AbstractIt has been known that 12-O-tetradecanoyl phorbol-13-acetate-inducible sequence 21 (TIS21), ortholog of human B-cell translocation gene 2, regulates expansions of stage-specific thymocytes and hematopoietic progenitors. In the present study, lineage-negative (Lin−)/stem cell antigen-1-positive (Sca-1+)/c-Kit+ (LSK) cell content was significantly elevated in bone marrow (BM) of TIS21-knockout (TIS21−/−) female mice, suggesting 17β-estradiol (E2)-regulated progenitor expansion. E2 induced DNA synthesis and cell proliferation of mouse embryonic fibroblasts (MEFs) isolated from TIS21−/− mice, but not wild type (WT). In contrast to WT, E2 failed to activate protein kinase B (Akt) in the TIS21−/− MEFs, independent of extracellular signal-regulated kinase 1/2 (Erk1/2) activation. Despite attenuation of Akt activation, mammalian target of rapamycin (mTOR) was constitutively activated in the TIS21−/− MEFs. Furthermore, mitogen-activated protein kinase 1/2 inhibitor or knockdown of Erk1 could restore activation of Akt and downregulate mTOR. Immunoprecipitation showed Akt preferentially bound to phosphorylated Erk1/2 (p-Erk1/2) in TIS21−/− cells, but reconstitution of TIS21 inhibited their interaction. E2-injected TIS21−/− male mice also increased LSK cells in BM. Taken together, expansion of hematopoietic progenitors in TIS21−/− female mice might be through inhibition of Akt activation, and constitutive activation of mTOR via preferential binding of TIS21 to E2-induced p-Erk1/2, compared with that of Akt. Our results suggest that TIS21 plays a pivotal role in maintaining the hematopoietic stem cell compartment and hematopoiesis.Disclosure of potential conflicts of interest is found at the end of this article.