LINC00665 is associated with CAF-induced LN metastasis of BCa.
Lymphangiogenesis contributes to the uncontrolled formation of a dysfunctional lymphatic network with incomplete basement membranes, which is considered the rate-limiting step in the metastasis of malignant cells through LNs23. Therefore, we collected BCa tissues from 228 cases at Sun Yat-Sen Memorial Hospital and subjected them to immunohistochemistry (IHC) analysis for lymphangiogenesis. The results showed that LN metastatic BCa tissues were presented with higher microlymphatic vessel density (MLD) than those without LN metastasis in both intratumoral and peritumoral regions (Fig. 1a). Considering that CAFs are the major stromal population widely implicated in the formation of the premetastatic microenvironment in BCa18, we explored the regulatory effect of CAFs in mediating lymphangiogenesis and LN metastasis. As shown in Fig. 1b, alpha-smooth muscle actin (α-SMA)-indicated CAFs enrichment in TME was significantly increased in LN metastatic BCa tissues compared with those without LN metastasis. Strikingly, double immunostaining followed by correlation analysis revealed that higher CAF infiltration was related positively to more lymphatic vessel endothelial hyaluronan receptor 1 (LYVE1)-indicated MLD in LN metastatic BCa tissues (Fig. 1c-e), suggesting that substantial CAF infiltration in TME is involved in LN metastasis of BCa.
Previous work by our lab uncovered the critical role of lncRNAs in mediating the TME for promoting BCa LN metastasis12. Given that tumor-induced EVs are intensively involved in regulating the activation of fibroblasts24, we utilized RNA sequencing of urinary EVs from five patients with BCa and five participants without cancer (Gene Expression Omnibus ID: GSE156308) to identify the essential EV-associated lncRNAs contributing to BCa LN metastasis. As shown in Fig. 1f, 255 lncRNAs were upregulated by >2-fold in EVs from the patients compared with those from the controls. We then integrated these data with the previous sequencing analysis results of BCa tissues and paired normal adjacent tissues (NATs) as well as LN-positive and LN-negative BCa (GEO: GSE106534). The results showed that the expression of 12 lncRNAs were consistently higher in EVs from the patients than in the controls and in LN-positive than in LN-negative BCa tissues (Supplementary Table 1). Further evaluation of our 228-case BCa cohort demonstrated that BCa tissues markedly overexpressed LINC00665 compared with NATs (Fig. 1g). Consistently, analysis of The Cancer Genome Atlas (TCGA) database demonstrated that LINC00665 was overexpressed in various human cancers and was related to poor prognosis of patients (Fig. 1h, Extended Data Fig. 1a–n). Therefore, LINC00665 was chosen for further analysis.
Next, we assessed the clinical relevance of LINC00665 in BCa. Supplementary Table 2 shows the detailed clinicopathological information of the patients with BCa in our clinical cohort. Quantitative real-time PCR (qRT-PCR) analysis revealed that LINC00665 was overexpressed in LN-positive (n = 53) compared with LN-negative BCa tissues (n = 175) (Fig. 1i). Metastatic LNs had higher LINC00665 expression levels than primary tumor (Fig. 1j). Importantly, in situ hybridization (ISH) analysis further demonstrated that LINC00665 was greatly overexpressed in BCa tissues with LN metastasis and mildly upregulated in tissues without LN metastasis, whereas it was hardly detected in NATs, suggesting that LINC00665 is closely involved in BCa LN metastasis (Fig. 1k). Moreover, LINC00665 overexpression was associated with shorter overall survival (OS) and disease-free survival (DFS) of the patients with BCa in our clinical cohort (Fig. 1l, 1m). Univariate and multivariate Cox analysis demonstrated that LINC00665 expression was an independent prognostic factor for the OS and DFS of patients with BCa (Supplementary Table 3, 4).
To determine the relationship of LINC00665 with the TME in LN metastatic BCa, we conducted ISH and double immunostaining analysis in BCa patients with LN metastasis. As shown in Fig. 1n and 1o, LINC00665 overexpression related positively to CAF infiltration and around MLD in both the intratumoral and peritumoral regions of LN metastatic BCa tissues, indicating that LINC00665 is associated with CAF-induced BCa LN metastasis.
