SALL4, a Stem Cell Factor, Affects the Side Population by Regulation of the ATP-Binding Cassette Drug Transport Genes

Abstract

Our previous work shows that the stem cell factor SALL4 plays a central role in embryonic and leukemic stem cells. In this study, we report that SALL4 expression was higher in drug resistant primary acute myeloid leukemic patients than those from drug-responsive cases. In addition, while overexpression of SALL4 led to drug resistance in cell lines, cells with decreased SALL4 expression were more sensitive to drug treatments than the parental cells. This led to our investigation of the implication of SALL4 in drug resistance and its role in side population (SP) cancer stem cells. SALL4 expression was higher in SP cells compared to non-SP cells by 2–4 fold in various malignant hematopoietic cell lines. Knocking down of SALL4 in isolated SP cells resulted in a reduction of SP cells, indicating that SALL4 is required for their self-renewal. The SP phenotype is known to be mediated by members of the ATP-binding cassette (ABC) drug transport protein family, such as ABCG2 and ABCA3. Using chromatin-immunoprecipitation (ChIP), quantitative reverse transcription polymerase chain reaction (qRT-PCR) and electrophoretic mobility shift assay(EMSA), we demonstrated that SALL4 was able to bind to the promoter region of ABCA3 and activate its expression while regulating the expression of ABCG2 indirectly. Furthermore, SALL4 expression was positively correlated to those of ABCG2 and ABCA3 in primary leukemic patient samples. Taken together, our results suggest a novel role for SALL4 in drug sensitivity, at least in part through the maintenance of SP cells, and therefore may be responsible for drug-resistance in leukemia. We are the first to demonstrate a direct link between stem cell factor SALL4, SP and drug resistance in leukemia.

Significance of SALL4 as a drug-resistant factor in lung cancer

“…A previous study has shown that the drug transporters ABCG2 and ABCA3 can be induced by SALL4 in leukemic cells (21). however, our microarray analysis using SALL4-overexpressed lung cancer cells did not reveal any alteration in the expression levels of either of these mRNAs (data not shown).…”

“…SALL4 is already highly expressed at the early-stage ⅠA, especially in lung cancer, suggesting that it has an essential role in carcinogenesis (20). SALL4 expression may characterize a feature of drug resistance, which has been observed in the stem cell population; evidence for this hypothesis has been demonstrated in a recent study using leukemic cells (21). however, the role of SALL4 in drug resistance has not yet been reported in other cancers.…”

Identification of the nuclear localization signal of SALL4B, a stem cell transcription factor

“…13,16,17 For example, SALL4 expression is correlated with a poor prognosis in AML patients. 18,19 SALL4 is also found to be a biomarker for a subclass of hepatocellular carcinoma with an aggressive phenotype. 20 Through physical and/or functional interaction with OCT4, SOX2, and NANOG, SALL4 plays an essential role in maintaining pluripotency and self-renewal of embryonic stem cells and hematopoietic stem cells.…”

“…20 Through physical and/or functional interaction with OCT4, SOX2, and NANOG, SALL4 plays an essential role in maintaining pluripotency and self-renewal of embryonic stem cells and hematopoietic stem cells. 4,5,19,21,22 SALL4 positively regulates OCT4 gene expression through binding to the conserved regulatory region of the OCT4 promoter. 5,11 On the other hand, SALL4…”

A SALL4/MLL/HOXA9 pathway in murine and human myeloid leukemogenesis

“…Moreover, SALL4 expression correlates with disease progression in human chronic myeloid leukemia (11). Its expression in AML patients correlates with treatment status (12). Further exploration revealed the role of SALL4 in drug resistance, in which SALL4 was involved in the maintenance of side population (SP) cells by regulating ATP-binding cassette drug transport genes (12).…”