BRAF-inhibitor Associated MEK Mutations Increase RAF-Dependent and -Independent Enzymatic Activity.

Alterations in MEK1/2 occur in cancers, both in the treatment-naïve state and following targeted therapies, most notably BRAF and MEK inhibitors in BRAF-V600E-mutant melanoma and colorectal cancer. Efforts were undertaken to understand the effects of these mutations, based upon protein structural location, and MEK1/2 activity. Two categories of MEK1/2 alterations were evaluated, those associated with either the allosteric pocket or helix-A. Clinically, MEK1/2 alterations of the allosteric pocket are rare and we demonstrate that they confer resistance to MEK inhibitors, while retaining sensitivity to BRAF inhibition. Most mutations described in patients fall within, or are associated with, helix-A. Mutations in this region reduce sensitivity to both BRAF and MEK inhibition and display elevated phospho-ERK1/2 levels, independent from increases in phospho-MEK1/2. Biochemical experiments with a representative helix-A variant, MEK1-Q56P, reveal both increased catalytic efficiency of the activated enzyme, and phosphorylation-independent activity relative to wild-type MEK1. Consistent with these findings, MEK1/2 alterations in helix A retain sensitivity to downstream antagonism via pharmacologic inhibition of ERK1/2. This work highlights the importance of classifying mutations based on structural and phenotypic consequences, both in terms of pathway signaling output and response to pharmacologic inhibition.Implications: This study suggests that alternate modes of target inhibition, such as ERK inhibition, will be required to effectively treat tumors harboring these MEK1/2-resistant alleles. Mol Cancer Res; 15(10); 1431-44. ©2017 AACR.

Involvement of cathepsin D in chemotherapy-induced cytochrome c release, caspase activation, and cell death.

Treatment of cells with chemotherapy drugs activates the intrinsic mitochondrial pathway of apoptosis and the caspase protease cascade. Recently, the lysosomal protease cathepsin D has been implicated in apoptosis caused by oxidative stress, inhibition of protein kinase C, and stimulation of the TNFR1 and Fas death receptors. However, the role of cathepsin D in chemotherapy-induced cell death has remained largely unexplored. In this report, we show that treatment of U937 leukemia cells with the chemotherapy drug etoposide (VP-16) results in cathepsin D release into the cytosol within 4 hours after initiation of drug treatment. VP-16-induced cathepsin D release was not inhibited by z-VAD-FMK or pepstatin A, suggesting that it occurred independently of the activities of caspase proteases or cathepsin D. Down-regulation of cathepsin D expression in suspension U937 cells or adherent HeLa cells using cathepsin D small interfering RNA partially inhibited cell death resulting from treatment of cells with tumor necrosis factor-alpha, tumor necrosis factor-related apoptosis inducing ligand, or the chemotherapy drugs VP-16, cisplatin, and 5-fluorouracil. Moreover, cathepsin D down-regulation significantly delayed cytochrome c release and caspase-3 activation in response to chemotherapy treatment. Incubation of isolated mitochondria with cathepsin D-treated cytosolic extracts resulted in potent release of cytochrome c, indicating that a cytoplasmic substrate mediates the effects of cathepsin D on mitochondria. Together, these findings show that cathepsin D plays an important role in chemotherapy-induced cell death, and that cathepsin D lies upstream of cytochrome c release and caspase-3 activation in the chemotherapy-induced execution pathway.

Whole genome amplification of DNA from laser capture-microdissected tissue for high-throughput single nucleotide polymorphism and short tandem repeat genotyping.

Genome-wide screening of genetic alterations between normal and cancer cells, as well as among subgroups of tumors, is important for establishing molecular mechanism and classification of cancer. Gene silencing through loss of heterozygosity is widely observed in cancer cells and detectable by analyzing allelic loss of single nucleotide polymorphism and/or short tandem repeat markers. To use minute quantities of DNA that are available through laser capture microdissection (LCM) of cancer cells, a whole genome amplification method that maintains locus and allele balance is essential. We have successfully used a ø29 polymerase-based isothermal whole genome amplification method to amplify LCM DNA using a proteinase K lysis procedure coupled with a pooling strategy. Through single nucleotide polymorphism and short tandem repeat genotype analysis we demonstrate that using pooled DNA from two or three separate amplification reactions significantly reduces any allele bias introduced during amplification. This strategy is especially effective when using small quantities of source DNA. Although a convenient alkaline lysis DNA extraction procedure provided satisfactory results from using 1500 to 3000 LCM cells, proteinase K digestion was superior for lower cell numbers. Accurate genotyping is achieved with as few as 100 cells when both proteinase K extraction and pooling are applied.