischemic heart disease (IHD) or dilated cardiomyopathy (DCM) has not been previously addressed. METHODS: We assessed the tyrosine phosphorylation of STAT1, 3, 5 and 6 in ventricular samples of explanted human hearts with IHD (n=11) and DCM (n=9) as an indication of STAT activation. Samples from normal donor hearts (n=9) acted as controls. In parallel, we also assessed protein expression and phosphorylation of three major families of mitogen-activated protein kinases (MAPKs); ERK, p38 MAPK and c-Jun NH(2)-terminal kinase (JNK). RESULTS: All STAT isoforms were significantly phosphorylated in DCM. In contrast, only the phosphorylation of STATs 1 and 5 were significantly enhanced in IHD. Expression of total STAT protein remained unchanged. For the MAPKs, significant phosphorylation of p38(MAPK) was only observed in IHD. In contrast, there was no change in ERK or JNK activation despite abundant protein expression. CONCLUSIONS: We have shown that different members of the STAT transcription factor family are chronically phosphorylated in the failing heart as a result of IHD (STAT1 and 5) or DCM (STAT1, 3, 5 and 6). In contrast, IHD but not DCM showed significant p38(MAPK) phosphorylation. Whilst the differences noted between IHD and DCM may reflect different initiating events, the common activation of STATs 1 and 5 suggests that these transcription factors may play a common role regulating the progression of heart failure.

Cardiac expression and subcellular localization of the p38 mitogen-activated protein kinase member, stress-activated protein kinase-3 (SAPK3).

Despite the interest in the roles that mitogen-activated protein kinases (MAPKs) play in the heart, the role of the different MAPK isoforms has been relatively poorly defined. A third isoform of p38 MAPK, known variously as stress-activated protein kinase-3 (SAPK3), p38- gamma or ERK6, has been previously shown to differ from p38- alpha/ beta both in its molecular weight and its lack of inhibition by the compound SB203580. We have generated monoclonal antibodies with specificity for SAPK3 demonstrated by immunoblot analysis, immunofluorescence studies, and cloning of SAPK3 from a rat heart cDNA expression library. By immunoblotting, we confirmed high expression of SAPK3 in fast, slow and mixed fibre types of murine skeletal muscle and observed significant expression restricted to heart, lung, thymus and testes. In addition to expression in normal heart (human, mouse, rat, dog and pig), we observed constant expression in diseased human heart, as well as control and hypertrophic cultured neonatal rat cardiac myocytes. Immunolocalization in cultured cardiac myocytes followed by confocal microscopy showed punctate, non-nuclear SAPK3 staining. In contrast, p38- alpha/ beta staining was non-punctate and distributed throughout the cytosol and nucleus. Whereas treatment with Leptomycin B to prevent nuclear export processes promoted higher levels of p38- alpha/ beta staining in cardiac myocyte nuclei, there was no apparent change in SAPK3 localization under these conditions. These differences between p38- alpha/ beta and SAPK3 probably reflect the specialized functions of SAPK3 and emphasize the need to evaluate SAPK3 upstream activators and downstream targets in the heart.