Introduction

The bone tissue structure of the oral and maxillofacial region is an important basis for supporting the facial shape. However, maxillofacial bone defects due to trauma, tumor resection, infection, and congenital malformation can cause not only facial deformities, but also difficulty in chewing, swallowing, speaking, and breathing. This could affect a patient’s physiological and psychological well-being [1,2,3]. Nowadays, autologous bone grafts are commonly employed as the standard procedure to repair bone defects. Yet, the disadvantages such as secondary injury of the donor site, limited supply, and poor prognosis in patients with osteoporosis are prominent [4, 5]. Although synthetic bone can be an alternative, its limitations include immune rejection, infection, and low capacity for osteoinduction and osteo-integration [6, 7]. Lately, bone tissue engineering has been the focus and is receiving extensive attention and research. Bone tissue engineering is a combination of seed cells, specific osteogenic factors, and biological scaffold materials aiming to create a reconstructed tissue with a similar function as a natural bone tissue; it is anticipated to offer a successful procedure for bone reconstruction [8,9,10]. Among the various types of cells, mesenchymal stem cells (MSCs) are generally preferred for bone repair. Human bone marrow mesenchymal stem cells (hBMSCs), as adult stem cells, have promising potential as seed cells in bone tissue engineering owing to their ability to self-renew, pluripotency, and mild immunogenicity [11,12,13]. Osteogenic differentiation of MSCs primarily includes cell differentiation, cell maturation, extracellular matrix formation, and mineralization. This process can be regulated by growth factors belonging to the Wnt family and bone morphogenetic protein (BMP) family, transcription factors that include β-catenin, runt-related transcription factor 2 (RUNX2), osterix (OSX), and certain microRNAs (miRNAs) [14, 15].

In some recent studies, long noncoding RNAs (lncRNAs) are reported to regulate how hBMSCs differentiate into osteoblasts [16, 27, 40]. KCNMA1-AS1 was previously shown to possess oncogenic activity and promote the proliferation of ovarian cancer cells [25]. No relevant reports to date have reported KCNMA1-AS1 to have any effect on osteogenic differentiation. In the current study, the increased expression level of KCNMA1-AS1 was observed during the osteogenic differentiation of hBMSCs. Accordingly, the prospective function of KCNMA1-AS1 in regulating osteogenic differentiation was investigated during this work. Gain/loss function assays were performed with lentiviral transfection models in subsequent experiments, which revealed that KCNMA1-AS1 overexpression promoted osteogenesis of hBMSCs both in vivo and in vitro. In contrast,KCNMA1-AS1 deficiency gave rise to the opposite outcome. These results indicated a positive influence of KCNMA1-AS1 on osteogenic differentiation taking place in the hBMSCs.

Further, the underlying osteogenic differentiation mechanism involving KCNMA1-AS1 was investigated using FISH assay and the results indicated that KCNMA1-AS1 was mainly localized in the nuclei of hBMSCs. The function of lncRNAs depends on subcellular localization. Nuclear lncRNAs have been found to influence gene expressions by interacting with transcription factors and proteins in diverse biological processes [41]. For instance, the interaction between lncRNA HOTTIP and WDR5 results in the activation of the Wnt/β-catenin pathway, which enhances the osteogenic differentiation of BMSCs [42]. LncRNA MEG3 regulates chondrogenic differentiation by inhibiting TRIB2, which is achieved by binding with EZH2 [43] as well as LINC02273; it is associated with hnRNPL, which promotes metastasis of breast cancer by increasing AGR2 transcription [44]. Furthermore, using RNA pull-down assay, mass spectrometry, and RIP assay, we were able to detect a tight junction between KCNMA1-AS1 and SMAD9.

