Novel View on Stem Cell Compartment and Aging - a Number of Pluripotent Very Small Embryonic Like Stem Cells and Hematopoietic Stem Cells in Murine Bone Marrow Is High in Several Long-Living and Reduced in Short-Living Murine Strains and Unexpectedly Inversely Correlates in These Animals with Plasma Growth Hormone/Insulin-Like Growth Factor-1 Level

It is known that an increase in caloric intake leads to increase in plasma growth hormone (GH) level that subsequently induces secretion of insulin-like growth factor-1 (IGF-1) from liver, what leads to accelerated aging (Nature 2010;464:504). On the other hand, caloric restriction and a resulting decrease in plasma IGF-1 level has the opposite effect and extends lifespan. It also known that adult tissues contain a population of pluripotent very small embryonic like stem cells (VSELs) that play as postulated an important role in rejuvenation of long term hematopoietic stem cells (LT-HSCs) in bone marrow (BM) Leukemia 2011; doi:10.1038/leu.2011.73, Exp. Hematology 2011;39:225–237). As we observed previously, the number of these cells in murine BM decreases with age and VSELs are kept quiescent in BM and protected from premature depletion by erasure of the somatic imprint in differentially methylated regions (DMRs) of some paternally imprinted genes involved in insulin/insulin growth factors signaling (IIS) such as e.g., Igf2-H19 and RasGRF1 (Leukemia 2009;23:2042).

To explain and connect these phenomena together, we hypothesized that prolonged insulin/insulin growth factors signaling (IIS) prematurely depletes VSELs from the adult tissues and in BM may negatively impact on population of HSCs.

The number of VSELs and HSCs in long living murine strains with inborn low level of circulating IGF-1 (Laron- and Ames- dwarfs) as well as in short living mice with high levels of circulating IGF-1 (e.g., transgenic mice that overexpress bovine growth hormone; bGH) was evaluated by FACS. VSELs were isolated and epigenetic status of genes regulating pluripotency (e.g., Oct-4) as well as imprinted genes regulating IIS was evaluated by employing bisulfate modification of DNA followed by sequencing and by COBRE assay. We also challenged long living mice with low IGF-1 plasma level by daily injections of recombinant GH or IGF-1.

We found that the number of VSELs and HSCs residing in BM inversely correlates with plasma GH/IGF-1 level. To support this, mice with low circulating plasma IGF-1 levels (Laron- and Ames-dwarf mice) have higher numbers of VSELs and HSCs in BM that, in contrast to aged-matched normal littermates, are maintained at high levels even into advanced age. The analysis of molecular signature of VSELs in these animals revealed prolonged retention of hypomethylation in the DMRs within the Igf2-H19 and RasGRF1 loci, which attenuates IIS signaling in these cells. The number of VSELs, however, decreased in these animals after prolonged treatment with GH or recombinant IGF-I. Conversely, mice with elevated IGF-I level in plasma due to expression of the GH transgene or normal wild type mice injected for a sustained period with recombinant GH both exhibit significant decreases in the number of VSELs and HSCs in BM compared to control animals. These decreases were paralleled by epigenetic changes in Igf2-H19 and RasGRF1 loci in which DMRs became hypermethylated over time. These changes in methylation lead to increases in IGF-2 and RasGRF1 expression and may explain why GH transgenic mice have an increase in IIS that leads to shortening of life span in these animals.

Our data shed new light on the relationships between senescence, GH/IGF-1 level, prolonged IIS, and number of VSELs and LT-HSCs. Accordingly, we propose a new paradigm in which a decrease in IIS (e.g., due to caloric restriction that lowers plasma IGF-1 level) may delay the age-dependent elimination of VSELs from adult tissues. By contrast, chronic IIS (e.g., due to chronic high caloric intake and the resulting elevated GH and IGF-1 levels) prematurely depletes VSELs residing in adult organs, which for example in BM leads to a decrease in the number of LT-HSCs. This study also indicates that GH-based anti-aging therapies need careful re-evaluation of their potentially uncontrolled stimulation of VSELs in BM that may lead to development of hematological malignancies. In support of this, elevated GH and IIS lead to hematological malignancies, while, by contrast, Laron dwarf mice and Laron dwarf patients, which have a low plasma IGF-1 levels, do not develop leukemias.

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