:Malignant pleural effusion is a common complication in metastatic breast cancer (MBC); however, changes in the pleural microenvironment are poorly characterized, especially with respect to estrogen receptor status. Histologically, MBC presents with increased microvessels beneath the parietal and visceral pleura, indicating generalized angiogenic activity. Breast cancer-associated pleural fluid (BAPF) was collected and cultured with HUVECs to recapitulate the molecular changes in subpleural endothelial cells. The clinical progression of triple-negative breast cancer (TNBC) is much more aggressive than that of hormone receptor-positive breast cancer (HPBC). However, BAPF from HPBC (BAPF-HP) and TNBC (BAPF-TN) homogeneously induced endothelial proliferation, migration, and angiogenesis. In addition, BAPF elicited negligible changes in the protein marker of endothelial-mesenchymal transition. Both BAPF-HP and BAPF-TN exclusively upregulated JNK signaling among all MAPKs in HUVECs. By contrast, the response to the JNK inhibitor was insignificant in Transwell and tube formation assays of the HUVECs cultured with BAPF-TN. The distinct contribution of p-JNK to endothelial angiogenesis was consequently thought to be induced by BAPF-HP and BAPF-TN. Due to increased angiogenic factors in HUVECs cultured with BAPF, vascular endothelial growth factor receptor 2 (VEGFR2) inhibitor was applied accordingly. Responses to VEGFR2 blockade were observed in both BAPF-HP and BAPF-TN concerning endothelial migration and angiogenesis. In conclusion, the above results revealed microvessel formation in the pleura of MBC and the underlying activation of p-JNK/VEGFR2 signaling. Distinct responses to blocking p-JNK and VEGFR2 in HUVECs cultured with BAPF-HP or BAPF-TN could lay the groundwork for future investigations in treating MBC based on hormone receptor status.