PROJECT SUMMARY Breast cancer (BC) lethality is primarily caused by metastasis, which accounts for >90% of BC-related deaths. The epigenetic mechanisms contributing to metastasis remain largely unknown. CARM1 is overexpressed in triple-negative breast cancer (TNBC) and high level of CARM1 expression correlates with poor prognosis. We have previously shown that either CARM1 knockout or treatment with CARM1 inhibitor (CARM1i) significantly decreased the growth and invasion of TNBC in vivo. We recently identified MAP2K4, a stress-activated protein kinase family member, as a substrate of CARM1 in both BC cell lines and primary TNBC tumors. Our results showed that either mutation of the MAP2K4 single methylation site or inhibition of CARM1 affects MAP2K4 activation. MAP2K4 is known to elicit the oncogenic functions in TNBC. We postulate that MAP2K4 methylation by CARM1 potentiates the oncogenic function of MAP2K4. Moreover, loss-of-function MAP2K4 mutations have been linked to the therapeutic response to clinically investigated drugs such as PI3K inhibitors (PI3Ki). We found that CARM1i and PI3Ki synergistically inhibits proliferation and migration of TNBC cells, supporting a TNBC cell autonomous crosstalk between activated MAP2K4 and PI3K to promote tumor growth. Although CARM1i enhances the activity and tumoral infiltration of CD8+ T cells in a 4T1 syngenetic mouse model, CARM1i does not appear to synergize with the anti-PD-1 antibody. We hypothesize that combinatory CARM1 and PI3K inhibition potentiates antitumor immunity and sensitizes TNBC tumors to immune checkpoint inhibitors (ICIs). We propose three specific aims: (1) Determine if MAP2K4 methylation augments the kinase activity of MAP2K4 and enhances JNK activation and cell migration; (2) Delineate the cell autonomous transcriptional network and signaling pathways affected by dual inhibition of CARM1 and PI3K; (3) Develop combined targeted and immunotherapy regimen for treating metastasis in syngeneic mouse models. The objectives of this study are to better understand how MAP2K4 methylation by CARM1 serves as an epigenetic event that contributes to breast cancer metastasis and therapeutically target CARM1 and PI3K to sensitize tumors to ICIs for treating metastatic BC.