Targeting Nicotinamide N-Methyltransferase and miR-449a in EGFR-TKI-Resistant Non-Small-Cell Lung Cancer Cells

Abstract

Epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) are used clinically as target therapies for lung cancer patients, but the occurrence of acquired drug resistance limits their efficacy. Nicotinamide N-methyltransferase (NNMT), a cancer-associated metabolic enzyme, is commonly overexpressed in various human tumors. Emerging evidence also suggests a crucial loss of function of microRNAs (miRNAs) in modulating tumor progression in response to standard therapies. However, their precise roles in regulating the development of drug-resistant tumorigenesis are still poorly understood. Herein, we established EGFR-TKI-resistant non-small-cell lung cancer (NSCLC) models and observed a negative correlation between the expression levels of NNMT and miR-449a in tumor cells. Additionally, knockdown of NNMT suppressed p-Akt and tumorigenesis, while re-expression of miR-449a induced phosphatase and tensin homolog (PTEN), and inhibited tumor growth. Furthermore, yuanhuadine, an antitumor agent, significantly upregulated miR-449a levels while critically suppressing NNMT expression. These findings suggest a novel therapeutic approach for overcoming EGFR-TKI resistance to NSCLC treatment.

Keywords: EGFR-TKI; NNMT; NSCLC; PTEN/PI3K/Akt; docking; gefitinib resistance; methylation; miR-449a; non-small-cell lung cancer; yuanhuadine.

Graphical abstract

Figure 1

NNMT Expression Is Inversely Related to that of miR-449a in gef-Resistant NSCLC Tissues and Cell Lines (A and B) Characterization of the indicated parental and gef-resistant phenotype cell lines (A) or erl-resistant phenotype cell lines for NNMT expression at mRNA levels (B). (C) Characterization of the indicated parental or gef-resistant phenotype tissues for NNMT expression at the mRNA levels. Total RNA was isolated and analyzed by real-time PCR using NNMT-specific primers and normalized to β-actin expression. (D and E) Confirmation of NNMT protein overexpression in gef-resistant cell lines (D) or erl-resistant cancer cell lines (E). (F) Confirmation of NNMT protein overexpression in gef-resistant tissues. The expression of NNMT protein was investigated by western blotting using β-actin as the loading control. (G) Immunohistochemistry of NNMT in tumor tissue sections. Immunohistochemical analysis of NNMT was performed using anti-NNMT antibody in tumor tissue sections. (H–J) Characterization of the indicated parental and gef-resistant cells (H) or erl-resistant cells (J) and tissues (I) for miR-449a expression. miR-449a levels were quantified by TaqMan real-time PCR and normalized to RNU6B. Data are representative of three independent experiments. *p < 0.05; **p < 0.01; ***p < 0.001 by the t test. ns, not significant.

Figure 2

NNMT Stimulates gef-Resistant NSCLC Cell Growth by Targeting miR-449a (A) Gefitinib sensitivity of the indicated gef-resistant phenotype cell lines. Cells were transiently post-transfected with scramble siRNA or NNMT siRNA for 48 hr and then incubated with the indicated concentrations of gef. Cell viability was assessed by the SRB assay. (B) Cell-cycle progression of gef-resistant phenotype cell lines. Cells were transiently transfected with either scramble siRNA or NNMT siRNA for 48 hr. Transfected cells were subjected to FACS analysis. (C) Colony formation of gef-resistant phenotype cell lines. Cells were transiently post-transfected with either scramble siRNA or NNMT siRNA for 48 hr and then cultured with the indicated concentrations of gef and subjected to colony formations assays. (D) Effects of miR-449a mimic on the miR-449a expression in gef-resistant cell lines. The indicated gef-resistant cell lines were cultured in six-well plates and then transfected with NC miRNA or miR-449a for 48 hr (50 pmol/well). The levels of miR-449a expression were determined by TaqMan real-time PCR with specific primers for mature miR-449a. Samples were normalized to RNU6B. (E) Gefitinib sensitivity of the indicated gef-resistant phenotype cell lines. Cells were transiently post-transfected with NC miRNA or miR-449a for 48 hr and then incubated with the indicated concentrations of gef. Cell viability was assessed by the SRB assay. (F) Bioinformatics analysis. The possible binding sites between NNMT and hsa-miR-449a were determined using a bioinformatics tool (http://www.targetscan.org/vert_71/). (G) Effects of siNNMTs on miR-449a expression by transfection with siNNMTs [N(1): siNNMT #1; N(2): siNNMT #2; N(3): siNNMT #3]. (H) Effects of siNNMT and/or miR-449a on cell proliferation by transfection with miR-449a and/or siNNMT in PC9-Gef cells. (I) Effects of miR-449a and/or siNNMT on tumor growth in PC9-Gef cells. (J) Immunohistochemistry of Ki-67 in tumor tissue sections. *p < 0.05; **p < 0.01; ***p < 0.001 by the t test.

