The Functional Role of Retinoic Acid and the Cellular Retinoic Acid Binding Protein 1 (Crabp1) in Tumor Suppression
2018-01
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The Functional Role of Retinoic Acid and the Cellular Retinoic Acid Binding Protein 1 (Crabp1) in Tumor Suppression
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2018-01
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Vitamin A is one of the most essential vitamins for human survival with important biological roles in embryogenesis, growth, development, cell differentiation and proliferation, reproduction, neuron function and vision. The pleotropic effects of Vitamin A are primarily attributable to the action of its most active derivative, all-trans retinoic acid (RA). The broad spectrum of RA activities is classically credited to regulation of target gene expression mediated by nuclear RA receptors (RARs). However, a multitude of extra-nuclear, non-genomic activities of RA have been increasingly detected, but the physiological relevance and mechanism of these actions has not been examined. Of particular importance is the rapid, non-genomic RA induced regulation of the mitogen activated protein kinase (MAPK) pathway that is critical for proper cellular homeostasis. The study identified cellular retinoic acid binding protein (Crabp)1 as the key mediator for the non-genomic action of RA, specifically in activating extracellular regulated kinase (ERK)1/2. Binding of RA or certain RA-like compounds to Crabp1 stimulates ERK1/2 activation to slow the ESC cell cycle. This is attributable to the activation of protein phosphatase 2A (PP2A) by ERK, ultimately lengthening the cell cycle. For stem cells, this contributes to the homeostasis of stem cell pools; for cancer stem cell, this sensitizes cancer cells to apoptosis triggered by the canonical, genomic action of RA mediated by RAR. The results suggest a new cancer therapeutic strategy whereby compounds specifically targeting Crabp1 can modulate cell cycle and cancer cell apoptosis. The physiological relevance of this finding was also expanded to the control of neural stem cell (NSC) pool in the hippocampus, that impacted animal behavior. In the Crabp1 knockout mouse hippocampus, neurogenesis and NSC proliferation increased and hippocampus-dependent brain functions such as learning and memory correspondingly improved. This study also established the physiological role of Crabp1 in modulating neural stem cell proliferation and hippocampus-dependent brain activities such as learning and memory. Finally, mechanistic studies were carried out, and identified RAF kinase as the direct target of RA-bound Crabp1 in regulating MAPK signaling pathway. Structural studies revealed the mechanism of RA-bound Crabp1’s action, which is by competing with Ras for RAF interaction. The results suggest a new signal crosstalk between the endocrine (RA) and growth factor (activating RAF-MEK-ERK), mediated by Crabp1 which specifically and rapidly engages RA in dampening RAF activation thereby reducing cellular sensitivity to growth factor stimulation. Data mining of human cancers genetically associated with reduced Crabp1 expression supports the currently suggested tumor suppressor role for Crabp1. Taken together, these studies elucidate a new function of RA action through its cytosolic binding protein, Crabp1, in modulating the MAPK signaling pathway, and reveal the physiological and pathological relevance of this non-canonical RA signaling in stem and cancer cell biology. These studies also establish Crabp1 as a tumor suppressor, and suggest Crabp1 as a new therapeutic target in cancer prevention and intervention. By targeting this new signaling pathway, it is feasible to apply retinoid therapy without eliciting RAR-mediated retinoid toxicity.
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University of Minnesota Ph.D. dissertation. January 2018. Major: Pharmacology. Advisor: Li-Na Wei. 1 computer file (PDF); ix, 130 pages.
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Persaud, Shawna. (2018). The Functional Role of Retinoic Acid and the Cellular Retinoic Acid Binding Protein 1 (Crabp1) in Tumor Suppression. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/202158.
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