LINC00665 promotes EV production to endow fibroblasts with the CAF phenotype.
Strikingly, ISH analysis revealed that LN metastatic BCa tissues had a higher extracellular expression level of LINC00665 than those without LN metastasis (Fig. 1k), indicating that extracellular LINC00665 might play an essential role in BCa LN metastasis. It has been proposed that, as natural nanoscale vesicles in cell-to-cell communication, EVs can carry abnormally expressed lncRNAs through the stromal matrix entering the lymphatic circulation25. Therefore, we determined whether LINC00665 exerted its function in BCa LN metastasis via EVs. The culture medium (CM) was collected to isolate and purify the BCa cell-secreted EVs. Transmission electron microscopy (TEM) and nanoparticle tracking analysis (NTA) indicated that LINC00665 overexpression markedly increased the EV concentrations in BCa cells compared with the control (Fig. 2a–c, Extended Data Fig. 2a–e), suggesting that LINC00665 may mediate EV production of BCa cells. Considering that EVs in the BCa TME were prominently internalized by stromal cells and affected their biological activity, we investigated the recipient cells of LINC00665-induced EVs. The BCa cell-secreted EVs were labelled with PKH67 and incubated with stromal cells in the BCa TME. Subsequent confocal analysis showed that the LINC00665-induced EVs were mainly internalized by fibroblasts (Fig. 2d). As tumor cell-produced molecular signals, including large extracellular vesicles, EVs, and cytokines, have been reported to induce stromal cell phenotypic features26, we explored the effect of the LINC00665-induced EVs on the fibroblast phenotype by detecting the expression of CAF phenotypic proteins, namely α-SMA and fibroblast activation protein (FAP). As shown in Fig. 2e-h, overexpressing LINC00665 dramatically upregulated α-SMA and FAP expression in the fibroblasts compared with the control, indicating that LINC00665-induced EVs trigger fibroblast transition to the CAF phenotype. Together, these results demonstrate that LINC00665 overexpression promotes EV secretion to endow fibroblasts with the CAF phenotype.
LINC00665 promotes CAF infiltration to mediate BCa lymphangiogenesis and LN metastasis in vitro and in vivo.
To evaluate the biological function of fibroblasts pretreated with LINC00665-induced EVs, the tube formation and Transwell assays in human lymphatic endothelial cells (HLECs) were performed. As shown in Fig. 2i and 2j, the CM from fibroblasts treated with LINC00665-induced EVs significantly promoted tube formation and migration of HLECs compared with the control, indicating that LINC00665-mediated fibroblast transition to CAFs stimulated BCa cell lymphangiogenesis in vitro. Next, we established a xenograft popliteal LN metastasis model in nude mice, as described previously27,28, to investigate the effect of LINC00665-induced CAF infiltration on BCa LN metastasis. The EVs were isolated from equal volumes of CM of BCa cells and verified by NTA assays for intratumoral injection into the primary footpad tumor (Fig. 3a, 3b). Prominently, the LINC00665-induced EVs markedly enhanced the metastasis of luciferase-labeled T24 cells to the popliteal LNs as compared with the control group, and the luminescence of the popliteal LNs gradually increased during the 6-week experiment, as indicated by the In Vivo Imaging System (IVIS) (Fig. 3c, 3d, Extended Data Fig. 2f, 2g). A higher metastatic rate of popliteal LNs in the LINC00665-induced EV group than the control group was observed (Fig. 3e–g), indicating that the LINC00665-induced EV promoted BCa LN metastasis. Moreover, the popliteal LNs of nude mice were enucleated for further IHC staining analysis. As shown in Fig. 3h and 3i, increased CAFs infiltration was observed around the interstitial tissues of popliteal LNs in the LINC00665-induced EV group compared with the control group. Interestingly, double immunofluorescent staining revealed that the LINC00665-induced EVs group had consistently increased α-SMA-indicated CAF infiltration and LYVE1-indicated MLD (Fig. 3j–l), confirming that the LINC00665-induced EVs promoted BCa lymphangiogenesis by stimulating CAF infiltration in vivo. Together, these results demonstrate that LINC00665-induced EV secretion promotes CAF infiltration to stimulate BCa lymphangiogenesis and LN metastasis.
