The following research descriptions pertain to recent work in the laboratory with substantial potential for discovery of new modalities for the modeling and treatment of cancers.
1. The pulmonary neuroendocrine cancers: Current understanding of the role of oxygen in the progression of cancers has indicated that solid tumors contain zones of hypoxia reflecting the degree of vascularization. Here extreme hypoxia (<2% O2) upregulates the O2 sensing HIF mediated pathway to engage a large transcriptional program, for example, critical for angiogenesis. However, all tissues respire. Much less is known about how tumor cells can respond to transitions from acute to sustained (chronic) levels of pO2 and pCO2 (carbon dioxide sensing) similar to those experienced by normal tissues. The pulmonary neuroendocrine tumors (NETs), carcinoids and small cell lung carcinoma (SCLC) are remarkable in recapitulating the plasma membrane localized O2 sensing complex found in their cell of origin, the pulmonary neuroendocrine cells (PNEC). These function as polymodal airway sensors, detecting changes in O2/CO2/H+ via O2/CO2 sensor complexes. NETs resemble native PNEC in possessing a membrane delimited O2 sensor mechanism (NADPH oxidase/NOX2 sensitive K+ channel molecular complex) and the CO2/H+ (pH) sensor mechanism (via carbonic anhydrase (CA)). Stimulus-secretion coupling links O2/CO2/H+ sensors to release of amine (serotonin, 5-HT) by NET cells, acting as an autocrine growth factor for NET. Our preliminary studies using SCLC and carcinoid cell line models show that O2/CO2/H+ sensors in NET cells are activated at mild to moderate levels of these stimuli causing secretion of bioactive 5-HT. We have evidence that the HIF pathway integrates with the sensor pathway. Manipulating O2 levels, and altering CO2 metabolism by downregulating CAs, pharmacologically and by shRNA, dramatically affects endocrine functions and tumor cell proliferation and survival, both in vitro and in informative xenograft models.
Our studies will exploit the O2/CO2 cell sensor mechanisms to understand how these drive tumor growth and survival. We posit that O2/CO2/H+sensors enhance the ability of NET to: i) adapt to hypoxia/ elevated CO2 (hypercapnia) /acidosis, and ii) preferentially select for hypoxia/acidosis-resistant tumor cells. Thus understanding the biology of NET will provide new insights into how cancer cells sense and respond to fluctuating O2/CO2 microenvironments found in all cancers.
2. Modeling and novel therapeutic approaches for pediatric cancer: Here we focus on modeling in vivo the tumor biology of neuroblastoma (NB) with emphasis on metastatic progression and targeting by novel therapeutics. Despite previous work using the immunocompromised mouse to model NB progression, it would be of great advantage to both more closely approximate tumor masses and metastatic sites found in patients. This is made feasible by recapitulating NB tumor growth and metastaic patterns in the nude rat. The larger nude rat model affords non-invasive MR imaging with higher resolution and better interpretation. We have been working with co-applicant Dr H-L Cheng at SickKids to model human tumor growth and metastasis in the nude rat model including interrogation of vascularization patterns, temporally and spatially using selected contrast agents. In addition, aggressive tumor cells can be sensitively imaged by MRI. Thus we propose to demosntrate the advantages of non-invasive and continuous monitoring of tumor parameters by MRI that will help to enhance NB studies at SickKids. In addition, and importantly, we will xenograft patient direct tumor tissue representing pre- and post treatment relapse will be evaluated. These cases of comparable advanced stage NB harbor the full spectrum in cellular heterogeneity of metastatic subpopulations that reside within tumors and can be further identified and selected for the study. We will select current NB tumor stem cell markers and in vitro enhancement methods, established in our laboratory, to identify selected cancer stem cell(CSC) - like fractions. The CSC subpopulations are associated with the metastatic potential and intrinsic resistance to chemotherapies and this approach will provide us with a stringent model to validate new therapeutic approaches that can reduce or eliminate this subpopulation associated clinically with minimal residual disease (MRD) and relapse.
Finally, in all our xenograft work modeling different cancers in immunocompromised animals we are now expoiting the increased versatility and monitoring potential of MRI [collaboration with Dr H-L cheng at HSC]. This non-invasive imaging tool permits tracking of tumor progression events temporally and spatially without moleculary disturbing tumor cells and concomitantly examing the vascularization of the tumor masses, a critical prognostic component.