Circ_0000915 reduced propranolol sensitivity of HemSCs
In an attempt to explore the effects of Circ_0000915 on propranolol resistance of HemSCs, two specific siRNAs against Circ_0000915 (si-Circ#1 and si-Circ#2) were transfected into HemSCs to knockdown the endogenic Circ_0000915 (p = 0.0013 for si-Circ#1 and p = 0.0010 for si-Circ#2, Fig. 1A). CCK-8 assay showed silence of Circ_0000915 significantly decreased cell viability of HemSCs with the treatment of indicated dose of propranolol (p < 0.0001 for si-Circ#1 and p = 0.0027 for si-Circ#2, Fig. 1B), IC50 values of propranolol obviously reduced in Circ_0000915-depleted HemSCs as compared with the negative control counterpart (p = 0.0021 for si-Circ#1 and p = 0.0034 for si-Circ#2, Fig. 1C). Knockdown of Circ_0000915 inhibited the expression of proliferative-related markers (Cyclin D1 and PCNA) at both mRNA (Cyclin D1: p = 0.0074 for si-Circ#1 and p = 0.0026 for si-Circ#2; PCNA: p = 0.0053 for si-Circ#1 and p = 0.0158 for si-Circ#2, Fig. 1D) and protein levels (Fig. 1E). Moreover, cell apoptosis of HemSCs induced by propranolol exposure was enhanced with Circ_0000915 ablation, as determined by Caspase-3 activity assay (p = 0.0027 for si-Circ#1 and p = 0.0067 for si-Circ#2, Fig. 1F) and Annexin-V/PI double staining assay (p = 0.0099 for si-Circ#1 and p = 0.0098 for si-Circ#2, Fig. 1G). Furthermore, hemangioma derived from Circ_0000915-depleted HemSCs exhibited smaller mean volume (p < 0.0001 for si-Circ#1 and p < 0.0001 for si-Circ#2, Fig. 1H and p = 0.0004 for si-Circ#1 and p = 0.0007 for si-Circ#2, Fig. 1I) and lower weight (p = 0.0018 for si-Circ#1 and p = 0.0074 for si-Circ#2, Fig. 1J).
On the other hand, gain-of-function experiments were subsequently conducted to verify the effects of Circ_0000915 overexpression on propranolol sensitivity of HemSCs. We first confirmed the transfection efficiency of Circ_0000915 plasmid (Circ-oe) in HemSCs by qRT-PCR (p = 0.0027, Fig. 2A). Overexpression of Circ_0000915 promoted cell viability of HemSCs with the treatment of indicated dose of propranolol (p = 0.0148, Fig. 2B), increased IC50 values of propranolol (p = 0.0054, Fig. 2C) and enhanced the expression of Cyclin D1 and PCNA (Cyclin D1: p = 0.0048; PCNA: p = 0.0050, Fig. 2D, E). Forcing expression of Circ_0000915 decreased cell apoptosis of HemSCs treated with propranolol (p = 0.0050, Fig. 2F and p = 0.0123, Fig. 2G). In murine hemangioma model, bigger mean tumor volume (p < 0.0001, Fig. 2H and p = 0.0011, Fig. 2I) and higher tumor weight (p = 0.0034, Fig. 2J) were observed in hemangioma derived from Circ_0000915-overexpressing HemSCs. Taken together, these results suggested Circ_0000915 decreased propranolol sensitivity of HemSCs both in vitro and in vivo.
Circ_0000915 was transcriptionally regulated by IL-6/STAT3 pathway
We subsequently investigated the regulatory mechanisms of Circ_0000915 involved after its functions in IHs were validated. Molecules which mediated expression of Circ_0000915 upstream were first explored. Potential transcription factors targeting promoter of FKBP8 were analyzed via rVista 2.0 (https://rvista.dcode.org/instr_rVISTA.html), STAT3 was included. We thereby further investigated if STAT3 transcriptionally regulated expression of Circ_0000915 indeed. HemSCs was treated with recombinant IL-6 for 24 h, Circ_0000915 was up-regulated in an IL-6 dose-dependent manner (p = 0.0090 for 20 ng/ml and p = 0.0040 for 40 ng/ml, Fig. 3A), and so was the expression level of phosphorylated STAT3 (Fig. 3B). The markedly increased Circ_0000915 induced by IL-6 could be abrogated by transfection with STAT3-specific siRNAs (p = 0.0048 for IL-6 + si-NC and p = 0.0040 for IL-6 + si-STAT3, Fig. 3C and Fig. 3D). Besides, we found that Circ_0000915 promoter-fused luciferase activity was significantly inhibited upon STAT3 knockdown in HemSCs (p = 0.0034, Fig. 3E). Furthermore, we examined direct binding of STAT3 to Circ_0000915 promoter by ChIP assay, and noted significant enrichment of Circ_0000915 promoter fragments after STAT3 immunoprecipitation in HemSCs (p = 0.0066, Fig. 3F). Collectively, these results indicated that Circ_0000915 was transcriptionally regulated by IL-6-activated STAT3.
