SUN5 interacts with TRIM28, enhancing IκBα ubiquitination to promote glycolysis in colorectal cancer cells

Abstract

Glycolysis provides the main energy source for the rapid proliferation and migration of colorectal cancer (CRC) cells. In our previous studies, we reported that SUN5, a nuclear membrane protein, promotes proliferation and migration. However, whether SUN5 is involved in the process of glycolysis is unclear. Here, we demonstrate that overexpression of SUN5 enhances glucose uptake and lactate production in CRC cells, whereas the opposite results are observed in SUN5-knockdown cells. Mechanistically, SUN5 activates the NF-κB signaling pathway, which can be inhibited by the IKK inhibitor BAY11-7082. Further studies reveal that SUN5 interacts with TRIM28 to increase IκBα ubiquitination, leading to the nuclear translocation of phosphorylated P65 (phos-P65) and subsequent increases in the transcription of GLUT1 and LDHA, accelerating glycolysis. Moreover, xenograft transplantation experiments reveal that the knockdown of SUN5 inhibits glycolysis and tumorigenesis in vivo. Taken together, these findings indicate that SUN5 enhances the glycolysis and tumorigenesis of CRC cells via interaction with TRIM28, which provides a potential target for the diagnosis and treatment of CRC.

1. Introduction

Colorectal cancer (CRC) is the third most common malignant tumor and the second leading cause of cancer death worldwide [1]. In China, 608,000 new cases of CRC occur, and 202,000 deaths occur every year [2]. The molecular mechanism underlying the development of CRC is not fully understood. Therefore, elucidating the molecular mechanisms driving CRC progression is crucial for identifying novel and effective therapeutic targets for CRC patients.

Cancer cells prefer glycolysis, which is characterized by elevated glucose consumption and lactate production even with adequate oxygen [3]. This phenomenon is known as the Warburg effect [4,5], which regulates various biological processes in CRC, including cell viability [6], invasive migration [7,8], chemotherapy resistance [9] and stemness [10]. The regulation of the Warburg effect is associated with abnormal activation of multiple signaling pathways, such as the PI3K/Akt/mTOR [11], Wnt/β-catenin [12], Hippo/YAP [13] and nuclear factor kappa B (NF-κB) signaling pathways [14–16]. The NF-κB complex consists of five members, including RelA (P65), RelB, c-Rel, NF-κB1 (P50), and NF-κB2 (P52). The P50/P65 complex is sequestered in the cytoplasm by the IκB protein complex in the quiescent state. A canonical mechanism of NF-κB activation involves IκBα phosphorylation, ubiquitination, and subsequent proteasome-mediated degradation [17]; thus, the P65/P50 dimer translocate to the nucleus to induce the expression of related genes, including glucose transporter 1 (GLUT1) [14,16] and hexokinase 2 (HK2) [18]. NF-κB signaling pathway also regulates the promoter activity and expression of hypoxia-induced factor-1α (HIF-1α) [19], contributing to metabolic reprogramming in cancer cells by activating the expression of glycolytic enzymes, including GLUT1/3, pyruvate kinase M2 (PKM2) [20] and lactate dehydrogenase A (LDHA) [21,22]. Investigations into the activation of the NF-κB signaling pathway may facilitate a more comprehensive understanding of glycolysis in CRC.

Recently, a novel family of nuclear membrane proteins, the Sad1/UNC84 domain (SUN) proteins, was shown to be involved in the glucose metabolism of tumors. The structures of the SUN family members tend to be conserved, with a transmembrane (TM) region at the N-terminus, a helical convoluted structural domain in the middle, and the SUN structural domain at the C-terminus. To date, five SUN proteins have been identified, namely, SUN1, SUN2, SUN3, SUN4 (SPAG4), and SUN5 (SPAG4L, TSARG4) [23–27]. SUN1 and SUN2 are downregulated in cancer, whereas SUN4 and SUN5 are enriched in cancer cells [24,27–29]. In oral cancer [28] and lung cancer [25], SUN2 exerts tumor suppressor functions by inhibiting the Warburg effect via SIRT5. SUN4 is known as a tumor marker in different cancers, including lung cancer [30], renal cell carcinoma (RCC) [31], hepatocellular carcinoma (HCC) [32] and glioblastoma [33]. SUN4 enhances mitochondrial respiration and aerobic glycolysis in CRC cells by activating the PI3K/Akt signaling pathway [34]. SUN5, the fifth member of the SUN gene family, was first identified by our group.