c-MYC Functions as a Molecular Switch to Alter the Response of Human Mammary Epithelial Cells to Oncostatin M

Abstract

Cytokines play an important role in creating an inflammatory microenvironment, which is now considered a hallmark of cancer. While tumor cells can exploit cytokine signaling to promote growth, invasion and metastasis, the response of normal and premalignant epithelial cells to cytokines present in a developing tumor microenvironment remains unclear. Oncostatin M (OSM), an IL-6 family cytokine responsible for STAT3 activation, has been implicated in cancer development, progression, invasion and metastasis. Paradoxically, OSM can also suppress the growth of normal cells and certain tumor-derived cell lines. Using isogenic human mammary epithelial cells (HMEC) at different stages of neoplastic transformation, we found that OSM signaling suppressed c-MYC expression and engaged a p16- and p53-independent growth arrest that required STAT3 activity. Inhibition of STAT3 activation by expressing a dominant-negative STAT3 protein or a STAT3-shRNA prevented the OSM-mediated arrest. In addition, expression of c-MYC from a constitutive promoter also abrogated the STAT3-mediated arrest, and strikingly, cooperated with OSM to promote anchorage-independent growth (AIG), a property associated with malignant transformation. Cooperative transformation by c-MYC and OSM required PI3K and AKT signaling, demonstrating the importance of multiple signaling pathways downstream of the OSM receptor in defining the cellular response to cytokines. These findings identify c-MYC as an important molecular switch that alters the cellular response to OSM-mediated signaling from tumor suppressive to tumor promoting.

STAT3-mediated SMAD3 activation underlies Oncostatin M-induced Senescence

“…41,42 Likewise, persistent Oncostatin M (OSM)-mediated activation of STAT3 also induces a p16/p53-independent senescence. 19 Given that RAS-induced OIS requires functional Transforming Growth Factor-b (TGF-b) signaling, we hypothesized that persistent OSM/STAT3-induced senescence would also utilize the TGF-b signaling pathway. 41 To test this hypothesis, post-selection shp53-HMEC (lacking p16 expression due to endogenous p16 promoter methylation and p53 due to shRNA-mediated ablation) were infected with retroviruses encoding either a dominant-negative STAT3 (shp53/DN-STAT3-HMEC) or a dominant-negative TGF-b type II receptor (shp53/ DN-TGFbR2-HMEC) to inhibit TGF-b signaling.…”

“…[13][14][15][16][17][18] Elevated levels of OSM in the TME are associated with highly aggressive metastatic cancers, increased risk of tumor recurrence, and a poor prognosis. [19][20][21][22][23][24] In breast cancer, OSM is concentrated at the invasive edges of highly metastatic tumors where cells often display mesenchymal cell characteristics and express the cancer stem cell marker CD44. 22,25 Whereas OSM drives proliferation, epithelial-mesenchymal transition (EMT), invasion, and metastasis in breast cancer cells and transformed human mammary epithelial cells (HMEC), it engages senescence in normal and non-transformed HMEC.…”

“…22,25 Whereas OSM drives proliferation, epithelial-mesenchymal transition (EMT), invasion, and metastasis in breast cancer cells and transformed human mammary epithelial cells (HMEC), it engages senescence in normal and non-transformed HMEC. 13,[19][20][21][26][27][28][29][30][31][32][33][34] Senescence is an irreversible, proliferationinhibiting response that occurs in 2 steps: 1) reversible cell cycle arrest followed by 2) futile growth-induced gerogenic conversion, or geroconversion. [35][36][37] Moreover, senescence is a major tumorsuppressive mechanism that acts as an early barrier to thwart cancer development.…”

“…38 In the case of OSM-induced senescence, persistent STAT3 activation represses MYC (c-MYC) expression and engages a p16-and p53-independent cell cycle arrest that is accompanied by many senescence-associated characteristics, including an enlarged flattened morphology and b-galactosidase (b-Gal) activity. 19,39,40 While OSM-induced MYC repression is associated with senescence, ectopic expression of MYC from a constitutive promoter prevents senescence and alters the response to OSM from tumor-suppressive to tumor-promoting. 19 Understanding how developing tumor cells adapt to senescenceinducing cytokine signals and circumvent the normal, growthsuppressive response is critical for understanding tumorigenesis.…”

“…19,39,40 While OSM-induced MYC repression is associated with senescence, ectopic expression of MYC from a constitutive promoter prevents senescence and alters the response to OSM from tumor-suppressive to tumor-promoting. 19 Understanding how developing tumor cells adapt to senescenceinducing cytokine signals and circumvent the normal, growthsuppressive response is critical for understanding tumorigenesis.…”