LINC00665 directly interacts with heterogeneous nuclear ribonucleoprotein L (hnRNPL).
To explore the underlying mechanism of LINC00665 in inducing CAF infiltration in BCa, the 5’ and 3’ rapid amplification of cDNA ends (RACE) assays were performed. The results showed that the full-length of LINC00665 was 2856 nucleotides (nt) in BCa cells (Extended Data Fig. 2h–k). Subsequently, fluorescence in situ hybridization (FISH) and subcellular fractionation assays indicated that LINC00665 was located in both the BCa cell cytoplasm and nucleus, but was mainly in the cytoplasm (Extended Data Fig. 2l, 2m). RNA pull-down assay revealed that the biotinylated LINC00665 group had an obviously different band with a molecular weight of 55–70 kDa compared with the control, which was further identified as hnRNPL through Mass spectrometry (MS) analysis (Fig. 4a, 4b). Consistently, western blotting analysis after the RNA pull-down assays demonstrated that LINC00665 specifically enriched hnRNPL (Fig. 4c, 4d). Confocal microscopy analysis validated the colocalization of LINC00665 and hnRNPL in the T24 and 5637 cells (Fig. 4E). RNA immunoprecipitation (RIP) assays verified the significant enrichment of LINC00665 by hnRNPL (Fig. 4f), confirming the interaction between LINC00665 and hnRNPL.
Next, we performed sequential deletion experiments to demonstrate that the 2250–2400 nt region of LINC00665 was essential for its interaction with hnRNPL (Fig. 4g–i). We used POSTAR2, a comprehensive database for exploring the RNA motif for the interaction with RNA-binding proteins29, to predict the preferred sequence motif of the hnRNPL binding site that formed a stem-loop structure in the 2285–2360 nt region of LINC00665 (Fig. 4j). Mutation of the 2285-2360 nt region in LINC00665 markedly attenuated the enrichment of LINC00665 by hnRNPL (Fig. 4k), indicating that the region is indispensable for the interaction between LINC00665 and hnRNPL. Taken together, these findings demonstrate the direct interaction between hnRNPL and the 2285–2360 nt region in LINC00665.
LINC00665 promotes RAB27B transcription by forming a DNA–RNA triplex structure with the RAB27B promoter.
Given that lncRNAs frequently act as molecular drivers of gene transcriptional regulation, leading to tumor initiation and progression30, we performed RNA sequencing to profile the target genes of LINC00665. As shown in Fig. 5a, 542 genes were markedly upregulated by >2-fold in the LINC00665-overexpressing BCa cells compared with control. As LINC00665 widely participates in the release of EVs, the gene expression of the Ras-related Rab protein family associated with EV secretion was analyzed (Fig. 5b, Extended Data Fig. 3a, Supplementary Table 5). The results showed that RAB27B was the most significantly upregulated gene associated positively with LINC00665 overexpression in BCa cells by qRT-PCR and western blotting analyses (Fig. 5c, 5d, Extended Data Fig. 3b–e). To explore the potential mechanism of LINC00665 in regulating the transcriptional activation of RAB27B, the truncated RAB27B promoter sequences (-2000 to +200 bp) were cloning into the pGL3 luciferase plasmids and subjected into the luciferase assays. Interestingly, LINC00665 increased the transcriptional activity of the constructs containing the -250 to -500 bp sequence in the RAB27B promoter (Fig. 5e, Extended Data Fig. 3f). Subsequently, chromatin isolation by RNA purification (ChIRP) assays verified that LINC00665 physiologically interacted with the P2 region (-315 to -327 bp) in the RAB27B promoter in T24 and 5637 cells (Fig. 5f, 5g, Extended Data Fig. 3g). Moreover, a lncRNA–DNA binding motif prediction tool, LongTarget31, was used to identify the potential triplex-forming oligonucleotides (TFOs) and corresponding triplex target sites (TTS) in LINC00665 and the RAB27B promoter, among which TFOs were further labeled with 5-carboxy tetramethyl-rhodamine (TAMRA) and TTS with fluorescein amidite (FAM) for the subjection into circular dichroism (CD) spectroscopy and fluorescence resonance energy transfer (FRET) analysis. As shown in Fig. 5h and Extended Data Fig. 3h, the results of CD spectroscopy revealed an obvious positive peak at 270–280 nm and a negative peak at 210 nm in the LINC00665 TFO2/RAB27B TTS2 group, which was similar to that of the FENDRR/PITX2 positive control32. In addition, FRET analysis showed a significant increase in fluorescence intensity at 570–580 nm and decrease at 520 nm in the LINC00665 TFO2/RAB27B TTS2 group compared with the control group (Fig. 5i, Extended Data Fig. 3i), suggesting the formation of a triplex structure between LINC00665 and the RAB27B promoter.
