Until recently, PTH (1-34), or teriparatide, remained the only FDA-approved osteoanabolic therapy for the treatment of osteoporosis. Prolonged use of teriparatide causes levels of bone resorption markers to rise to that of formation, thus limiting its efficacy. This “anabolic window” justifies the search for therapies that maximize anabolism, while limiting resorption. Miller et al. (JAMA, 2016;​316:​722) reported the phase III trial results of abaloparatide (ABL), a novel analog of parathyroid hormone-related protein (PTHrP 1-34), where similar anabolic effects were observed, but a lesser stimulation of resorption was induced with ABL compared with teriparatide or placebo. The results of this trial led to the FDA approval of ABL and, thus, it became the second osteoanabolic therapy brought to market. This study aims to elucidate the mechanisms that underlie the actions of PTH (1-34), PTHrP (1-36), and ABL in the osteoblast and what differences, if any, can be observed therein. My work has shown that in primary murine calvarial osteoblasts, PTHrP (1-36) and ABL result in a significantly lower cyclic AMP (cAMP) response compared with PTH (1-34). Subsequently, peak activation of protein kinase A (PKA) was achieved at 1-2.5min and dose response analyses demonstrated 1/2 max values of 1 nM, 13 nM, and 30 nM for PTH (1-34), PTHrP (1-36), and ABL, respectively. Downstream of this signaling cascade, time course and dose response analyses showed similar relative differences in the phosphorylation of cAMP response element binding protein (CREB), and extracellular signal-regulated protein genes was performed to determine the effects of these three peptides and while many genes were similarly regulated such as the anti-osteoblastogenic Wnt inhibitor, SOST, two genes, the transcription factor c-Fos and receptor activator of nuclear factor kappa-B ligand (RANKL), were differentially regulated in time and dose-dependent manners. Activation of PKA by 8-bromo-cAMP or inhibition by heat-stable PKA inhibitor (PKI) proved that cAMP and PKA are involved in the regulation of RANKL. Additionally, siRNA knockdown of downstream effectors SIK2 and CRTC3 increase and decrease RANKL mRNA, respectively, and show that in the osteoblast, the PTH (1-34), PTHrP (1-36), and ABL-mediated regulation of RANKL involves the cAMP/PKA/SIK2/CRTC3 signaling axis. However, the difference in RANKL expression appears to be due to the relative difference in the regulation of this pathway. To examine the global effects of these three peptides on the osteoblast, RNA-Sequencing was performed and revealed that PTH (1-34) regulates 540 genes, PTHrP (1-36) regulates 323 genes, and ABL regulates 304 genes (fold change ≥ 2, False Discovery Rate < 0.05). Ingenuity Pathway Analysis showed that a subset of cAMP and PKA signaling molecules are regulated by all three peptides. Additionally, PTH (1-34) and ABL down-regulate Wnt/β-catenin signaling molecules, while PTHrP (1-36) does not. To examine the effects of these peptides in vivo, 16-week old C57BL/6 male mice were given 80 ug/kg daily injections for 6 weeks. Whole body bone mineral density (BMD) was measured every two weeks and at the end of the study, femurs, tibiae, and sera were harvested for analyses (n=10/group). PTH (1-34) and ABL resulted in increased whole body BMD, and also increases in cortical thickness as observed by micro-CT analysis. ABL had significantly greater effects on serum-P1NP, a marker for bone formation, compared with PTH (1-34). Additionally, PTH (1-34) and ABL exerted similar effects on serum-CTX, a marker for bone resorption, while PTHrP (1-36) did not. qRT-PCR analyses were performed on cortical, trabecular, and bone marrow fractions and showed that the differences in RANKL regulation exerted by the three peptides in vitro is mirrored in cortical bone. Taken together, these data show that PTH (1-34), PTHrP (1-36), and ABL result in the differential activation of cAMP and PKA, which lead to differential effects on SIK2 and CRTC3, and these events modulate the expression of RANKL. RNA-Sequencing shows that biased signaling occurs between the three peptides, since there are opposing effects on Wnt/β-catenin signaling by PTH (1-34)/ABL and PTHrP. In vivo, ABL is superior to PTH (1-34) and PTHrP (1-36) in raising serum-P1NP levels, while PTH (1-34) and ABL similarly increase serum-CTX levels and result in similar increases in cortical thickness. This study illuminates the differences between PTH (1-34), PTHrP (1-36), and ABL and provides explanations as to how ABL is an equal, if not superior, osteoanabolic therapy compared with PTH (1-34).