# Shedding of GPP130 by PC7 and Furin: Potential Implication in Lung Cancer Progression
###### Abstract
Additionally, manganese (Mn\({}^{2+}\)) treatment has been shown to induce GPP130 oligomerization, thereby redirecting it into multivesicular bodies (MVBs) en route to lysosomal degradation [21; 22]. Thus, GPP130 maintains intracellular Mn\({}^{2+}\) homeostasis by binding excess Mn\({}^{2+}\) in the Golgi lumen, thereby initiating the routing of Mn\({}^{2+}\)-bound GPP130 to lysosomes for degradation. Indeed, Mn\({}^{2+}\) treatment inhibits the progression of multiple types of 3q-amplified malignancies by degrading GPP130, resulting in a secretory blockade that interrupts pro-survival autocrine loops and attenuates pro-metastatic processes in the tumor microenvironment [23]. GPP130 is thus considered a Golgi-resident protein with a distinctive trafficking route, facilitating cargo transport within the secretory pathway. Moreover, the GPP130/GOLIM4 gene has been implicated in head and neck cancer through a study that identified it as a downstream target of stromal interaction molecule 1 (STIM1), a Ca\({}^{2+}\) channel protein involved in store-operated calcium entry (SOCE)--a pathway activated in response to decreased Ca\({}^{2+}\) levels in the ER [24]. In this study, GPP130 expression was found to be elevated in head and neck cancer cells and was proposed as a downstream target of STIM1 [25]. Furthermore, GPP130 has been implicated in regulating cell cycle progression and proliferation by modulating the expression of MDM2 and CDK6 in these cancer cells [25]. Recently, it was also shown that miR-105-3p acts as an oncogene to promote the proliferation and metastasis of breast cancer cells by silencing GOLIM4 [26] and that targeting miR-942-5p/GPP130 axis suppressed breast cancer malignant behavior [27]. Finally, a non-biased proteomic analysis identified GPP130 as a novel target for the treatment of endometrial cancer [28]. Thus, so far, the available data suggest that depending on the cancer type, GPP130 can act as a tumor suppressor or an oncogene. However, the specific role of GPP130 or its fragments in lung cancer has not been studied. Figure 1: PC7 and Furin cleave GPP130. **(A)** A schematic explaining the identification of three PC7 substrates using mass spectrometry analysis. **(B)** Analysis of genetic alterations in the GOLM2 gene (protein product: CASC4) and GOLIM4 (protein product: GPP130) using cBioPortal data. The figure was plotted using the cBioPortal website and depicts the frequency and type of genetic alterations for the two genes. **(C)** Schematic representation of human Golgi Phosphoprotein of 130 kDa (GPP130) protein. Depicted are the cytosolic tail (CT), the transmembrane domain (TM), the luminal domain, and the C-terminal V5-tag. The blue circle depicts a potential N-glycosylation site. **(D)** Western blot analysis of cell lysates and media from HEK293 cells expressing GPP130-V5 with either pIRES-empty vector (EV) or human PC7 [\(n=3\)]. **(E)** Western blot analysis of cell lysates and media from HEK293 cells expressing GPP130-V5 with all the basic aa PCs or pIRES-empty vectors [\(n=3\)]. The images shown are representative of three independent experiments. Because of the amplified expression of GPP130 in lung cancer patients (Figure 1B), in the present study, we concentrated on the ex vivo growth function activity of GPP130 in the lung carcinoma A549 cell line. The functional implications of PC7/Furin in the cleavage of GPP130, as well as its cytosolic tail Cys-palmitoylation, were studied in more detail. We also compared the growth-promoting activities of GPP130 to those of CASC4 and their shed products. The data showed that GPP130 cleavage by PC7 and/or Furin results in enhanced cellular growth, likely due to the production of a secreted soluble form and an N-terminal membrane-bound fragment. The potential Cys-palmitoylation of the latter was found to reduce its growth-promoting activity. Conversely, blocking the PC7/Furin processing of GPP130 results in a substantial reduction in cellular growth.