HnRNPL has been demonstrated as a key regulator in catalyzing H3K4 trimethylation (H3K4me3), the common manner of regulating target gene transcriptional activation33. Therefore, we explored whether hnRNPL participates in LINC00665-activated RAB27B transcription by mediating H3K4me3 at the RAB27B promoter in BCa cells. LINC00665 overexpression markedly increased the enrichment of hnRNPL and H3K4me3 at the RAB27B promoter, while muting the hnRNPL-binding region on LINC00665 reduced it (Fig. 5j, 5k, Extended Data Fig. 3j, 3k). Conversely, downregulating LINC00665 significantly decreased hnRNPL and H3K4me3 enrichment at the RAB27B promoter (Extended Data Fig. 3l–o). Moreover, silencing hnRNPL greatly impaired the ability of LINC00665 to upregulate RAB27B expression (Fig. 5l, 5m). Together, these results demonstrate that LINC00665 directly binds to the RAB27B promoter to form a DNA–RNA triplex and activate RAB27B transcription.
LINC00665-induced EVs endow fibroblasts with the CAF phenotype by activating the TGF-β pathway.
Considering that RAB27B is an essential mediator in EV secretion by enhancing the fusion of late endocytic compartments with the plasma membrane34, we detected whether RAB27B is crucial for LINC00665-induced EV secretion. The results showed that LINC00665 increased the amount of EVs secreted by T24 and 5637 cells while downregulating RAB27B-impaired LINC00665-induced EV secretion (Fig. 5n, 5q). Since RABs were reported to participate in vesicle docking and fusion with recipient cells35, we evaluated the essential role of RAB27B underlying fibroblast internalization of LINC00665-induced EVs. As shown in Fig. 5r and 5s, an increased green fluorescence signal was observed in the cytoplasm of fibroblasts incubated with PKH67-labeled LINC00665-induced EVs compared with the control, which was attenuated by downregulating RAB27B, suggesting that LINC00665 remarkedly enhances fibroblast internalization of BCa cell-secreted EVs via RAB27B.
It was previously reported that TGF-β signaling pathway plays a crucial role in regulating the fibroblast phenotype in cancers36, we explored the effect of TGF-β signaling pathway in fibroblast transition to CAFs as mediated by LINC00665-induced EVs. Analysis of the alterations of crucial proteins in TGF-β signaling pathway showed that the phosphorylation of SMAD2 and SMAD3 was markedly increased in fibroblasts treated with LINC00665-induced EVs compared with the control, while fewer change in the expression levels of TGF-βR1 and TGF-βR2 was observed (Fig. 6a), indicating that LINC00665-induced EVs specifically promoted SMAD2 and SMAD3 phosphorylation to activate the TGF-β signaling pathway in fibroblasts. Subsequently, the treatment with SIS3, which specifically inhibits SMAD2 and SMAD3 phosphorylation, was conducted to validate whether TGF-β signaling pathway activation is indispensable for the LINC00665-induced EV-mediated phenotype transition of fibroblasts. Immunofluorescence analysis and flow cytometry showed a significant increase in α-SMA and FAP expression in fibroblasts treated with EVs secreted by LINC00665-overexpressing BCa cells, which was markedly attenuated by treating with SIS3 (Fig. 6b–d). Collectively, our findings suggest that LINC00665-induced EVs trigger fibroblast transition to the CAF phenotype by regulating SMAD2 and SMAD3 phosphorylation in the fibroblasts.
