Browsing by Subject "leukemia"

Tissue homeostasis is controlled by the availability of growth factors, which sustain exogenous nutrient uptake and prevent apoptosis. Cancer cells, however, can express constitutively active oncogenic kinases such as BCR-Abl that promote these processes independent of extrinsic growth factors. When cells are deprived sufficient growth signals or when oncogenic kinases are inhibited, glucose metabolism decreases and cells activate the self-digestive process of autophagy, which clears damaged organelles and provides degradation products as an alternate fuel to support mitochondrial metabolism. Importantly, loss of growth signals can also lead to apoptosis mediated through Bcl-2 family proteins, and Bcl-2 has been reported to interfere with autophagy, potentially disrupting a key nutrient source just as glucose uptake becomes limiting. Since autophagy may support survival or lead to death depending on context, the role of this pathway in apoptosis-competent growth factor deprived cells remains unclear.

In this thesis, I examine the interactions of autophagy with Bcl-2 family proteins and apoptosis upon inhibition of growth signals in hematopoietic cells. In contrast to other studies, I found autophagy was rapidly induced in growth factor deprived cells regardless of Bcl-2 or Bcl-xL expression, and this led to increased production of fatty acids and amino acids for metabolism. While these data suggested autophagy may play a key role to support metabolism of growth factor deprived cells, provision of exogenous pyruvate or lipids as alternate fuel had little affect on cell survival. Instead, I found that autophagy modulated cell stress pathways and Bcl-2 family protein expression in a context specific fashion to impact cell fate.

My results show that autophagy's effect on cell survival is dependent on its level of induction within a cell. I observed that partial suppression of autophagy protects cells from stress and induction of pro-apoptotic Bcl-2 family expression, while complete inhibition of autophagy enhances stress and is pro-apoptotic. In experiments using shRNAi to partially suppress autophagy, I found increased survival upon growth factor deprivation in several different types of cells expressing anti-apoptotic Bcl-2 or Bcl-xL, indicating that autophagy promoted cell death in these instances. Cell death was not autophagic, but apoptotic, and relied on direct Chop-dependent transcriptional induction of the pro-apoptotic Bcl-2 family protein Bim. In contrast, complete acute disruption of autophagy through conditional Cre-mediated excision of the autophagy-essential gene Atg3 led to p53 phosphorylation, upregulation of p21 and the pro-apoptotic Bcl-2 family protein Puma, and rapid cell death of cells the presence or absence of growth factor. Importantly, transformed BCR-Abl-expressing cells had low basal levels of autophagy but were highly dependent on this process. Deletion of Atg3 or treatment with chemical autophagy inhibitors led to rapid apoptosis, and BCR-Abl expressing cells were unable to form leukemia in mice in without autophagy. Together, my data demonstrate a dual role for autophagy in cell survival or cell death and suggest that the level of autophagy in a cell is critical in determining its role in apoptosis and cell fate. Ultimately, these results may help to determine future approaches to modulate autophagy in cancer therapy.

Background: Acute myeloid leukemia (AML), a rare blood cancer affecting white blood cells, currently impacts approximately 20,380 people living in the U.S. Currently, the 5-year survival rate of AML is 31.7% with Black patients experiencing higher rates of mortality compared to White patients. Black patients have been disproportionately impacted by socioeconomic distress related to historical trauma, social injustices, higher levels of poverty, minimal insurance coverage, and experience higher cancer burden compared to Whites. However, additional research is needed to understand factors that may have contributed to racial disparities among adults with AML. The impact of the cancer care system on health disparities among Black patients has not been well described in the literature. The purpose of this dissertation is to increase understanding of racial disparities in adults with AML and explore factors that may impact disparate outcomes among Black patients.

