Activated c-Abl tyrosine kinase in malignant solid tumors

Evidence for a direct involvement of hMSH5 in promoting ionizing radiation induced apoptosis

Although increasing evidence has suggested that the hMSH5 protein plays an important role in meiotic and mitotic DNA recombinational repair, its precise functions in recombination and DNA damage response are presently elusive. Here we show that the interaction between hMSH5 and c-Abl confers ionizing radiation (IR)-induced apoptotic response by promoting c-Abl activation and p73 accumulation, and these effects are greatly enhanced in cells expressing hMSH5(P29S) (i.e. the hMSH5 variant possessing a proline to serine change within the N-terminal (Px)(5) dipeptide repeat). Our current study provides the first evidence that the (Px)(5) dipeptide repeat plays an important role in modulating the interaction between hMSH5 and c-Abl and alteration of this dipeptide repeat in hMSH5(P29S) leads to increased IR sensitivity owing to enhanced caspase-3-mediated apoptosis. In addition, RNAi-mediated hMSH5 silencing leads to the reduction of apoptosis in IR-treated cells. In short, this study implicates a role for hMSH5 in DNA damage response involving c-Abl and p73, and suggests that mutations impairing this process could significantly affect normal cellular responses to anti-cancer treatments.

Saracatinib Impairs Head and Neck Squamous Cell Carcinoma Invasion by Disrupting Invadopodia Function

Elevated Src kinase activity is linked to the progression of solid tumors, including head and neck squamous cell carcinoma (HNSCC). Src regulates HNSCC proliferation and tumor invasion, with the Src-targeted small molecule inhibitor saracatinib displaying potent anti-invasive effects in preclinical studies. However, the pro-invasive cellular mechanism(s) perturbed by saracatinib are unclear. The anti-proliferative and anti-invasive effects of saracatinib on HNSCC cell lines were therefore investigated in pre-clinical cell and mouse model systems. Saracatinib treatment inhibited growth, cell cycle progression and transwell Matrigel invasion in HNSCC cell lines. Dose-dependent decreases in Src activation and phosphorylation of the invasion-associated substrates focal adhesion kinase, p130 CAS and cortactin were also observed. While saracatinib did not significantly impact HNSCC tumor growth in a mouse orthotopic model of tongue squamous cell carcinoma, impaired perineural invasion and cervical lymph node metastasis was observed. Accordingly, saracatinib treatment displayed a dose-dependent inhibitory effect on invadopodia formation, extracellular matrix degradation and matrix metalloprotease 9 activation. These results suggest that inhibition of Src kinase by saracatinib impairs the pro-invasive activity of HNSCC by inhibiting Src substrate phosphorylation important for invadopodia formation and associated matrix metalloprotease activity.

Abl kinases regulate autophagy by promoting the trafficking and function of lysosomal components

Autophagy is a lysosome-dependent degradative pathway that regulates the turnover of intracellular organelles, parasites, and long-lived proteins. Deregulation of autophagy results in a variety of pathological conditions, but little is known regarding the mechanisms that link normal cellular and pathological signals to the regulation of distinct stages in the autophagy pathway. Here we uncover a novel role for the Abl family kinases in the regulation of the late stages of autophagy. Inhibition, depletion, or knockout of the Abl family kinases, Abl and Arg, resulted in a dramatic reduction in the intracellular activities of the lysosomal glycosidases alpha-galactosidase, alpha-mannosidase and neuraminidase. Inhibition of Abl kinases also reduced the processing of the precursor forms of cathepsin D and cathepsin L to their mature, lysosomal forms, which coincided with the impaired turnover of long-lived cytosolic proteins and accumulation of autophagosomes. Furthermore, defective lysosomal degradation of long-lived proteins in the absence of Abl kinase signaling was accompanied by a perinuclear redistribution of lysosomes and increased glycosylation and stability of lysosome-associated membrane proteins, which are known to be substrates for lysosomal enzymes and play a role in regulating lysosome mobility. Our findings reveal a role for Abl kinases in the regulation of late-stage autophagy and have important implications for therapies that employ pharmacological inhibitors of the Abl kinases.

Proteomic analysis of breast cancer molecular subtypes and biomarkers of response to targeted kinase inhibitors using reverse-phase protein microarrays

Although breast cancer molecular subtypes have been extensively defined by means of gene expression profiling over the past decade, little is known, at the proteomic level, as to how signaling pathways are differentially activated and serve to control proliferation in different breast cancer subtypes. We used reverse-phase protein arrays to examine phosphorylation status of 100 proteins in a panel of 30 breast cancer cell lines and showed distinct pathway activation differences between different subtypes that are not obvious from previous gene expression studies. We also show that basal levels of phosphorylation of key signaling nodes may have diagnostic utility in predicting response to selective inhibitors of phosphatidylinositol 3-kinase and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase. Finally, we show that reverse-phase protein arrays allow the parallel analysis of multiple pharmacodynamic biomarkers of response to targeted kinase inhibitors and that inhibitors of epidermal growth factor receptor and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase result in compensatory up-regulation of the phosphatidylinositol 3-kinase/Akt signaling pathway.