Figure 3

miR-449a Modulates the Growth of gef-Resistant NSCLC Cells through Induction of the PTEN Pathway (A) Characterization of the indicated parental or drug-resistant phenotype cell lines for PTEN expression at protein levels. (B) Immunohistochemistry of PTEN in tumor tissue sections. Immunohistochemical analysis of PTEN was performed using anti-PTEN antibody in tumor tissue sections. (C and D) Effects of 5-Aza on the levels of NNMT (C) and miR-449a (D) expression. The drug-resistant cell lines were incubated in the presence or absence of 5-Aza (10 or 20 μM) for 24 hr. (E) Reversal of PTEN promoter methylation by miR-449a in gef-resistant cells. Top, methylation-specific PCR (MSP) analysis of the PTEN gene and map of the promoter region of the PTEN gene. Bottom, methylation-specific PCR analysis of the PTEN gene in H292-Gef cells. Bottom right, Cells were transfected with or without miR-449a (50 pmol/well) and furthered analyzed by using EpiTect MethyLight PCR kit as described in the Materials and Methods. Bottom left, Results of MSP analysis of PTEN gene in H292-Gef cells after post-transfection with or without miR-449a mimic and miR-449a inhibitor (50 pmol/well). M and U represent PCR products of methylated and unmethylated alleles, respectively. (F) Effects of miR-449a on the protein expression of PTEN. The gef-resistant cells were transfected with NC miR-449a or miR-449a for 48 hr. PTEN protein levels were determined from cell lysates by immunoblotting. (G) Effects of PTEN siRNA on the Akt pathway. The drug-resistant cell lines were incubated with scramble siRNA or PTEN siRNA for 48 hr and then analyzed by western blotting. (H) Effects of PI3K inhibitor on the Akt pathway. Cells were incubated with PI3K inhibitor (LY294002, 20 μM) for 24 hr and then analyzed by immunoblotting. (I) Effects of PI3K inhibitor on miR-449a. The gef-resistant cell lines were incubated with LY294002 for 24 hr; then the levels of miR-449a were determined using TaqMan real-time PCR. (J) Effects of NNMT siRNA on the Akt pathway. The gef-resistant cell lines were incubated with scramble siRNA or NNMT siRNA for 48 hr, and then the indicated protein levels were further analyzed by immunoblotting. (K) Immunohistochemistry of p-Akt in tumor tissue sections. Immunohistochemical analysis of p-Akt was performed using anti-p-Akt antibody in tumor tissue sections with or without NNMT siRNA. (L) Effects of NNMT plasmid on the Akt pathway. NNMT plasmid was transfected to the indicated parental cells within 48 hr; then further immunoblotting was performed to detect protein expressions. *p < 0.05; **p < 0.01; ***p < 0.001 by the t test. ns, not significant.

Figure 4

YD Regulates miR-449a and NNMT Overexpression in gef-Resistant NSCLC Cells (A) Effects of YD on NNMT and the related protein expression. NNMT, p-Akt, Akt, and β-actin protein levels in cell lysates were assayed by immunoblotting after 24 hr of YD treatment. (B) Effects of YD on NNMT mRNA expression. NNMT mRNA expression was evaluated by real-time PCR after 24 hr of YD treatment. (C) Effects of YD on miR-449a expression. miR-449a levels were further analyzed using the TaqMan PCR kit after 24 hr of YD treatment. (D) The indicated cells were implanted subcutaneously into the flanks of BALB/c nude mice. Dosing of YD (1 mg/kg body weight) or gef (10 mg/kg body weight) was initiated when the tumor volumes reached approximately 400 mm³ for H1993-Gef cells or 150 mm³ for PC9-Gef cells. YD and gef were administrated orally once daily for 21 days continuously. The tumor volumes were measured every 4 days (n = 5 mice per group). The error bars represent the means ± SD. (E) H1993-Gef and PC9-Gef tumors were excised from animals on day 21 after treatment, and tumor weights were calculated. (F) Protein expression levels of NNMT. The proteins from cell lysates (right panel) or small portions of tumors from each group were homogenized in Complete Lysis Buffer (Active Motif) (left panel) for immunoblotting. β-Actin was used as an internal standard. (G and H) Immunohistochemistry of NNMT in tumor tissue sections of H292 and H292-Gef (G) or H1993-Gef (H). Immunohistochemical analysis of NNMT was performed using anti-NNMT antibody in each of the indicated groups. (I and J) Relative expression of NNMT and miRNA-449a in tumor transcripts of the indicated xenograft tissues. The levels of NNMT (I) or miR-449a (J) expression were analyzed by real-time PCR or with the TaqMan PCR kit using specific primers, respectively. *p < 0.05; **p < 0.01; ***p < 0.001 by the t test.

Figure 5

YD Interacts with the Binding Site of NNMT (A) In vitro activity of purified NNMT was assessed in the presence of increasing concentrations of YD. Data are reported as a percentage of NNMT activity with respect to the control. Histograms represent the mean ± SD of three independent experiments. (B) Binding site of hNNMT complexed with S-adenosylhomocysteine (SAH) and nicotinamide (NCA). (C) Docked pose of YD with the highest binding score. Hydrogen bonds within the binding site are shown as yellow lines. (D) Residues forming hydrophobic interactions with YD are shown as gray lines. Among them, previously reported conserved aromatic residues (Phe15, Tyr20, Tyr86, and Tyr204) are presented as spheres. *p < 0.05.