p38 gamma and RhoC interact to drive breast cancer metastasis

DEVIN ROSENTHAL; SILVIA ESCUDERO; SM THOMAS; LIWEI BAO; ZHIFEN WU; ALEJANDRA C VENTURA; SOFIA D MERAJVER

Simposio; New Era in Medicine; 2009

Breast cancer is the second leading cause of cancer-related deaths in women in the US each year. Thevast majority of these fatalities are not caused by primary tumor burden but rather by metastases tovital organs. The clinical shift from localized to metastatic breast cancer entails a requirement thatcancer cells activate an invasive program and be able to adapt to changing extracellular stimuli. The p38mitogen activated protein kinase (MAPK) pathway represents a potential signaling switch for thetransition from primary to metastatic cancer. p38 is a member of the MAPK family of stress andmitogen-responsive protein kinases and consists of four closely related isoforms: alpha, beta, gamma,and delta. p38 serves as a major signaling hub in the cell, integrating signals from a variety of signalingpathways and channeling these stimuli into cellular responses through an array of effector proteins. Thefour isoforms show unique expression patterns in normal tissue: alpha, beta, and delta are the mostubiquitously expressed, while gamma is restricted primarily to skeletal muscle. Surprisingly, we foundhigh levels of phosphorylated p38 gamma in a myoepithelial-derived aggressive breast cancer cell line,MDA-MB-231, compared to the non-tumorigenic mammary epithelial cell line MCF-10A, despite bothcell lines having similar levels of total p38 gamma. This led us to hypothesize that, based on its normalfunction in skeletal muscle, p38 gamma may play a role in mediating metastatic behavior in basallyderivedbreast cancers. To test this hypothesis, we used isoform-specific shRNA to knock down p38gamma 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 cellsrevealed a change in the mode of motility from mesenchymal-like in control cells to lamellipodia-drivenmotility in the shGamma cells. This altered motility appears to be the result of a dramatically modifiedactin cytoskeleton, characterized by loss of lengthwise stress fibers and bundled, rather than branched,actin in the lamellipodia. When we investigated gene expression changes responsible for this phenotypewe discovered that expression of the metastatic oncogene RhoC GTPase is significantly reduced at theprotein, but not mRNA, level in shGamma cells due to decreased RhoC protein stability. These findingshave great potential clinical significance, as early evidence indicates that p38 gamma is overexpressed incases of invasive breast cancer, and appears to correlate with RhoC expression in these samples. Ourcurrent and future work includes determining which breast cancer subtypes utilize p38 gamma forinvasion and metastasis, and characterizing in more detail the dynamic actin cytoskeletal changesaffected by p38 gamma.