The microRNA miR-34a inhibits non-small cell lung cancer (NSCLC) growth and the CD44hi stem-like NSCLC cells

Abstract

Lung cancer is among the most lethal malignancies with a high metastasis and recurrence rate, which is probably due to the existence of lung cancer stem cells (CSCs). CSCs in many tumors including non-small cell lung cancer (NSCLC) have been identified using adhesion molecular CD44, either individually or in combination with other marker(s). MicroRNAs (miRNAs) regulate both normal stem cells and CSCs and dysregulation of miRNAs has been implicated in tumorigenesis. Recently, miR-34a was found to be downregulated in NSCLC cells but the biological functions of miR-34a in regulating NSCLC cell behavior have not been extensively studied. Here we show that transfection of synthetic miR-34a, but not the negative control (NC) miRNA oligonucleotides (oligos) in three NSCLC cell lines, i.e., A549, H460, and H1299, inhibited their holoclone formation, clonogenic expansion, and tumor regeneration in vivo. Furthermore, the lentiviral vector-mediated overexpression of miR-34a in purified CD44hi H460 cells also inhibited tumor outgrowth. In contrast, expression of miR-34a antagomirs (i.e., antisense oligos) in the CD44lo H460 cells promoted tumor development. Our study shows that miR-34a is a negative regulator of the tumorigenic properties of NSCLC cells and CD44hi lung CSCs, and establishes a strong rationale for developing miR-34a as a novel therapeutic agent against NSCLC.

Figure 1. miR-34a inhibits NSCLC cell clonal and clonogenic properties.

(A–C) Clonal assays. Cells transfected with miR-34a or miR-NC oligos (33 nM) were plated in triplicate at 50 cells/well in 6-well plates. The experiment was terminated at 12 d and wells were Giemsa-stained (A). Shown in B are representative images. Results shown in A and B were representative of two independent experiments. (C) Quantitative presentation of results in A. Bars represent the mean ± S.D. (D–E) Clonogenic assays in MC. A total of 1,000 cells per well were plated for clonogenic assay. Photos were taken on d 15 after plating and shown in D are representative fields. (E) Quantitative presentation of results in D. Bars represent the mean ± S.D.

Figure 2. miR-34a inhibits NSCLC tumor growth.

(A) NSCLC cells freshly transfected with miR-34a oligos showed miR-34a levels several orders of magnitude higher than those transfected with miR-NC oligos. The indicated NSCLC cells were transfected with miR-34a or miR-NC oligos, and 48 h later, were harvested and used in tumor experiments (below) whereas a small number of cells were set aside and used in qRT-PCR measurement of miR-34a mRNA levels. Shown are the mean miR-34a levels (in log scale; n = 2) in miR-34a transfected cells relative to those in the miR-NC transfected cells (actual mean values indicated in the bars). (B–D) miR-34a oligo transfection inhibited A549 tumor growth. Indicated are tumor incidence (tumors developed/numbers of injections; %), harvest time (including actual injection and termination dates), mean tumor weight (in grams), and the P values for tumor weights. Gross tumor images are not to the same scale. (E–G) miR-34a oligo transfection inhibited H460 tumor growth. (H–L) miR-34a oligo transfection inhibited H1299 tumor growth.

Figure 3. Functional assay (ALDH) and analysis of CD44 and CD133 expression using flow cytometry.

(Top) The Aldefluor assay in 3 NSCLC cells. DEAB-treated samples served as negative controls. ∼1–2% H460 and H1299 cells were Aldefluor-positive whereas >90% of A549 cells were Aldefluor-positive. (Middle) Representative flow cytometry profile of CD44 (FITC) expression in 3 NSCLC cells. Virtually 100% of A549, H460, and H1299 cells were CD44-positive (mean values being 97.2%, 99.3%, and 99.2%, respectively; n = 3). (Bottom) Flow cytometry analysis of CD133 (PE) expression in 3NSCLC cells. There was almost no expression of CD133 in these three NSCLC cell lines.

Figure 4. Effects of miR-34a on the growth of tumors derived from purified CD44^hi or CD44^lo cells.

(A–B) CD44 expression level. (A) Representative diagrams of flow cytometry analysis of CD44 (FITC) expression in H460 cells. (B) CD44 mRNA levels in purified CD44^hi and CD44^lo H460 cells assessed by qRT-PCR. (C, E, F) miR-34a overexpression in purified CD44^hi H460 cells by lentiviral infection inhibited tumor regeneration. (C) Indicated are tumor incidence (tumors developed/numbers of injections; %), harvest time (including actual injection and termination dates), mean tumor weight (in grams, F), and the P values for tumor weights. Gross tumor images are not to the same scale. (E) The tumor growth curve. (D, G–J) Anti-miR-34a promoted tumor growth of purified CD44^lo H460 cells. (D) Indicated are tumor incidence (tumors developed/numbers of injections; %), harvest time (including actual injection and termination dates), mean tumor weight (in grams, G), and the P values for tumor weights. Gross tumor images are not to the same scale. (H–J) The tumor growth curve at three different cell doses.