Erica L. Jackson

Identification of Bronchioalveolar Stem Cells in Normal Lung and Lung Cancer

Injury models have suggested that the lung contains anatomically and functionally distinct epithelial stem cell populations. We have isolated such a regional pulmonary stem cell population, termed bronchioalveolar stem cells (BASCs). Identified at the bronchioalveolar duct junction, BASCs were resistant to bronchiolar and alveolar damage and proliferated during epithelial cell renewal in vivo. BASCs exhibited self-renewal and were multipotent in clonal assays, highlighting their stem cell properties. Furthermore, BASCs expanded in response to oncogenic K-ras in culture and in precursors of lung tumors in vivo. These data support the hypothesis that BASCs are a stem cell population that maintains the bronchiolar Clara cells and alveolar cells of the distal lung and that their transformed counterparts give rise to adenocarcinoma. Although bronchiolar cells and alveolar cells are proposed to be the precursor cells of adenocarcinoma, this work points to BASCs as the putative cells of origin for this subtype of lung cancer.

Endogenous oncogenic K-rasG12D stimulates proliferation and widespread neoplastic and developmental defects

Activating mutations in the ras oncogene are not considered sufficient to induce abnormal cellular proliferation in the absence of cooperating oncogenes. We demonstrate that the conditional expression of an endogenous K-ras(G12D) allele in murine embryonic fibroblasts causes enhanced proliferation and partial transformation in the absence of further genetic abnormalities. Interestingly, K-ras(G12D)-expressing fibroblasts demonstrate attenuation and altered regulation of canonical Ras effector signaling pathways. Widespread expression of endogenous K-ras(G12D) is not tolerated during embryonic development, and directed expression in the lung and GI tract induces preneoplastic epithelial hyperplasias. Our results suggest that endogenous oncogenic ras is sufficient to initiate transformation by stimulating proliferation, while further genetic lesions may be necessary for progression to frank malignancy.

PDGFRα-Positive B Cells Are Neural Stem Cells in the Adult SVZ that Form Glioma-like Growths in Response to Increased PDGF Signaling

Neurons and oligodendrocytes are produced in the adult brain subventricular zone (SVZ) from neural stem cells (B cells), which express GFAP and have morphological properties of astrocytes. We report here on the identification B cells expressing the PDGFRalpha in the adult SVZ. Specifically labeled PDGFRalpha expressing B cells in vivo generate neurons and oligodendrocytes. Conditional ablation of PDGFRalpha in a subpopulation of postnatal stem cells showed that this receptor is required for oligodendrogenesis, but not neurogenesis. Infusion of PDGF alone was sufficient to arrest neuroblast production and induce SVZ B cell proliferation contributing to the generation of large hyperplasias with some features of gliomas. The work demonstrates that PDGFRalpha signaling occurs early in the adult stem cell lineage and may help regulate the balance between oligodendrocyte and neuron production. Excessive PDGF activation in the SVZ in stem cells is sufficient to induce hallmarks associated with early stages of tumor formation.

Malignant Astrocytomas Originate from Neural Stem/Progenitor Cells in a Somatic Tumor Suppressor Mouse Model

Malignant astrocytomas are infiltrative and incurable brain tumors. Despite profound therapeutic implications, the identity of the cell (or cells) of origin has not been rigorously determined. We previously reported mouse models based on conditional inactivation of the human astrocytoma-relevant tumor suppressors p53, Nf1, and Pten, wherein through somatic loss of heterozygosity, mutant mice develop tumors with 100% penetrance. In the present study, we show that tumor suppressor inactivation in neural stem/progenitor cells is both necessary and sufficient to induce astrocytoma formation. We demonstrate in vivo that transformed cells and their progeny undergo infiltration and multilineage differentiation during tumorigenesis. Tumor suppressor heterozygous neural stem/progenitor cultures from presymptomatic mice show aberrant growth advantage and altered differentiation, thus identifying a pretumorigenic cell population.

The differential effects of mutant p53 alleles on advanced murine lung cancer.

We report a direct comparison of the differential effects of individual p53 mutations on lung tumor growth and progression, and the creation of a murine model of spontaneous advanced lung adenocarcinoma that closely recapitulates several aspects of advanced human pulmonary adenocarcinoma. We generated compound conditional knock-in mice with mutations in K-ras combined with one of three p53 alleles: a contact mutant, a structural mutant, or a null allele. p53 loss strongly promoted the progression of K-ras-induced lung adenocarcinomas, yielding a mouse model that is strikingly reminiscent of advanced human lung adenocarcinoma. The influence of p53 loss on malignant progression was observed as early as 6 weeks after tumor initiation. Furthermore, we found that the contact mutant p53R270H, but not the structural mutant p53R172H, acted in a partially dominant-negative fashion to promote K-ras-initiated lung adenocarcinomas. However, for both mutants, loss-of-heterozygosity occurred uniformly in advanced tumors, highlighting a residual tumor-suppressive function conferred by the remaining wild-type allele of p53. Finally, a subset of mice also developed sinonasal adenocarcinomas. In contrast to the lung tumors, expression of the point-mutant p53 alleles strongly promoted the development of sinonasal adenocarcinomas compared with simple loss-of-function, suggesting a tissue-specific gain-of-function.

