Breast cancer accounts for 30% of female cancers, with an increasing incidence rate of 0.5% per year . Surgery and chemotherapy are the predominant therapy options for breast cancer treatment. However, drug-resistant breast tumor is the main challenge for the current treatment protocols [2, 3]. Drug resistance can be associated with various mechanisms, including increased efflux of drugs, DNA repair, and drug metabolism. Although, the elevated drug efflux by ABC transports has been considered the primary mechanism of drug resistance in cancer . ABC transporters are a family of membrane pumps that hydrolyze ATP and use the energy to transport various substrates across the cell membrane . The overexpression of ABC transporters has decreased the drug concentration inside the cell, producing multidrug resistance (MDR). It is well known that the members of the ABCB (ABCB1), ABCC (ABCC1-6, 10, 12), and ABCG (ABCG2) subfamilies are significant players in the development of cancer drug resistance [5–7].
The genetic mechanisms of ABC transporters upregulation include mutation, chromosomal rearrangement, and gene amplification [3, 8, 9]. Recently, non-coding RNAs (ncRNAs) have been proposed to be involved in transporter-mediated drug resistance in cancer [10, 11]. Accumulating evidence showed that dysregulation of long non-coding RNAs (lncRNAs) could increase the expression of ABC transporters in chemo-resistant cells. The up-regulation of several lncRNAs including XIST, UCA1, FOXD2-AS1, HOTAIR, LUCAT1, FTH1P3, CACS15, KCNQ1OT1, NR2F1-AS1, DANCR, OIP5-AS1, FOXC2-AS1, MALAT1, CASC9, and MRUL has been reported in cancer MDR [12–25]. The regulatory mechanisms of lncRNA are diverse; however, the most common one is that lncRNA competes with the ABC transporter RNA to bind to common miRNAs. Therefore, lncRNAs, as a miRNA sponge, increase the expression of the ABC transporter. This mechanism, known as competing endogenous RNA (ceRNA), has a fundamental role in cancer development . Nevertheless, none of the lncRNAs ever studied in ABC transporter activation are directly related to the ABC transporter genes.
Processed pseudogenes are a group of lncRNAs that evolved from their original genes . Due to the high similarity to the parental gene, pseudogenes regulate parental gene expression via the ceRNA mechanism . The human genome has 22 ABC transporter pseudogenes that nearly one-half of them are transcriptionally active . One of these pseudogenes, ABCC6P1, has co-expression with the parental gene (ABCC6) in various human tissues .
ABCC6 is a glutathione conjugate pump that can induce resistance to chemotherapeutic drugs [30, 31]. ABCC6 and ABCC6P1 have the same regulatory elements, and knockdown of the ABCC6P1 decreases the ABCC6 mRNA expression . Moreover, copy number variation and Q378X mutation co-exist in the ABCC6 and ABCC6P1 genes in patients with pseudoxanthoma elasticum [32, 33]. The expression of ABCC6P1 is upregulated in papillary thyroid cancer, and the knockdown of ABCC6P1 induces cell cycle arrest in cancer cells .
This study hypothesized that increased ABCC6P1 RNA level could increase ABCC6 expression levels and eventually lead to drug resistance. We observed that upregulation of the ABCC6P1 pseudogene increases the ABCC6 mRNA level. Moreover, we demonstrated that ABCC6P1 overexpression increases the drug efflux and survival of the cell.