Forty-seven years ago, the parathyroid hormone (PTH) in one injection of Lilly's old bovine parathyroid extract, PTE, was found to greatly increase the 30-day survival of heavily X-irradiated rats when given from 18 h before to as long as 3 h after irradiation but no later. This was the first indication that PTH might stimulate hematopoiesis. Recent studies have confirmed the relation between PTH and hematopoiesis by showing that hPTH-(1-34)OH increases the size of the hematopoietic stem cell pool in mice. The peptide operates through a cyclic AMP-mediated burst of Jagged 1 production in osteoblastic cells lining the stem cells' niches on trabecular bone surfaces. The osteoblastic cells' Jagged 1 increases the hematopoietic stem cell pool by activating Notch receptors on attached stem cells. PTH-triggered cyclic AMP signals also directly stimulate the proliferation of the hematopoietic stem cells. However, the single PTH injection in the early experiments using PTE probably increased the survival of irradiated rats mainly by preventing the damaged hematopoietic progenitors from irreversibly initiating self-destructive apoptogenesis during the first 5 h after irradiation. It has also been shown that several daily injections of hPTH-(1-34)OH enable lethally irradiated mice to survive by stimulating the growth of transplanted normal bone marrow cells. If the osteogenic PTHs currently entering or on the verge of entering the market for treating osteoporosis can also drive hematopoiesis in humans as well as rodents, they could be potent tools for reducing the damage inflicted on bone marrow by cytotoxic cancer chemotherapeutic drugs and ionizing radiation.
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