Arl4c, a member of the ADP-ribosylation factor (ARF) small GTP-binding protein subfamily, is expressed upon the simultaneous activation of pathways like Wnt-β-catenin and growth factor-Ras-mitogen-activated protein kinase signalling. It is involved in multiple biological functions such as cell proliferation, epithelial morphogenesis, and has implications in various disease conditions including cancer and tooth germ development [1,3,4]. Genetic models, especially knockout (KO) or conditional knockout (CKO) mouse models, can be valuable in further elucidating its functions.

In tooth germ development, loss-of-function experiments in mouse tooth germ rudiment culture using siRNAs and shRNA demonstrated that Arl4c is involved in cell proliferation and osteoblastic cytodifferentiation in odontogenic epithelial cells. Treatment with SecinH3 (an inhibitor of the ARNO/Arf6 pathway) reduced tooth germ size, width, cusp height, and the thickness of the eosinophilic layer, suggesting the Arl4c-ARNO/Arf6 pathway axis contributes to tooth germ development through osteoblastic/ameloblastic differentiation [3].

In colorectal cancer, Arl4c expression was higher in cells with the epithelial-to-mesenchymal transition (EMT) phenotype, and its expression in cancer stromal cells was stronger in cases with high-grade tumor budding and high histological grade, indicating its association with CRC prognosis [2].

In pancreatic cancer, high Arl4c expression is associated with cell proliferation, drug resistance, and pancreatic stellate cell (PSC) activation. Arl4c regulates connective tissue growth factor (CTGF) paracrine, induces autophagic flux in PSCs, and the TGFβ1 secreted by activated PSCs enhances cancer cell stem cell properties, increasing drug resistance [5].

In clear cell renal cell carcinoma (ccRCC), downregulation of Arl4c significantly inhibited cell proliferation, migration, and invasion. As an essential downstream effector of the Wnt signalling pathway, it increases the expression of cyclin D1 and c-myc, promoting EMT and ccRCC progression [6].

In ameloblastoma, loss-of-function experiments using inhibitors or siRNAs showed that ARL4C elevation depended on the RAF1-MEK/ERK pathway, and ARL4C-depleted tumour cells had decreased proliferation and migration capabilities. Also, ARL4C elevation in ameloblastoma promoted osteoclast formation [7].

In conclusion, Arl4c is a key regulator in multiple biological processes. Model-based research, especially KO/CKO mouse models, has revealed its significant roles in diseases such as tooth-related disorders, colorectal, pancreatic, and renal cell carcinomas, as well as ameloblastoma. Understanding Arl4c's functions provides insights into disease mechanisms and potential therapeutic targets.

References:

1. Fujii, Shinsuke, Kiyoshima, Tamotsu. 2023. The role of Wnt, ARL4C, and Sema3A in developmental process and disease pathogenesis. In Pathology international, 73, 217-233. doi:10.1111/pin.13325. https://pubmed.ncbi.nlm.nih.gov/37098842/

2. Kanai, Ryo, Uehara, Takeshi, Yoshizawa, Takahiro, Nagaya, Tadanobu, Ota, Hiroyoshi. 2023. ARL4C is associated with epithelial-to-mesenchymal transition in colorectal cancer. In BMC cancer, 23, 478. doi:10.1186/s12885-023-10958-4. https://pubmed.ncbi.nlm.nih.gov/37237373/

3. Truong, Thinh Thi Kim, Fujii, Shinsuke, Nagano, Ryoko, Fukumoto, Satoshi, Kiyoshima, Tamotsu. 2023. Arl4c is involved in tooth germ development through osteoblastic/ameloblastic differentiation. In Biochemical and biophysical research communications, 679, 167-174. doi:10.1016/j.bbrc.2023.09.014. https://pubmed.ncbi.nlm.nih.gov/37703759/

4. Matsumoto, Shinji, Fujii, Shinsuke, Kikuchi, Akira. 2016. Arl4c is a key regulator of tubulogenesis and tumourigenesis as a target gene of Wnt-β-catenin and growth factor-Ras signalling. In Journal of biochemistry, 161, 27-35. doi:10.1093/jb/mvw069. https://pubmed.ncbi.nlm.nih.gov/28053143/

5. Chen, Xin, Zhang, Yanzhen, Qian, Weikun, Wang, Zheng, Ma, Qingyong. 2021. Arl4c promotes the growth and drug resistance of pancreatic cancer by regulating tumor-stromal interactions. In iScience, 24, 103400. doi:10.1016/j.isci.2021.103400. https://pubmed.ncbi.nlm.nih.gov/34849465/

6. Zhang, Peizhi, Xu, Yingkun, Chen, Shaoan, Wu, Guangzhen, Xia, Qinghua. 2022. ARL4C Regulates the Progression of Clear Cell Renal Cell Carcinoma by Affecting the Wnt/β-Catenin Signaling Pathway. In Journal of oncology, 2022, 2724515. doi:10.1155/2022/2724515. https://pubmed.ncbi.nlm.nih.gov/35774359/

7. Fujii, Shinsuke, Ishibashi, Takuma, Kokura, Megumi, Jimi, Eijiro, Kiyoshima, Tamotsu. 2021. RAF1-MEK/ERK pathway-dependent ARL4C expression promotes ameloblastoma cell proliferation and osteoclast formation. In The Journal of pathology, 256, 119-133. doi:10.1002/path.5814. https://pubmed.ncbi.nlm.nih.gov/34622442/