Resistance of bone marrow stroma to genotoxic preconditioning is determined by p53

Transplantation of bone marrow (BM) is made possible by the differential sensitivity of its stromal and hematopoietic components to preconditioning by radiation and/or chemotherapeutic drugs. These genotoxic treatments eliminate host hematopoietic precursors by inducing p53-mediated apoptosis but keep the stromal niche sufficiently intact for the engraftment of donor hematopoietic cells. We found that p53-null mice cannot be rescued by BM transplantation (BMT) from even the lowest lethal dose of total body irradiation (TBI). We compared structural changes in BM stroma of mice differing in their p53 status to understand why donor BM failed to engraft in the irradiated p53-null mice. Irradiation did not affect the general structural integrity of BM stroma and induced massive expression of alpha-smooth muscle actin in mesenchymal cells followed by increased adiposity in p53 wild-type mice. In contrast, none of these events were found in p53-null mice, whose BM stroma underwent global structural damage following TBI. Similar differences in response to radiation were observed in in vitro-grown bone-adherent mesenchymal cells (BAMC): p53-null cells underwent mitotic catastrophe while p53 wild-type cells stayed arrested but viable. Supplementation with intact BAMC of either genotype enabled donor BM engraftment and significantly extended longevity of irradiated p53-null mice. Thus, successful preconditioning depends on the p53-mediated protection of cells critical for the functionality of BM stroma. Overall, this study reveals a dual positive role of p53 in BMT: it drives apoptotic death of hematopoietic cells and protects BM stromal cells essential for its functionality.Subject terms: Haematopoietic stem cells, Stem-cell research     

p63 suppresses the ability of pregnancy-identified mammary epithelial cells (PIMECs) to drive HER2-positive breast cancer

While pregnancy is known to reduce a woman’s life-long risk of breast cancer, clinical data suggest that it can specifically promote HER2 (human EGF receptor 2)-positive breast cancer subtype (HER2+ BC). HER2+ BC, characterized by amplification of HER2, comprises about 20% of all sporadic breast cancers and is more aggressive than hormone receptor-positive breast cancer (the majority of cases). Consistently with human data, pregnancy strongly promotes HER2+ BC in genetic mouse models. One proposed mechanism of this is post-pregnancy accumulation of PIMECs (pregnancy-identified mammary epithelial cells), tumor-initiating cells for HER2+ BC in mice. We previously showed that p63, a homologue of the tumor suppressor p53, is required to maintain the post-pregnancy number of PIMECs and thereby promotes HER2+ BC. Here we set to test whether p63 also affects the intrinsic tumorigenic properties of PIMECs. To this end, we FACS-sorted YFP-labeled PIMECs from p63+/−;ErbB2 and control p63+/+;ErbB2 females and injected their equal amounts into immunodeficient recipients. To our surprise, p63+/− PIMECs showed increased, rather than decreased, tumorigenic capacity in vivo, i.e., significantly accelerated tumor onset and tumor growth, as well as increased self-renewal in mammosphere assays and proliferation in vitro and in vivo. The underlying mechanism of these phenotypes seems to be a specific reduction of the tumor suppressor TAp63 isoform in p63+/− luminal cells, including PIMECs, with concomitant aberrant upregulation of the oncogenic ΔNp63 isoform, as determined by qRT-PCR and scRNA-seq analyses. In addition, scRNA-seq revealed upregulation of several cancer-associated (Il-4/Il-13, Hsf1/HSP), oncogenic (TGFβ, NGF, FGF, MAPK) and self-renewal (Wnt, Notch) pathways in p63+/−;ErbB2 luminal cells and PIMECs per se. Altogether, these data reveal a complex role of p63 in PIMECs and pregnancy-associated HER2+ BC: maintaining the amount of PIMECs while suppressing their intrinsic tumorigenic capacity.Subject terms: Breast cancer, Mechanisms of disease     

The HSP-RTK-Akt axis mediates acquired resistance to Ganetespib in HER2-positive breast cancer

Breast cancer is the leading cause of cancer-related death in women worldwide. Human epidermal growth factor receptor 2 (HER2)-positive subtype comprises 20% of sporadic breast cancers and is an aggressive disease. While targeted therapies have greatly improved its management, primary and acquired resistance remain a major roadblock to making it a curable malignancy. Ganetespib, an Hsp90 (Heat shock protein 90) small molecule inhibitor, shows preferential efficacy in HER2-positive breast cancer, including therapy-refractory cases, and has an excellent safety profile in ongoing clinical trials (38 in total, six on breast cancer). However, Ganetespib itself evokes acquired resistance, which is a significant obstacle to its clinical advancement. Here, we show that Ganetespib potently, albeit temporarily, suppresses HER2-positive breast cancer in genetic mouse models, but the animals eventually succumb via acquired resistance. We found that Ganetespib-resistant tumors upregulate several compensatory HSPs, as well as a wide network of phospho-activated receptor tyrosine kinases (RTKs), many of which are HSP clients. Downstream of p-RTKs, the MAPK pathway remains suppressed in the resistant tumors, as is HER2 itself. In contrast, the p-RTK effector Akt is stabilized and phospho-activated. Notably, pharmacological inhibition of Akt significantly delays acquired Ganetespib resistance, by 50%. These data establish Akt as a unifying actionable node downstream of the broadly upregulated HSP/p-RTK resistance program and suggests that Akt co-targeting with Ganetespib may be a superior therapeutic strategy in the clinic.Subject terms: Breast cancer, Cancer therapeutic resistance