Kruppel-Like Factor 4 Upregulates p21 and Downregulates Proliferation of Human and Mouse HSPCs, but Is Not Essential for Mouse HSPC Repopulation.

Several Kruppel-like factor family members, including KLF1, KLF2, KLF3, and KLF6 have pivotal roles in hematopoiesis. Experiments in zebrafish have suggested that KLF4 may play a similar role. Here we found that enforced expression of KLF4 in hematopoietic cells induced cell cycle arrest without triggering apoptosis. Based on the high levels of expression of KLF4 in mouse and human hematopoietic stem-progenitor cells (HSPCs), we hypothesized and demonstrated that KLF4 regulates proliferation of these cells through regulation of p21^cip1/waf1 (p21). Nevertheless, KLF4^−/− mouse fetal liver cells had normal numbers of all mature lineages and provided radioprotection, similar to wild type (wt) controls. Furthermore, in long-term competitive repopulation assays, KLF4^−/− mouse HSPCs demonstrated hematopoietic potency equivalent to wt. We found that KLF2 is expressed at higher levels than KLF4 in mouse HSPCs and is a more potent activator of p21, suggesting that KLF2 (and/or other KLF family members) may play a compensatory role in KLF4^−/− HSPCs. Thus, although is not essential for their normal development and function, KLF4 expression is sufficient to induce p21-mediated cell cycle arrest in hematopoietic cells.