Breast cancer cells that disseminate to the bone can either lie dormant or induce osteolytic bone destruction mediated, in part, through tumor cell expression of parathyroid hormone-related protein (PTHrP). Our lab found that PTHrP also promotes tumor cell exit from dormancy, since PTHrP overexpression downregulates known pro-dormancy genes (e.g. LIFR) and switches dormant MCF7 breast cancer cells to an osteolytic phenotype in vivo. PTHrP has biological domains that include a nuclear localization sequence (NLS) and C-terminal region with cytoplasmic activity, but the role of these domains in breast cancer dormancy and bone colonization is unknown. Since nuclear PTHrP stimulates cell cycle progression in muscle cells, we hypothesized that PTHrP NLS deletion would prevent tumor cell exit from dormancy in vivo. Surprisingly, we found that deletion of the PTHrP NLS (termed DNLS) or NLS + C-terminal region (DNLS+CTERM) in MCF7 cells dramatically increased radiographic bone destruction in vivo (up to 5.3-fold, p<0.05), even greater than overexpression of full-length secreted (FLSEC) PTHrP. RNAseq and Gene Set Enrichment Analysis revealed that hypoxia signaling was significantly enriched in DNLS (NES = 1.60, FDR = 0.017) and DNLS+CTERM (NES = 1.41, FDR = 0.058) cells, consistent with significant enrichment of additional hypoxia-inducible pathways (e.g. EMT). HIF1α mRNA was significantly elevated in the FLSEC cells only (1.7-fold, p<0.01), suggesting PTHrP may directly promote HIF1α transcription, since PTHrP is excluded from the nucleus in DNLS and DNLS+CTERM cells. Interestingly, HIF1α protein levels were significantly elevated in DNLS cells only (1.9-fold, p<0.05), suggesting that PTHrP in the cytoplasm stabilizes HIF1α protein. While PTHrP may drive HIF1α transcription, it is likely rapidly degraded, since protein levels were not elevated in FLSEC cells. These data reveal important insights into PTHrP regulation of HIF and a novel mechanism by which breast cancer cells exit dormancy and colonize the bone.