Relapse and drug resistance are the main causes of mortality in patients with small‑cell lung cancer (SCLC). Intratumoral heterogeneity (ITH) is a key biological mechanism that leads to relapse and drug resistance. Phenotypic plasticity is an important factor that leads to ITH in SCLC, although its mechanisms and key regulatory factors remain to be elucidated. In the present study, cell proliferation and cell switch assay were measured using trypan blue. Alamar Blue was used to test drug sensitivity. Differential genes were screened by RNA sequencing. Reverse transcription‑quantitative PCR and western blotting were performed to assess the expressions of CSF2/p‑STAT3/MYC pathway related molecules, neuroendocrine (NE)/non‑neuroendocrine (non‑NE), transcription factors and drug‑related targets. The present study found that SCLC cell line NCI‑H69 exhibited adherent (H69A) and suspensive (H69S) phenotypes, which could switch back and forth. The two phenotypic cells had significant differences in cellular NE and non‑NE characteristics, drug sensitivity and expression of drug‑related targets. RNA sequencing showed that granulocyte‑macrophage colony‑stimulating factor [i.e., colony‑stimulating factor 2 (CSF2)] was the main differentially expressed gene between the two phenotypes and that H69A cells highly expressed CSF2. The inhibition of CSF2 promoted the transformation from H69A to H69S, increased drug sensitivity and NE marker expression and decreased the non‑NE marker expression in H69A. The STRING, Pathway Commons and Reactome databases showed a potential regulatory relationship between CSF2 and phosphorylated signal transducer and activator of transcription 3 (p‑STAT3)/MYC. p‑STAT3 and MYC expression was higher in H69A cells than in H69S cells and CSF2 silencing inhibited their expression. Taken together, these results indicated that CSF2 may regulate the phenotypic plasticity of SCLC through the phosphorylated STAT3/MYC pathway, thereby limiting the transformation between cell clones with different phenotypes and changing the sensitivity of specific cell clones to targeted drugs. Targeting CSF2 may be a potential therapeutic strategy to overcome drug resistance in SCLC treatment by influencing ITH.
Keywords: drug resistance; granulocyte‑macrophage colony‑stimulating factor; intratumoral heterogeneity; phenotypic plasticity; small cell lung cancer.