Circ_0000915 suppressed miR-890 expression by acting as a sponge
We next explored the target molecules of Circ_0000915 downstream, which largely depends on its cellular sublocalization. Firstly, we demonstrated that Circ_0000915 resided predominantly in the cytoplasm in HemSCs by qRT-PCR analysis of nuclear and cytoplasmic RNAs (Fig. 4A), which strengthened the “ceRNA” regulatory pattern [35, 36] of Circ_0000915. We then predicted the potential target miRNAs of Circ_0000915 with Circular RNA Interactome (https://circinteractome.nia.nih.gov/), and miR-890 was chosen for further study due to its functional correlation. Luciferase reporter vectors containing the wild-type (Circ_0000915-wt) and mutant (Circ_0000915-mt) miR-890 binding site within Circ_0000915 were constructed (Fig. 4B), a dramatical inhibition of luciferase activity was detected in HemSCs and HEK 293t cells co-transfected with miR-890 mimics and Circ_0000915-wt, but not with miR-890 mimics and Circ_0000915-mt (p = 0.0036, Fig. 4C and p = 0.0096, Fig. 4D). To further consolidate the direct interaction between Circ_0000915 and miR-890, biotin-RNA pulldown assay was carried out and qRT-PCR analyses revealed Circ_0000915 and miR-890 were both abundantly enriched by biotin-labeled antisense probes against Circ_0000915 (Circ_0000915: p = 0.0013; miR-890: p = 0.0016, Fig. 4E). Besides, expression of miR-890 was significantly increased with Circ_0000915 depletion in HemSCs (p = 0.0019 for si-Circ#1 and p = 0.0041 for si-Circ#2, Fig. 4F). In contrast, Ectopic Circ_0000915 expression dramatically decreased miR-890 expression level in HemSCs (p = 0.0242, Fig. 4G). Collectively, our data indicated Circ_0000915 decreased the expression level of endogenous miR-890 by acting as a “sponge” in HemSCs.
RNF187 was targeted by miR-890
Next, we seek for potential target genes of miR-890 by bioinformatics using targetscan (http://www.targetscan.org/vert_71/). RNF187 was screened as a novel function-related target of miR-890 and the alignment between miR-890 and RNF187 3’UTR was illustrated in Fig. 5A. Then we verified the interplay between miR-890 and RNF187. Firstly, we found RNF187 mRNA was significantly enriched in the complex pulled down by biotinylated miR-890 mimics (Biotin-miR-890) compared with biotinylated negative control mimics (Biotin-miR-NC), as determined by biotin miRNA pull-down assay (p = 0.0001, Fig. 5B). Secondly, luciferase activity driven by the reporter vector containing wild-type RNF187 3’UTR (RNF187 3’UTR-wt) was notably suppressed in both HemSCs and HEK 293t cells co-transfected with miR-890 mimics, which was completely abolished when co-transfected with reporter vector containing mutant RNF187 3’UTR (RNF187 3’UTR-mt) and miR-890 mimics (p = 0.0072, Fig. 5C and p = 0.0099, Fig. 5D). Furthermore, as shown in Fig. 5E–G, both mRNA and protein levels decreased in HemSCs after forced expression of miR-890 (p = 0.0076, Fig. 5E, G) and increased after miR-890 inhibition (p = 0.0010, Fig. 5F, G). Moreover, in accordance with the competitive regulatory effect of Circ_0000915 on miR-890, expression of RNF187 markedly down-regulated in the Circ_0000915-silenced HemSCs (p = 0.0043 for si-Circ#1 and p = 0.0009 for si-Circ#2, Fig. 5H, J), and up-regulated in HemSCs with overexpression of Circ_0000915 (p = 0.0012, Fig. 5I, J), at both mRNA and protein levels. In summary, these results indicated Circ_0000915 sponged miR-890 and alleviated its inhibitory effect on RNF187 expression in HemSCs.
