The RNA-binding protein, LIN28B, has been implicated in various solid tumors and hematological malignancies, including AML [
8]. Emerging evidence have suggested LIN28B in contributing to the transformation of cancer stem cells [
27,
28]. The most well-studied molecular mechanism of LIN28B in oncogenesis is its ability in regulating let-7 miRNA biogenesis through a TUTase-independent mechanism by sequestering pri-let-7 miRNAs in the nucleoli [
29]. The repression of members of let-7 miRNAs could well elicit the de-repression of several oncogenes such as K-Ras, c-Myc, and HMGA2 [
30‐
32], in turn, promoting disease progression.
This study shows that by modulating LIN28B expression could lead to significant changes in cell proliferation and cell cycle. Notably, LIN28B overexpression renders AML cells growth independent of cytokines and enhances tumorigenicity in vivo, suggesting the role of LIN28B in more aggressive tumor phenotype. Reducing LIN28B not only causes AML cells less actively replicating but also induces G
2/M cell cycle arrest. Interestingly, it has been reported that overexpression of let-7a miRNA could induce a prominent G
2/M phase arrest in prostate cancer [
33].
Numerous human malignancies such as ovarian cancer, lung cancer, hepatocellular carcinoma, and melanoma have been shown to have repression of multiple members of the let-7 family of miRNAs, thus promoting oncogenesis by depressing targets such as c-Myc, Ras, and HMGA2 [
10,
34]. In this study, IGF2BP1 has been increased by LIN28B through repression of let-7 miRNA. IGF2BP1 is a novel target gene of let-7 miRNAs. The IGF2BP1 belongs to a conserved family of RNA binding, oncofetal proteins that consist of other isoforms, IGF2BP2, and IGF2BP3 [
35]. IGF2BP1 and IGF2BP3 are re-expressed in several aggressive cancers in colorectal and lymphomas with a high incidence of more than 70% [
35]. In particularly, IGF2BP1 has been shown to prevent cleavage of MYC mRNA from endonucleases by binding to the CRD (coding region stability determinant) in the MYC open reading frame [
36]. Furthermore, overexpression of IGF2BP1 is found to be associated with increased c-MYC and RAS expressions, while loss of IGF2BP1 could induce caspase-3 and PARP-mediated apoptosis in colorectal cancer cell lines [
36]. Hence, the regulation of IGF2BP1 plays an important role in LIN28B-mediated oncogenesis. As let-7 miRNA family concurrently regulates several pivotal oncogenic pathways, there is of great interest in re-expression of let-7 miRNAs as a therapeutic option for cancer. Strategies using viral vector, to overexpress let-7 miRNAs or by introducing artificial double-stranded miRNA (mimic of let-7 miRNAs), hold great promising for novel anti-AML therapy.
Studies in stem cell, growth and metabolic disorders, and cancers uncover LIN28/LIN28B as a central regulator of cellular metabolism through let-7-dependent or let-7-independent manner, where LIN28/LIN28B directly binds mRNA of glycolysis and mitochondrial OxPhos enzymes and enhances their translation [
37‐
40]. One of the hallmarks of cancer cells is rapid growth and proliferation by evading suppression signaling [
40]. In order to sustain high growth rate, cancer cells often demand more amino acid, nucleotide, ATP, etc., by reprogramming their metabolic pathways [
41]. Our gene expression profiling results uncover that LIN28B plays a significant role in wide ranges of metabolic processes of AML cells, involving amino acids, oxoacid, organic acids, carboxylic acids, and neutral amino acids transportation. These organic compounds are mainly utilized in essential metabolic pathways such as glycolysis, tricarboxylic acid (TCA) cycle, and gluconeogenesis [
42,
43]. These basic molecules are essential for increased DNA replication, RNA production, and protein synthesis in the LIN28B overexpressing cells. In addition to uncontrolled growth, the other characteristic of AML cells is arrested differentiation. A recent report demonstrates that LIN28/LIN28B regulates stem cell metabolism and facilitate conversion from naive to primed pluripotency [
37]. So, it is reasonable to suppose that these altered metabolic pathways induced by LIN28B also contributes to keep AML cells in differentiation blocked states. Particularly, it is worth of highlighting the top most downregulated genes: phosphoserine aminotransferase 1 (PSAT1), phosphoglycerate dehydrogenase (PHGDH), asparagine synthetase (ASNS), serine methyl transferase2 (SHMT2), and cystine-glutamate transporter, SLC7A11 in the LIN28B knockdown cells. These genes have been demonstrated to promote tumorigenesis in a variety type of cancers and associated with poor prognosis [
44]. PSAT1, PHGDH, and SHMT2 are three key enzymes in the serine and glycine pathway and enhanced serine and glycine biosynthesis provides sufficient precursors for the synthesis of proteins, nucleic acids, and lipids for highly proliferating cancer cells [
45,
46]. Meanwhile, the upregulation of ASNS allows asparagine biosynthesis, making the cancer cells to be less sensitive to asparagine depletion. Hashimoto K et al. demonstrated the importance of decreasing ASNS level for monocytic differentiation in HL-60 cells [
47]. On the other hand, Huang. Y et. al showed that the overexpression of SLC7A11 not only mediates cellular uptake of
l-alanosine but also confers glutathione-mediated chemoresistance [
48]. Thus, these findings open new opportunities for developing better biomarkers for predicting the efficacy of anti-leukemic treatments and selecting optimal drug therapies such as using
l-alanosine or other amino acid-related drugs as novel therapeutic strategies like differentiation therapy.