Rationale Tobacco use is associated with an increase in white blood cell count (WBC). This association has been attributed to bronchopulmonary inflammation and/or infection. It is not known if nicotine itself may play a role. Objectives We determined if nicotine itself could effect WBC count, and determined if this was due to a direct effect on hematopoietic stem cells (HSC). Methods and Measurements C57Bl6J mice received nicotine orally, and measurements of WBC count; bone marrow and spleen cellularity; and HSC count were made. To determine the functionality of HSCs, irradiated animals received bone marrow transplants from vehicle or nicotine treated mice. Main Results Nicotine increased leukocytes in the peripheral blood, bone marrow and spleen. Peripheral red cell and platelet count were unaffected. Nicotine increased the frequency of HSC in the bone marrow. Isolated long-term HSCs from nicotine-treated mice transplanted into irradiated mice regenerated all hematopoeitic cell lineages, demonstrating functional competence of those HSCs. HSC expressed nicotinic acetylcholine receptors (nAChRs), as documented by FITC-conjugated alpha-bungarotoxin binding. Nicotine increased soluble Kit ligand, consistent with stem cell activation. Conclusions The data suggest a new mechanism for the increased WBC associated with tobacco use. The effect of nicotine to activate hematopoiesis may contribute to tobacco-related diseases.

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SUMMARYAutosomal dominant mutations in telomere-associated factors elicit a disease known as dyskeratosis congenita (DKC), and patients suffer proliferative abnormalities associated with telomere erosion. Mice that are heterozygous for telomerase genes (Tert or Terc, hereafter referred to as mTert and mTerc) are useful models of telomerase haploinsufficiency, but do not strictly mimic DKC. In strains with long telomeres (>60 kbp), animals that are heterozygous for mTert undergo telomere erosion for nine generations and remain phenotypically normal. In an mTerc heterozygous strain with short telomeres (<15 kbp), early mortality arises after five to six generations, but dyskeratosis occurs only upon the further loss of mPot1b. We show that prolonged mTert heterozygosity (for greater than ten generations) did not elicit disease, even upon heterozygote interbreeding, and that telomeres reset to wild-type lengths. This lengthening did not occur in nullizygotes, and short telomeres inherited from mTert null parents were rescued only in heterozygous progeny. In the bone marrow, nullizygotes remained competent for radioprotection for three generations. Thus, gradual telomere erosion in the presence of telomerase may enable subsequent telomere extension, similar to that described in budding yeast. We speculate whether such adaptation occurs in normal human cells (or whether it could be induced in DKC-derived cells), and whether it might mitigate the impact of telomerase inhibition upon stem cells during cancer therapy.

The family of selenoproteins have a broad range of functions, including protection against oxidative damage. Previous studies have shown that elevated levels of oxidative damage can induce accelerated loss of telomeric DNA during proliferation of mammalian cells. The incorporation of selenocysteine (Sec) into proteins in mammalian cells requires the Sec insertion sequence (SECIS) binding protein 2 (SBP2). Thus in the present study we have assessed the effect of knocking down the expression of SBP2 on telomere length. Following knock-down of SBP2 expression in 2 different human cell lines, the MSTO mesothelioma cell line (~5 Kb average telomere length) and SY5Y neuroblastoma cell line (~4.2 Kb average telomere length), we observed a significant reduction (−0.6 to −1.1Kb; P≤0.01) in telomere length as compared to control cells. This reduction in telomere length was independent of affects on telomerase, since both telomerase activity levels and Tert mRNA expression levels were not altered by knockdown of SBP2 expression. Furthermore, telomeres were particularly sensitive to S1 nuclease digestion following SBP2 knock-down, indicating an increased frequency of oxidative damage induced lesions in the telomeric DNA in these cells. Together, these observations imply that selenoproteins may help protect telomeric reserve in mammalian cells.