The switch between stem/progenitor cell expansion and differentiation is critical for organ homeostasis. is usually correlated with IL6-enriched MaSC/basal-like breast malignancy (BLBC). Finally we show that this high SRF expression enables YAP to more efficiently induce IL6 and stemness in BLBC compared with luminal-type breast malignancy. Collectively our results establish the importance of SRF-YAP-IL6 signalling in promoting MaSC-like properties in a BLBC-specific manner. Adult stem cell regulation has been the subject of intense study in recent years. Adult stem cells have been detected in various organs including the intestine and mammary gland1 2 These adult stem cells play a critical role in maintaining organ homeostasis enabling tissue regeneration after organ injury. Adult stem cells also are important in cancer development and progression with a number of studies demonstrating that tumour-initiating cells share many molecular and cellular characteristics Formoterol hemifumarate in common with adult and embryonic stem cells3. This commonality places studies of adult stem cells at the crossroads of understanding both tissue regeneration and cancer mechanisms. Importantly targeting tumour-initiating cells is considered a promising anticancer strategy; thus understanding the regulation of adult stem cells may ultimately bear successful therapy. The transcriptional co-activator YAP (Yes-associated Formoterol hemifumarate protein) a downstream effector of the newly emerging Hippo pathway has recently come to the fore as a critical regulator of tissue regeneration cancer and adult stem cells. YAP is usually a putative oncogene located in the 11q22 amplicon found in various types of cancers4. Studies on YAP transgenic mice YAP-knockout mice and Hippo pathway-knockout mice have collectively revealed that YAP is required for adult stem cell activation during tissue damage and shown that aberrant YAP activation expands epithelial stem/progenitor cells homologue of YAP-IL6 (yorkie-unpaired) is usually important for intestinal stem cell PR65A activation20 28 30 We confirmed that YAP induced IL6 at the mRNA level (Fig. 1e and Supplementary Fig. 2b) and increased IL6 secretion (Fig. 1f). Depletion of IL6 decreased the proportion of CD44Hi/CD24Lo cells and decreased both the number and size of mammospheres (Fig. 1g h) while increasing Formoterol hemifumarate CTGF at the post-transcriptional level (Fig. 1e and Supplementary Fig. 3a). Notably IL6 depletion did not reverse EMT or alter cell proliferation or apoptosis (Supplementary Fig. 3a-c) thus IL6 while not influencing other transforming properties is usually specifically involved in promoting MaSC-like property. Depleting the YAP target CTGF failed to attenuate MaSC-like properties (Supplementary Fig. 4). Surprisingly an IL6-neutralizing antibody and an inhibitor of IL6 downstream JAK signalling increased mammosphere frequency (Fig. 1h). Since IL6 intracellular signalling has been exhibited in the senescence-associated secretory phenotype31 we hypothesized that intracellular IL6 may similarly be responsible for YAP-induced MaSC-like properties. Indeed although MCF-10A cells expressing nonsecretable mutant IL6 lacking a signal peptide (IL6 ΔS) failed to activate JAK signalling it generated mammospheres at a frequency comparable to that of MCF-10A cells expressing wild-type IL6 (Supplementary Fig. 3d-f) suggesting the predominant role of intracellular IL6 in promoting MaSC-like property. Accordingly treatment of recombinant human IL6 failed to increase mammosphere in untransformed MCF-10A cells (Supplementary Fig. 3g). This is in sharp contrast with IL6-JAK signalling being the major determinant of cancer stemness in transformed cells30 (Fig. 6g). We are uncertain why Formoterol hemifumarate inhibition of extracellular IL6 promotes mammosphere formation; the balance between intra- and extracellular IL6 signalling may determine MaSC-like property where extracellular IL6 signalling may possibly inhibit MaSC-like property. Physique 6 SRF-YAP-IL6 signalling is required for CSC formation. TEAD is usually dispensable for Formoterol hemifumarate YAP-induced MaSC-like properties Next we asked which transcription factors are responsible for YAP-induced MaSC properties. Since the vast majority of YAP’s physical associations with transcription factors depend on TEAD-binding and WW domains32 we utilized mutant YAPs that individually lacked each of these protein-interaction domains and examined induction of MaSC properties and expression of IL6 and CTGF as representative MaSC signature genes. Strikingly whereas YAP ΔWW was fully.