Characterization of Adult Neural Stem Cells and Their Relation to Brain Tumors

The adult mammalian brain contains neural stem cells that are capable of generating new neurons and glia over the course of a lifetime. Neural stem cells reside in 2 germinal niches, the subventricular zone (SVZ) and the dentate gyrus subgranular zone. These primary progenitors have been identified in their niche in vivo; these cells have characteristics of astrocytes. Recent studies have shown that adult SVZ stem cells are derived from radial glia, the stem cells in the developing brain, which in turn are derived from the neuroepithelum, the earliest brain progenitors. Thus, SVZ stem cells are a continuum from neuroepithelium to radial glia to astrocytes, and are contained within what has been considered the lineage for astrocytes. However, it seems that only a small subset of the astrocytes present in the adult brain have stem cell properties. Recent findings have shown that SVZ stem cell astrocytes express a receptor for platelet-derived growth factor (PDGF), suggesting that the ability to respond to specific growth factor stimuli, such as PDGF, epidermal growth factor and others, may be unique to these stem cell astrocytes. Intriguingly, activation of these same signaling pathways is widely implicated in brain tumor formation. Since the adult brain has very few proliferating cells capable of accumulating the numerous mutations required for transformation, the adult neural stem and/or progenitor cells may be likely candidates for the brain tumor cell of origin. Indeed, activation of the PDGF or epidermal growth factor pathways in adult neural stem or progenitor cells confers tumor-like properties on these cells, lending support to this hypothesis.

Discovery of a Potent and Selective Steroidal Glucocorticoid Receptor Antagonist (ORIC-101)

The glucocorticoid receptor (GR) has been linked to therapy resistance across a wide range of cancer types. Preclinical data suggest that antagonists of this nuclear receptor may enhance the activity of anticancer therapy. The first-generation GR antagonist mifepristone is currently undergoing clinical evaluation in various oncology settings. Structure-based modification of mifepristone led to the discovery of ORIC-101 (28), a highly potent steroidal GR antagonist with reduced androgen receptor (AR) agonistic activity amenable for dosing in androgen receptor positive tumors and with improved CYP2C8 and CYP2C9 inhibition profile to minimize drug-drug interaction potential. Unlike mifepristone, 28 could be codosed with chemotherapeutic agents readily metabolized by CYP2C8 such as paclitaxel. Furthermore, 28 demonstrated in vivo antitumor activity by enhancing response to chemotherapy in the GR+ OVCAR5 ovarian cancer xenograft model. Clinical evaluation of safety and therapeutic potential of 28 is underway.

Abstract 1968: A novel glucocorticoid receptor (GR) antagonist overcomes GR-mediated chemoresistance in triple-negative breast cancer

The glucocorticoid receptor (GR) is a member of the nuclear receptor superfamily, which is activated by its endogenous steroid hormone ligand cortisol, and by synthetic glucocorticoids such as dexamethasone. Several preclinical studies have shown that GR mediates resistance to both targeted therapies and conventional chemotherapies in a variety of epithelial cancers including prostate, lung, bladder, renal, ovarian and triple-negative breast cancers (TNBC) (Gassler et al., 2005; Li et al., 2017; Zhang et al., 2007). In TNBC, both GR activation and a disrupted cortisol secretion cycle are associated with chemotherapy resistance, increased disease recurrence and poor prognosis (Pan et al., 2011; Skor et al., 2013). Therefore a molecule that inhibits GR activation could attenuate the development of therapy resistance and improve patient outcomes. We have developed novel GR inhibitors that effectively block GR transcriptional activity in cells by competing for ligand binding and by blocking GR-coactivator interactions. In vitro, treatment of TNBC cells with the GR antagonist OP-3713 blocks GR transcriptional activity and enhances the efficacy of chemotherapeutic agents. In rodents the predominant glucocorticoid is corticosterone (Siswanto et al., 2008), which is a weak agonist of human GR. Therefore, to fully activate GR in human xenograft cancer models it is necessary to provide exogenous cortisol. Using xenograft models of TNBC, we find that tumors grown in mice with physiologically relevant circulating cortisol levels to activate GR are significantly less sensitive to chemotherapy than those grown in the absence of cortisol. Furthermore, inhibition of GR by OP-3713 prevents tumor relapse following chemotherapy treatment. We have begun to elucidate the mechanisms by which GR mediates chemoresistance in TNBC, and the basis for the reversal by OP-3713. Our findings underscore the important role of GR as a mediator of resistance in TNBC and highlight the therapeutic potential of GR inhibitors in combination with clinically relevant chemotherapeutic agents. Citation Format: Nadine Jahchan, Haiying Zhou, Wayne Kong, Dan Mc Weeney, Ted Tracy, James Stice, Dena Sutimantanapi, Chelsea Chen, Tom Huang, Yosup Rew, Xiauhui Du, Tatiana Zavorotinskaya, Daqing Sun, Qiuping Ye, Erica Jackson, Valeria Fantin. A novel glucocorticoid receptor (GR) antagonist overcomes GR-mediated chemoresistance in triple-negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1968.