Belonging to the transforming growth factor beta (TGF-β) superfamily, mothers against decapentaplegic (SMAD) proteins, which have found eight members in mammals, have been divided into three subgroups, namely, common SMAD (Co-SMAD or SMAD4), receptor-regulated SMADs or R-SMADs, and inhibitory SMADs or I-SMADs. SMAD9 (previously known as SMAD8), being an R-SMAD, is a prominent transcription factor. Also, SMAD9, along with SMAD1 and SMAD5, is phosphorylated and activated directly by BMP type I receptor to form a heterotrimeric complex with SMAD4; these complexes take part in the regulation of target genes and proteins [45,46,47]. It is important to note that the SMAD9 signaling pathway plays a significant role in regulating bone development as described before [48,49,50,51]. Rare lncRNAs have also been reported to be associated with the SMAD9 signaling pathway during osteogenic differentiation, in which lncRNA SNHG5 [52] is an example. Surprisingly, in this study, we discovered that KCNMA1-AS1 overexpression triggered the phosphorylation of SMAD9 both in vivo and in vitro activating the SMAD9 signaling cascade, whereas KCNMA1-AS1 interference attenuated the expression of p-SMAD9, thereby inhibiting the SMAD9 signaling pathway. To verify whether KCNMA1-AS1 impacts the osteogenic differentiation of hBMSCs by activating the SMAD9 signaling pathway, we performed rescue experiments using LDN193189 as a repressor of the SMAD9 signaling pathway. The expression levels of p-SMAD9 and osteogenic specific markers tested by western blot suggested that they were remarkably lowered in hBMSCs treated with KCNMA1-AS1 overexpression and LDN193189 compared to those treated with KCNMA1-AS1 overexpression and DMSO. In addition, the application of LDN193189 abrogated the promotional effects of KCNMA1-AS1 overexpression on ALP activity and mineralized calcium nodules deposition. These findings proved that KCNMA1-AS1 regulates osteogenic differentiation by modulating the SMAD9 signaling pathway. However, the effect of KCNMA1-AS1 on bone regeneration certainly needs to be elucidated further using bone defect models.

The present research showed that lncRNA KCNMA1-AS1 promoted osteogenic differentiation of HBMSCs by targeting the SMAD signaling pathway. LncRNA KCNMA1-AS1 was previously found to be implicated in the progression and migration of epithelial ovarian cancer by promoting proliferation, migration and inhibiting apoptosis [53]. Some other lncRNAs also exhibit a concerning duality - promoting osteoblast differentiation while also fueling cancer progression. Taking an example of LncRNA HOTTIP, it was found to enhance human osteogenic BMSCs differentiation via the activation of the Wnt/β-catenin signalling pathway [42], the activation of which induces enhanced epithelial-mesenchymal transition in cancer metastasis [54]. LncRNA HOTTIP also promoted pancreatic cancer cell proliferation, survival and migration [55]. Taking another example, LncRNA SNHG5 was shown to promote the osteogenic differentiation of BMSCs via the miR-212-3p/GDF5/SMAD pathway. SMAD signaling collaborates with oncogenic pathways like Wnt, NF-kB, and Notch to fuel epithelial-to-mesenchymal transition (EMT) and invasion. LncRNA SNHG5 was also found to play a tumor-promoting role in many cancer types including nasopharyngeal carcinoma [56], hepatocellular carcinoma [57], and cervical cancer [58]. The complex, context-specific functionality of these lncRNAs presents challenges for exploiting their bone regenerative potential while avoiding cancer promotion. There are a few key considerations when assessing whether lncRNAs implicated in both osteogenic differentiation and cancer progression could realistically be used as therapy for bone defects. The complexity of lncRNAs functioning divergently in different cell contexts, their specific roles in driving cancer progression, challenges with targeted delivery, and safety risks of potentially promoting tumors all pose significant barriers to clinical use. Innovative delivery methods, genetic screening, combination therapies with other bone anabolics, short-term treatment and extensive preclinical testing in bone defects models may help mitigate concerns, but far more research is needed. Close collaboration between bone biology and oncology experts will be critical to determine if any lncRNAs implicated in cancer may realistically be harnessed safely and effectively for bone defect therapies. Overall, the cancer risk poses a significant hurdle, and extensive further study is required before this novel genetic targeting approach could be clinically viable.

In summary, for the first time, we have reported here that lncRNA KCNMA1-AS1 has augmented expression during osteogenic differentiation. The overexpression of KCNMA1-AS1 results in enhanced hBMSCs’ osteogenic differentiation. Our research revealed a novel mechanism where the KCNMA1-AS1 was found to regulate osteogenic differentiation of hBMSCs via the SMAD9 signaling pathway (Fig. 7), which provides a novel perspective on translational research in bone tissue engineering.

Fig. 7
figure 7

Schematic diagram illustrating the regulation of osteogenic differentiation in hBMSCs by KCNMA1-AS1.

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