LINC00665-induced EVs enhance lymphangiogenesis by stimulating HGF secretion in CAFs.
To analyze the mechanisms of CAFs activated by LINC00665-induced EV (CAFsLINC00665−EVs) in enhancing lymphangiogenesis, we compared the cytokine profiles of CAFsLINC00665−EVs with the control. The results showed that five cytokines were dramatically increased, while one was decreased, in the CM from CAFsLINC00665−EVs compared with the control (Fig. 6e, 6f). Interestingly, we found that VEGF, the common regulator of lymphangiogenesis, was not significantly secreted by the CAFsLINC00665−EVs as compared with the control, indicating that CAFsLINC00665−EVs might stimulate BCa LN metastasis in a VEGF-independent manner (Fig. 6e, 6f). Moreover, the enzyme-linked immunosorbent assay (ELISA) yielded results consistent with that of the cytokine profiling, in which EVs from LINC00665-silenced BCa cells markedly reduced CAF secretion of cytokines, including IL-6, IL-8, Dkk-1, HGF, and CXCL10, while overexpressing LINC00665 had the opposite effect (Fig. 6G, 6h). Next, in vitro assays were performed to evaluate the role of the above changed cytokines in CAF-induced lymphangiogenesis. The results showed that the treatment with neutralizing antibody against HGF (αHGF) dramatically impaired the tube formation and migration of HLECs promoted by CAFsLINC00665−EVs, while inhibiting other cytokines had no significant effects (Fig. 6i), suggesting that LINC00665-induced EVs facilitate BCa lymphangiogenesis by stimulating HGF secretion by CAFs.
LINC00665 EV-mediated RAB27B–HGF–c-Myc positive loop is indispensable to BCa LN metastasis.
Accumulating evidence has shown that tumor cell-stimulated CAFs reciprocally regulate the aggressive biological behavior of tumor cells to support tumor metastasis14. Therefore, the biological effect of CAFsLINC00665−EVs in BCa cells was examined. An in vitro coculture model revealed that coculturing with CAFsLINC00665−EVs significantly upregulated LINC00665 expression in BCa cells (Fig. 7a, 7b), suggesting the potential formation of a positive feedback loop between BCa cells and CAFs.
It was recently shown that CAFs induce the activation of massive transcription factors in tumor cells to regulate lncRNA transcription13. Therefore, we hypothesized that CAFs may induce the transcriptional activation of the LINC00665 promoter to upregulate its expression. To address the hypothesis, we assessed the transcriptional factors that potentially interact with the LINC00665 promoter. The results showed that c-Myc was the most possible transcriptional factor to interact with the LINC00665 promoter, and downregulating c-Myc dramatically reduced the transcriptional activity of LINC00665 promoted by HGF in BCa cells (Fig. 7d, 7e, Extended Data Fig. 4a), confirming that c-Myc widely participates in LINC00665 transcription enhanced by CAFsLINC00665−EVs-secreted HGF. Moreover, chromatin immunoprecipitation (ChIP) analysis demonstrated that CAFsLINC00665−EVs increased the enrichment of c-Myc in the -351 to -358 bp region (referred as P2) of the LINC00665 promoter rather than another predicted binding site located in -106 to -113 bp region (referred as P1) (Fig. 7f–h). Mutating the LINC00665 promoter P2 region markedly impaired the transcriptional activity increased by the CAFsLINC00665−EVs, while P1 region mutation had rare effects (Fig. 7i, Extended Data Fig. 4b), suggesting that CAFsLINC00665−EVs-mediated c-Myc interact directly with -351 to -358 bp of the LINC00665 promoter to activate its transcription.
