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Angiogenesis, the growth of blood vessels from pre-existing vasculature, is primarily regulated by vascular endothelial growth factor receptors (VEGFRs). Dysregulated angiogenesis is associated with cancers, obesity, and over 70 vascular diseases. Upregulated VEGFR protein expressions in diseased vasculature are promising biomarkers for predicting clinical outcomes, as indicated by non-quantitative immunohistochemical studies in patients with impaired vascularization or tumor angiogenesis. While the quantitative characterization of VEGFRs is critical in identifying biomarkers for anti-angiogenic therapies, VEGFR biomarker development presents two particular challenges: (1) The invasive tissue biopsy needed limits the amount of VEGFR data that can be collected from both normal and diseased vasculatures, and (2) we poorly understand the significance of endothelial and various non-endothelial VEGFR-expressing cells in angiogenic therapies. To address these challenges, here I pioneer a blood biopsy-based proteomic approach that allows non-invasive VEGFR quantification. More significantly, I identify and establish age- and sex-specific basal levels of VEGFRs on endothelial cells and bone marrow-derived progenitor cells (Chapter 2). In recent years, blood biopsies have expanded our knowledge of vascular pathology. In particular, circulating angiogenic cells, such as circulating endothelial cells (cECs) and circulating progenitor cells (cPCs), are isolated and counted, and their elevated abundances are often correlated with vascular disease progression and cancer prognosis. However, cECs and cPCs have been overlooked as accessible proxies for profiling vascular biomarker expressions by activated or damaged vasculatures. For the first time, I show that cPCs and cECs exhibit heterogeneous plasma membrane expression of VEGFRs, which are correlated with donor sexes and ages, particularly pre- vs. post-menopausal status. Menopause is known to reduce regenerative and angiogenic capacities, as manifested by decreased capillary growth in skeletal muscle and increased risks for cardiovascular diseases. Here I provide baseline VEGFR expression ranges for these cells, showing that ~50% of cECs in premenopausal females exhibit intermediate-to-high plasma membrane expression (138,000 VEGFR1 and 39,000-236,000 VEGFR2/cell) and ~25% of cECs in males exhibit high VEGFR plasma membrane expression (206,000 VEGFR1 and 155,000 VEGFR2/cell). In marked contrast, nearly all cECs in postmenopausal females are VEGFR-low (2,900 VEGFR1 and 3,400 VEGFR2/cell), agreeing with the reduced angiogenic capacities after menopause. Additionally, VEGFR1 signaling is critical for cPC localization to activated or damged blood vessels. My data show that VEGFR1 plasma membrane localization in cPCs occurs only in postmenopausal females, suggesting menopause activates VEGFR1 signaling pathways in cPCs. Therefore, my data offer quantitative insights into how VEGFR-regulated regenerative and angiogenic capacities are altered due to menopause. Overall, these findings provide the first insights into how sex and age interactions, particularly menopause, influence VEGFR plasma membrane localization in circulating angiogenic cells. More importantly, the findings help establish age- and sex-specific VEGFR baselines for predicting vascular disease progression and therapeutic outcomes. The second challenge is quantitatively characterize how endothelial and non-endothelial VEGFR-expressing cells contribute to angiogenic regulation. Here, I quantitatively elucidate the changes in VEGFR expressions by endothelial cells and non-enodthelial cells in adipose tissues, and identify biomarkable adipose tissue cells that show altered VEGFR membrane expressions in normal versus high-adiposity states (Chapter 3). Obesity is a major risk factor for vascular disorders, including peripheral artery disease, critical limb ischemia, and several cancers. I hypothesize that VEGFR membrane expression by adipose tissue cells is altered as body fat accumulates (increased adiposity). The VEGFR quantification data presented here indicate that ~ 20% of activated lymphocytes upregulate their membrane expressions of VEGFR1 and VEGFR3 by tenfold in response to increased subcutaneous adiposity induced by lipedema, which is very commonly accompanied by impaired vascularization and chronic inflammation. On the other hand, in murine visceral adipose tissue, myeloid progenitor cells exhibit the highest VEGFR membrane expressions (16,000 ± 4,700 VEGFR1, 50,000 ± 6,200 VEGFR2, and 2,100 ± 460 VEGFR3/cell). Compared to myeloid progenitor cells, visceral endothelial cells exhibit an order of magnitude lower VEGFR1 and VEGFR2 levels (2,400 ± 710 VEGFR1/cell, 1,100 ± 190 VEGFR2/cell, and 1,200 ± 220 VEGFR3/cell, respectively). My approach and findings are foundational to a systematic understanding of how VEGFR-expressing adipose cells regulate adipose angiogenesis and adipogenesis. Future studies are warranted to compare how VEGFR membrane expressions differ in chow-fed and high fat-fed mice, and the quantitative proteomic findings will guide therapies for visceral obesity-associated vascular disorders. Last but not least, unlike VEGFRs, many receptors of clinical interest, particularly the oxytocin receptor (OXTR) and its genetic variants, do not have specific antibodies that enable quantitative characterization. To overcome this issue, I have designed a transfected cell model that is engineered to express HA-OXTR-GFP protein complexes, in which an N-terminal HA acts as a proxy for membrane OXTR detection and a C-terminal GFP acts as an indicator in selecting transfected cells from untransfected cells (Chapter 4). This transfected cell model is applied to characterize the varied dose-response profiles of OXTR wild-type and variant cells to oxytocin, a common labor induction drug. My OXTR quantification data show clear correlations to oxytocin-induced functional outcomes, including calcium release and cell desensitization, suggesting that the quantities of different OXTR variants are predictive of cell responses to administered oxytocin and should be considered when making personalized oxytocin dosing decisions. Overall, my results demonstrate that membrane expression of VEGFRs is significantly associated with physiological factors such as sex, age, and menopause, and with pathological adipose tissue expansion. Although VEGFR protein expression is a promising biomarker for many vascular diseases and cancers, quantitative and baseline VEGFR data are still needed for VEGFR-driven pathology. My work on both VEGFRs and other biomarkable receptors, such as OXTR, provides much-needed standardized approaches and quantitative data, a first step towards proteomic biomarker-driven precision medicine.