p38 gamma is a novel driver of breast cancer metastasis by modulating cellular motility

DEVIN ROSENTHAL; SILVIA ESCUDERO; LEWEI BAO; ZHIFEN WU; ALEJANDRA C VENTURA; SOFIA D MERAJVER

Conferencia; 2010 AACR Annual Meeting; 2010

Breast cancer is the second leading cause of cancer-related deaths in women in the US each year. The vast majority of thesefatalities are not caused by primary tumor burden but rather by metastases to vital organs. The clinical shift from localized tometastatic breast cancer entails a requirement that cancer cells activate an invasive program and be able to adapt to changingextracellular stimuli. The p38 mitogen activated protein kinase (MAPK) pathway represents a potential signaling switch forthe transition from primary to metastatic cancer. p38 is a member of the MAPK family of stress and mitogen-responsiveprotein kinases and consists of four closely related isoforms: alpha, beta, gamma, and delta. p38 serves as a major signalinghub in the cell, integrating signals from a variety of signaling pathways and channeling these stimuli into cellular responsesthrough an array of effector proteins. The four isoforms show unique expression patterns in normal tissue: alpha, beta, anddelta are the most ubiquitously expressed, while gamma is restricted primarily to skeletal muscle. Surprisingly, we found highlevels of phosphorylated p38 gamma in a myoepithelial-derived aggressive breast cancer cell line, MDA-MB-231, comparedto the non-tumorigenic mammary epithelial cell line MCF-10A, despite both cell lines having similar levels of total p38gamma. This led us to hypothesize that, based on its normal function in skeletal muscle, p38 gamma may play a role inmediating metastatic behavior in basally-derived breast cancers. To test this hypothesis, we used isoform-specific shRNA toknock down p38 gamma expression in MDA-MB-231. Inhibition of p38 gamma (referred to as shGamma) resulted indecreased cell motility and invasion, two hallmarks of cancer metastasis. Analysis of individual cells revealed a change in themode of motility from mesenchymal-like in control cells to lamellipodia-driven motility in the shGamma cells. This alteredmotility appears to be the result of a dramatically modified actin cytoskeleton, characterized by loss of lengthwise stressfibers and bundled, rather than branched, actin in the lamellipodia. When we investigated gene expression changesresponsible for this phenotype we discovered that expression of the metastatic oncogene RhoC GTPase is significantlyreduced at the protein, but not mRNA, level in shGamma cells due to decreased RhoC protein stability. These findings havegreat potential clinical significance, as early evidence indicates that p38 gamma is overexpressed in cases of invasive breastcancer, and appears to correlate with RhoC expression in these samples. Our current and future work includes determiningwhich breast cancer subtypes utilize p38 gamma for invasion and metastasis, and characterizing in more detail the dynamicactin cytoskeletal changes affected by p38 gamma.