We consequently explored if RNF187 mediated propranolol sensitivity of HemSCs. Endogenic RNF187 in HemSCs was significantly silenced with transfection of specific siRNAs against RNF187 (p = 0.0192 for si-RNF187#1 and p = 0.0158 for si-RNF187#2, Fig. 6A, B). In line with Circ_0000915, knockdown of RNF187 obviously impaired cell viability of HemSCs with the treatment of indicated dose of propranolol, and IC50 values of propranolol remarkably decreased in RNF187-depleted HemSCs as compared with the negative control counterpart, as determined by CCK-8 assay (p < 0.0001 for si-RNF187#1 and p < 0.0001 for si-RNF187#2, Fig. 6C and p = 0.0072 for si-RNF187#1 and p = 0.0058 for si-RNF187#2, Fig. 6D). Moreover, depletion of RNF187 observably decreased both mRNA (Cyclin D1: p = 0.0202 for si-RNF187#1 and p = 0.0212 for si-RNF187#2; PCNA: p = 0.0360 for si-RNF187#1 and p = 0.0094 for si-RNF187#2, Fig. 6E) and protein levels (Fig. 6F) of proliferative-related markers (Cyclin D1 and PCNA). Furthermore, RNF187 ablation promoted cell apoptosis rate of HemSCs induced by propranolol exposure. (p = 0.0184 for si-RNF187#1 and p = 0.0084 for si-RNF187#2, Fig. 6G and p = 0.0166 for si-RNF187#1 and p = 0.0167 for si-RNF187#2, Fig. 6H). Collectively, these data suggested RNF187 positively mediated propranolol resistance of HemSCs.
Circ_0000915 mediated propranolol sensitivity of HemSCs via miR-890/RNF187 axis
We have confirmed Circ_0000915 decreased propranolol sensitivity of HemSCs, but the relevance of the Circ_0000915/miR-890/RNF187 axis in propranolol resistance of HemSCs remained elusive. It was observed that the increased miR-890 and decreased RNF187 caused by Circ_0000915 depletion were obviously reverted following co-transfection with miR-890-inhibitor in HemSCs (miR-890: p = 0.0052 for si-Circ#1 + NC-inhibitor and p = 0.0037 for si-Circ#1 + miR-890-inhibitor; RNF187: p = 0.0068 for si-Circ#1 + NC-inhibitor and p = 0.0002 for si-Circ#1 + miR-890-inhibitor, Fig. 7A, B). Functionally, CCK-8 assay showed Circ_0000915 knockdown-induced reduced proliferation (p < 0.0001 for si-Circ#1 + NC-inhibitor and p < 0.0001 for si-Circ#1 + miR-890-inhibitor, Fig. 7C) and IC50 value of propranolol (p = 0.0028 for si-Circ#1 + NC-inhibitor and p = 0.0035 for si-Circ#1 + miR-890-inhibitor, Fig. 7D) in HemSCs were dramatically abolished by miR-890 depletion. Expression of proliferative-related markers (Cyclin D1 and PCNA) in HemSCs were markedly decreased after Circ_0000915 depletion and subsequently increased by miR-890 co-transfection (Cyclin D1: p = 0.0084 for si-Circ#1 + NC-inhibitor and p = 0.0020 for si-Circ#1 + miR-890-inhibitor; PCNA: p = 0.0060 for si-Circ#1 + NC-inhibitor and p = 0.0001 for si-Circ#1 + miR-890-inhibitor, Fig. 7E, F). In addition, Caspase-3 activity assay and Annexin-V/PI double staining assay revealed miR-890 ablation inhibited the apoptosis-promoting effect of Circ_0000915 knockdown in HemSCs with the treatment of propranolol (p = 0.0011 for si-Circ#1 + NC-inhibitor and p = 0.0021 for si-Circ#1 + miR-890-inhibitor, Fig. 7G and p = 0.0102 for si-Circ#1 + NC-inhibitor and p = 0.0164 for si-Circ#1 + miR-890-inhibitor, Fig. 7H). Taken together, these results demonstrated that Circ_0000915 decreased propranolol sensitivity of HemSCs via regulating miR-890/RNF187 pathway.
Circ_0000915, miR-890 and RNF187 were dysregulated and associated with propranolol resistance in hemangiomas
To strengthen the clinical significance of our study, 50 normal skin tissues and IH tissues in proliferative phase were collected. Expression levels of Circ_0000915, miR-890 and RNF187 in these samples were examined by qRT-PCR. Circ_0000915 was significantly up-regulated in IH tissues as compared with normal skin tissues (p = 0.0095, Fig. 8A). Furthermore, expression levels of Circ_0000915 were much higher in IH tissues from propranolol-resistant patients than propranolol-sensitive ones (p = 0.0001, Fig. 8B). Conversely, miR-890 exhibited an opposite expression pattern in IH tissues (p = 0.0004, Fig. 8C) and was negatively associated with propranolol resistance of patients with IH (p = 0.0017, Fig. 8D). In line with Circ_0000915, elevated RNF187 was also observed in IH tissues (p = 0.0020, Fig. 8E) and positively correlated to propranolol resistance of patients with IHs (p = 0.0005, Fig. 8F). Collectively, these data indicated Circ_0000915 and RNF187 may play oncogenic roles and promote propranolol resistance in IHs, while miR-890 may act as a tumor suppressor and inhibit propranolol resistance in IHs.