Methods: A literature review, a qualitative study and a secondary data analysis were used to understand racial disparities in AML and factors that contribute to poor outcomes. The literature review was completed using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 statement to explore disparities in health outcomes among adult survivors of acute leukemias. A total of twenty-five full text published articles were analyzed and included in the synthesis. A qualitative descriptive design was used to explore barriers and facilitators of navigating the cancer care system among a sample of 16 Black adult patients treated with AML at a large comprehensive cancer center. Study findings were analyzed using thematic analysis. Interviews were recorded and coded by two independent coders using Braun and Clark’s method of thematic analysis. A secondary data analysis of cancer registry data was conducted to assess the impact of facility type on the relationship between race and cancer health outcomes (i.e., time to death and time to hematopoietic stem cell transplant) among a sample of 1201 patients with AML. Descriptive statistics were used to explore the structure of the dataset and assess the distribution of the variables. Bivariate analysis was used to examine the differences in facility types across race. Mediation analysis was used to test the mediation effect of facility type on the relationships between race and the health outcomes (time to death and time to HSCT).Results: Identifying as Black, having Hispanic ethnicity, being male, older age, living in areas of high poverty, lack health insurance, and having Medicaid as primary insurance were least favorable conditions for survival among adult AML patients. However, there were limited published qualitative studies exploring the experience of navigating cancer care among adult Black patients with AML. There were also limited published quantitative studies examining causal factors of health disparities among Black patients with AML. The qualitative descriptive study found that Black patients perceived barriers to positive cancer care experiences were discriminatory practices and scarcity in available matched hematopoietic stem cell transplant (HSCT) donors. However, social support, patient-centered care, and empathy from others were perceived as facilitators to care. The secondary data analysis provided evidence the type of facility patients present to for cancer screening may not facility the relationship between race and health outcomes time to death and time to HSCT; however, structural racism does facility this relationship. More research is needed to identify how access to cancer care may contribute to structural racism.

Conclusion: Findings from this dissertation provide empirical data for expanding our understanding factors of the existing racial disparities in cancer care, barriers and facilitators to successful navigation of the cancer care system for Black patients and evidence to identify factors that contribute to structural racism within cancer care. Improving educational curriculum to help clinicians identify signs and symptoms of AML in smaller healthcare and primary care settings, improving insurance coverage for AML treatments and supportive care needs, increasing disease-specific cancer care navigators or social workers, and developing pathways to care for diagnosing AML may be needed to mitigate racial disparities in AML.

Healthy cells require input from growth factor signaling pathways to maintain cell metabolism and survival. Growth factor deprivation induces a loss of glucose metabolism that contributes to cell death in this context, and we have previously shown that maintenance of glycolysis after growth factor deprivation suppresses the activation of p53 and the induction of the pro-apoptotic protein Puma to prevent cell death. However, it has remained unclear how cell metabolism regulates p53 activation and whether this increased glycolysis promotes cell survival in the face of additional types of cell stress. To examine these questions, we have utilized a system in which stable overexpression of the glucose transporter Glut1 and hexokinase 1 in hematopoietic cells drives growth-factor independent glycolysis. This system allows us to examine the effects of glucose metabolism in the absence of other signaling events activated downstream of growth factor receptors. Here, we demonstrate that elevated glucose metabolism, characteristic of cancer cells, can suppress PKCδ-dependent p53 activation to maintain cell survival after growth factor withdrawal. In contrast, DNA damage-induced p53 activation was PKCδ-independent and was not metabolically sensitive. Both stresses required p53 serine 18 phosphorylation for maximal activity but led to unique patterns of p53 target gene expression, demonstrating distinct activation and response pathways for p53 that were differentially regulated by metabolism.

Unlike the growth factor-dependence of normal cells, cancer cells can maintain growth factor-independent glycolysis and survival and often demonstrate dramatically increased rates of glucose uptake and glycolysis, in part to meet the metabolic demands associated with cell proliferation. Given the ability of elevated glucose metabolism to suppress p53 activity in the context of metabolic stress, we examined the effect of increased glucose uptake on leukemogenesis using a mAkt-driven model of leukemia and adoptive transfer experiments. We show here that elevated glucose uptake promoted leukemogenesis in vivo, perhaps through suppression of p53 transcriptional activity. During the process of leukemogenesis, cancerous cells can acquire growth factor independent control over metabolism and survival through expression of oncogenic kinases, such as BCR-Abl. While targeted kinase inhibition can promote cancer cell death, therapeutic resistance develops frequently and further mechanistic understanding regarding these therapies is needed. Kinase inhibition targets the necessary survival signals within cancerous cells and may activate similar cell death pathways to those initiated by growth factor deprivation. As we have demonstrated that loss of metabolism promotes cell death after growth factor withdrawal, we investigated whether cell metabolism played a role in the induction of apoptosis after treatment of BCR-Abl-expressing cells with the tyrosine kinase inhibitor imatinib. Consistent with oncogenic kinases acting to replace growth factors, treatment of BCR-Abl-expressing cells with imatinib led to reduced metabolism and p53- and Puma-dependent cell death. Accordingly, maintenance of glucose uptake inhibited p53 activation and promoted imatinib resistance, while inhibition of glycolysis enhanced imatinib sensitivity in BCR-Abl-expressing cells with wild type p53 but had little effect on p53 null cells. Together, these data demonstrate that distinct pathways regulate p53 after DNA damage and metabolic stress and that inhibition of glucose metabolism may enhance the efficacy of and overcome resistance to targeted molecular cancer therapies.