Since CAFs conversely induced LINC00665 transcriptional activation, in vitro assays were performed to detect the role of the LINC00665-mediated positive feedback loop in maintaining CAF-induced lymphangiogenesis (Fig. 7j). The results showed that downregulating LINC00665 expression impaired the fibroblast transition to CAFs induced by coculturing with BCa cells (Fig. 7k–m). Moreover, αHGF treatment significantly blocked CAF-induced LINC00665 overexpression in BCa cells and inhibited the coculturing-induced activation of CAFs (Fig. 7k–m). Strikingly, either LINC00665 silencing or αHGF treatment suppressed the fibroblast-induced tube formation and migration of HLECs in the BCa cell and fibroblast coculture model (Fig. 7n), suggesting that the LINC00665-induced RAB27B–HGF–c-Myc positive feedback loop between BCa cells and fibroblasts promotes BCa lymphangiogenesis and LN metastasis.
Clinical relevance of the LINC00665-induced RAB27B–HGF–c-Myc positive feedback loop in LN metastatic BCa.
In light of the crucial regulatory role of HGF in LINC00665-induced fibroblast transition to CAFs and BCa lymphangiogenesis, double immunostaining and ISH analysis were conducted to determine the clinical relevance of HGF in BCa LN metastasis. The results showed that higher HGF expression levels were accompanied by more CAF infiltration and MLD in LINC00665-overexpressed regions of LN metastatic BCa tissues (Fig. 8a, 8b). Correlation analysis revealed that HGF overexpression in BCa tissues was associated positively with higher LINC00665 expression (Fig. 8c). Furthermore, qRT-PCR analysis demonstrated that HGF was significantly overexpressed in BCa tissues compared with the paired NATs (n = 228) (Fig. 8d). Moreover, LN-positive BCa tissues possessed higher HGF expression levels than the LN-negative BCa tissues (Fig. 8e). Furthermore, HGF overexpression was associated positively with shorter OS and DFS of patients (Fig. 8f, 8g).
To assess the effect of blocking HGF in suppressing lymphangiogenesis and LN metastasis mediated by the LINC00665-induced RAB27B–HGF–c-Myc positive feedback loop, in vitro coculture model and in vivo popliteal LN metastasis model was conducted. In vitro assays showed that both fibroblasts treated with LINC00665-induced EVs and primary CAFs dramatically promoted tube formation and migration of HLECs, while αHGF treatment dramatically abrogated these effects (Fig. 8h, Extended Data Fig. 4d–e). Moreover, in vivo popliteal LN metastasis assays demonstrated that LINC00665-induced EVs significantly promoted BCa cells to metastasize to the LNs compared with the control groups, whereas HGF inhibition dramatically reversed this effect (Fig. 6i, 6j). Statistical analysis showed that αHGF significantly attenuated the LN metastatic rate increased by the LINC00665-induced EVs (Extended Data Fig. 4f). Furthermore, blockage of HGF reduced the CAF infiltration and MLD mediated by the LINC0065-induced EVs in the tumor tissues (Fig. 8k).
The previous study has demonstrated that Cabozantinib, a small-molecule inhibitor specifically targeting the HGF–MET signaling pathway, exhibits a noticeable clinical effect in suppressing tumor angiogenesis7. As the essential role of HGF in BCa lymphangiogenesis and LN metastasis has been well-elucidated in a large clinical cohort and in vitro and in vivo experiments, we further constructed an orthotopic xenograft model to evaluate the clinical application potential of cabozantinib in LINC00665-mediated lymphangiogenesis and LN metastatic BCa (Fig. 8l). The results showed that the utilization of Cabozantinib decreased the orthotopic tumor volume in nude mice educated by LINC00665-induced EVs as compared with the control group (Fig. 8m). Importantly, cabozantinib combined with cisplatin, the main chemotherapeutic drug for advanced BCa5, showed a significant effect in reducing tumor volume and prolonging the tumor-bearing survival time of LINC00665-induced EV-educated mice (Fig. 8m, 8n), revealing the encouraging clinical potential of cabozantinib in treating LINC00665-induced LN metastatic BCa. Taken together, these results demonstrate that the LINC00665-induced RAB27B–HGF–c-Myc positive feedback loop between BCa cells and fibroblasts plays a significant role in BCa LN metastasis (Figure 8o).