ERBB receptor tyrosine kinases have important roles in human cancer. In particular, the expression or activation of epidermal growth factor receptor and ERBB2 are altered in many epithelial tumours, and clinical studies indicate that they have important roles in tumour aetiology and progression. Accordingly, these receptors have been intensely studied to understand their importance in cancer biology and as therapeutic targets, and many ERBB inhibitors are now used in the clinic. We will discuss the significance of these receptors as clinical targets, in particular the molecular mechanisms underlying response.

Stimulation of the insulin and insulin-like growth factor I (IGF-I) receptor activates the phosphoinositide-3-kinase/Akt/ mTOR pathway causing pleiotropic cellular effects including an mTOR-dependent loss in insulin receptor substrate-1 expression leading to feedback down-regulation of signaling through the pathway. In model systems, tumors exhibiting mutational activation of phosphoinositide-3-kinase/Akt kinase, a common event in cancers, are hypersensitive to mTOR inhibitors, including rapamycin. Despite the activity in model systems, in patients, mTOR inhibitors exhibit more modest antitumor activity. We now show that mTOR inhibition induces insulin receptor substrate-1 expression and abrogates feedback inhibition of the pathway, resulting in Akt activation both in cancer cell lines and in patient tumors treated with the rapamycin derivative, RAD001. IGF-I receptor inhibition prevents rapamycin-induced Akt activation and sensitizes tumor cells to inhibition of mTOR. In contrast, IGF-I reverses the antiproliferative effects of rapamycin in serum-free medium. The data suggest that feedback down-regulation of receptor tyrosine kinase signaling is a frequent event in tumor cells with constitutive mTOR activation. Reversal of this feedback loop by rapamycin may attenuate its therapeutic effects, whereas combination therapy that ablates mTOR function and prevents Akt activation may have improved antitumor activity. (Cancer Res 2006; 66(3): 1500-8)

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We have isolated a human homolog of Xenopus Eg5, a kinesin-related motor protein implicated in the assembly and dynamics of the mitotic spindle. We report that microinjection of antibodies against human Eg5 (HsEg5) blocks centrosome migration and causes HeLa cells to arrest in mitosis with monoastral microtubule arrays. Furthermore, an evolutionarily conserved cdc2 phosphorylation site (Thr-927) in HsEg5 is phosphorylated specifically during mitosis in HeLa cells and by p34cdc2/cyclin B in vitro. Mutation of Thr-927 to nonphosphorylatable residues prevents HsEg5 from binding to centrosomes, indicating that phosphorylation controls the association of this motor with the spindle apparatus. These results indicate that HsEg5 is required for establishing a bipolar spindle and that p34cdc2 protein kinase directly regulates its localization.

Loss of PTEN function leads to activation of phosphoinositide 3-kinase (PI3K) signaling and Akt. Clinical trials are now testing whether mammalian target of rapamycin (mTOR) inhibition is useful in treating PTEN-null cancers. Here, we report that mTOR inhibition induced apoptosis of epithelial cells and the complete reversal of a neoplastic phenotype in the prostate of mice expressing human AKT1 in the ventral prostate. Induction of cell death required the mitochondrial pathway, as prostate-specific coexpression of BCL2 blocked apoptosis. Thus, there is an mTOR-dependent survival signal required downstream of Akt. Bcl2 expression, however, only partially restored intraluminal cell growth in the setting of mTOR inhibition. Expression profiling showed that Hif-1 alpha targets, including genes encoding most glycolytic enzymes, constituted the dominant transcriptional response to AKT activation and mTOR inhibition. These data suggest that the expansion of AKT-driven prostate epithelial cells requires mTOR-dependent survival signaling and activation of HIF-1 alpha, and that clinical resistance to mTOR inhibitors may emerge through BCL2 expression and/or upregulation of HIF-1 alpha activity.

Abstract. Mammalian polo-like kinase 1 (Plkl) is structurally related to the polo gene product of Drosophila melanogaster, Cdc5p of Saccharomyces cerevisiae, and plol ÷ of Schizosaccharornyces pombe, a newly emerging family of serine-threonine kinases implicated in cell cycle regulation. Based on data obtained for its putative homologues in invertebrates and yeasts, human Plkl is suspected to regulate some fundamental aspect(s) of mitosis, but no direct experimental evidence in support of this hypothesis has previously been reported. In this study, we have used a cell duplication, microinjection assay to investigate the in vivo function of Plkl in both immortalized (HeLa) and nonimmortalized (Hs68) human cells. Injection of anti-Plkl antibodies (Plkl +) at various stages of the cell cycle had no effect on the kinetics of DNA replication but severely impaired the ability of cells to divide. Analysis of Plkl +-injected, mitotically arrested HeLa cells by fluorescence microscopy revealed abnormal distributions of condensed chromatin and monoastral microtubule arrays that were nucleated from duplicated but unseparated centrosomes. Most strikingly, centrosomes in Plkl +-injected cells were drastically reduced in size, and the accumulation of both ~/-tubulin and MPM-2 immunoreactivity was impaired. These data indicate that Plkl activity is necessary for the functional maturation of centrosomes in late G2/early prophase and, consequently, for the establishment of a bipolar spindle. Additional roles for Plkl at later stages of mitosis are not excluded, although injection of Plkl + after the completion of spindle formation did not interfere with cytokinesis. Injection of Plkl + into nonimmortalized Hs68 cells produced qualitatively similar phenotypes, but the vast majority of the injected Hs68 cells arrested as single, mononucleated cells in G2. This latter observation hints at the existence, in nonimmortalized cells, of a centrosome-maturation checkpoint sensitive to the impairment of Plkl function. CELt cycle progression in all eukaryotes depends on the periodic activation and inactivation of cyclindependent kinases (for reviews see Norbury and Nurse, 1992;Morgan, 1995;Nigg, 1995). However, it is becoming increasingly clear that protein kinases structurally distinct from cyclin-dependent kinases also play important roles in the regulation of cell cycle events. One prominent, newly emerging family of serine/threonine-kinases with a likely role in cell cycle control is represented by the polo gene product of Drosophila melanogaster (Sunkel and Glover, 1988;Llamazares et al., 1991) and its putative homologues Cdc5p of Saccharomyces cerevisiae (Kitada et al., 1993), plol + of Schizosaccharomyces pombe (Ohkura et al., 1995), and polo-like kinase 1 (Plkl, also referred to as Plk) 1 of mouse and human (Clay et al., 1993; Lake and JelAddress all correspondence to Erich A.

Everolimus significantly increased letrozole efficacy in neoadjuvant therapy of patients with ER-positive breast cancer.

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