Downregulation of HOXA1 gene affects small cell lung cancer cell survival and chemoresistance under the regulation of miR-100.

Chemoresistance is often developed in small cell lung cancer (SCLC) patients and leads to poor prognosis. Hox genes, a highly conserved family, play a crucial role in apoptosis, receptor signalling and differentiation. MicroRNAs (miRNAs) have also been shown to play a crucial role in these biological processes by regulating the target genes. Several studies reported that both Hox genes and miRNAs are involved in chemoresistance. The aim of our study is to characterise the clinical significance and functional roles of HOXA1 in SCLC. Expression of HOXA1 was examined in 63 cases of SCLC tissues and 29 cases of blood by immunohistochemistry and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) methods. Multivariate analysis confirmed the prognostic significance of HOXA1 in SCLC patients. Restoration of HOXA1 expression was carried out in SCLC multidrug resistant cell line H69AR and its parental cell line H69 to assess its influence on chemoresistance. Luciferase reporter assay was used to assess HOXA1 as a target of miR-100. The results showed that HOXA1 was expressed in 46% (29/63) of SCLC. Low HOXA1 expression was associated with the poor prognosis of SCLC (P<0.05 by the Fisher's Exact Test) and the shorter survival rate (P<0.001 by the Kaplan-Meier method). HOXA1 expression on both mRNA and protein levels significantly correlated with chemotherapy response. Enforced expression of HOXA1 in resistant H69AR cells led to increased chemosensitivity through increasing cell apoptosis and cell-cycle arrest. Inhibition of HOXA1 expression using HOXA1 siRNA in H69 cells resulted in cell resistance to therapeutic drugs through reducing drug-induced cell apoptosis accompanied with cell cycle arrest. Expression of endogenous miR-100 was significantly elevated in resistant H69AR cells and negatively related with HOXA1 expression. The expression of HOXA1 in SCLC tissues correlated inversely with the expression levels of miR-100. Reporter assays confirmed that miR-100 targeted predicted sites in 3'-untranslated region (3'-UTR) of HOXA1 gene. Our data suggested that HOXA1-mediated SCLC chemoresistance is under the regulation of miR-100. HOXA1 may be a prognostic predictor and potential therapeutic target in human SCLC.