TRIM52 promotes colorectal cancer cell proliferation through the STAT3 signaling

Colorectal cancer (CRC) is the third most common form of gastrointestinal cancers, with more than one million newly diagnosed cases each year in the world [1]. The incidence of CRC in women and men is 9.2% and 10%, respectively, which makes CRC the second most common among women, and the third most common cancer among men [2‐4]. Although continued efforts have made to improve the understanding of tumorigenesis, and numerous oncogenes and tumor suppressor genes involved in CRC tumorigenesis have been identified, the biological and molecular mechanisms of CRC are still far from being fully understood.

Signal transducer and activator of transcription 3 (STAT3), a transcription factor, normally resides in the cytoplasm. When its tyrosine-705 residue is phosphorylated in response to cytokine signaling and tyrosine kinase oncoproteins, STAT3 translocates to the nucleus and controls the transcription of downstream genes that are involved in cell cycle transition and cell survival [5, 6]. Activated phospho-STAT3 (p-STAT3) is found to be increased in CRC samples compared to normal mucosae [7‐9]. The Janus family kinases (JAK) are known to mediate the activation of STAT3 [10]. AG490, a pharmacological inhibitor of JAK, and STAT3 small interfering RNA (siRNA) could suppress CRC cell growth and invasion, and induce CRC cell apoptosis [9].

The tripartite motif (TRIM) family protein is characterized by its tripartite motif, which contains a RING domain, one or two B-box domains and a coiled-coil domain [11]. More than 80 members have been identified in human until now. Substantial evidence has accumulated and supported the roles of TRIM proteins in innate immune response, cell proliferation, apoptosis and autophagy [11, 12]. The dysfunction of TRIM family proteins is known to be associated with the pathogenesis of several diseases, including various human cancers [12‐14]. TRIM52 is a novel member of the TRIM protein family [15]. TRIM52 can degrade the viral nonstructural protein 2A (NS2A), thus exerting antiviral activity against Japanese encephalitis virus (JEV) infection [16]. TRIM52 is able to active the NF-κB signaling pathway [17]. Recently, the oncogenic role of TRIM52 has been described in hepatocellular carcinoma (HCC) [18, 19]. Hepatitis B virus X protein (HBx) may regulate TRIM52 expression [19]. TRIM52 can facilitate cell proliferation, migration and invasion of HCC cells [18].

Previous studies have revealed the significance of TRIM52 in HCC, but no investigation has focused on the effects of TRIM52 on CRC. In the current study, we examined the expression and function of TRIM52 in CRC. We found that the STAT3 signaling was involved in the abilities of TRIM52 to induce the proliferation and inhibit apoptosis in CRC cells.

The dysfunction of TRIM family proteins implicates in the pathogenesis of various human cancers [12‐14]. Recently, studies have investigated the functions of TRIM proteins in CRC. TRIM24 [28] and TRIM29 [29] are up-regulated in CRC tissues and significantly correlated with poor prognosis. Studies have demonstrated the oncogenic roles of TRIM24 [30], TRIM27 [31], TRIM29 [29, 32] and TRIM59 [28] in CRC. TRIM52, a novel member of TRIM family proteins, has been reported to be up-regulated in HCC and to promote cell proliferation, migration and invasion of HCC cells [18, 19]. Nevertheless, the roles and mechanisms of TRIM52 in colorectal carcinogenesis have not been investigated. In present study, we provided new evidence that TRIM52 expression was elevated in CRC tissues (Fig. 1a) and cell lines (Fig. 2a), and that TRIM52 expression was significantly correlated with tumor size, tumor stage and overall survival of CRC patients (Fig. 1b and Table 2). Furthermore, knockdown of TRIM52 expression in CRC cells significantly suppress cell proliferation due to the induction of apoptosis in vitro (Figs. 2, 3) and in nude mice (Fig. 4). Evidence suggested that cellular senescence is another key event contributing to the anticancer response [33]. The possible effect of TRIM52 knockdown on cell senescence could not be ruled out and further work will be necessary. Nevertheless, all of these findings indicate that TRIM52 may serve as an oncogene in CRC, which was consistent with its role in HCC [18, 19].

Activated STAT3 is found to be increased in CRC samples compared to normal mucosae [7‐9]. AG490 and STAT3 knockdown could suppress CRC cell growth and invasion, and induce CRC cell apoptosis [9]. A recent study reported that TRIM8 interacts with protein inhibitor of activated STAT3 (PIAS3), thus enhancing the STAT3-dependent signaling [34]. TRIM29 knockdown in CRC cells led to a notable reduce in the phosphorylation levels of STAT3 [32]. Here, we tried to explore the association of TRIM52 and the STAT3 signaling in CRC. TRIM52 knockdown reduced the phosphorylation of STAT3 at Tyr705 in both CRC cells and xenograft tumors (Fig. 5). SHP2, a tyrosine phosphatase, has been shown to negatively regulate the STAT3 signaling [27]. TRIM52 interacted with SHP2 and promotes its ubiquitination, whereas the ubiquitination site on SHP2 is to be identified. Further, we found that JAK2 inhibitor AG490 can block the promotional effects of TRIM52 overexpression on CRC cell proliferation (Fig. 6). These data suggest that TRIM52 may promote SHP2 ubiquitination, thus inactive the STAT3 signaling and serve as an oncogene in CRC.

Bcl-2 family proteins, including anti-apoptotic factor Bcl-2 and proapoptotic factor Bax, are important for the regulation of apoptosis. It seems that abnormal activation of the bcl-2 gene is an early event in colorectal tumorigenesis [35]. The protein levels of Bcl-2 [36] and Bax [37] may be potential prognostic indicator for CRC although controversial results exist. Inhibition of the STAT3 pathway could decrease the Bcl-2 expression and increase Bax expression in CRC cells [9, 38]. Here, TRIM52 knockdown decreased Bcl-2 expression and increase Bax expression (Fig. 3), which may due to the decreased phosphorylation of STAT3. Complementary data were obtained in TRIM52 overexpressed cells (Fig. 6c). The increased apoptosis rate of CRC cells with TRIM52 knockdown may ascribed to the decreased ratio of Bcl2/Bax.

Our data also revealed that TRIM52 could promote CRC cell proliferation by inhibiting cell apoptosis through the STAT3 signaling pathway. Collectively, these findings provide new insight into the role of TRIM52 in CRC, which might serve as a prognostic indicator and a novel therapeutic target for CRC treatment.