Acidosis induces cell invasions in renal cancer cells
We recently reported that acidosis induced elevated level of Snail ,leading to invasion of prostate cancer via ASIC1 in vitro[6]. Therefore, we supposed acidosis also promoted cell invasion in renal cancer cells. We then determine the invasion ability of 786-O and 769P cell lines upon acidosis. In one hour exposed to acid medium (pH 6.6) the average invasive 769P cells number was increased three times as much as that in cells of pH7.4(Fig. 1A). In addition, similar results were found in 786-O cells(Fig. 1B). Moreover, acidosis-induced increased level of Snail also provided extra evidence(Fig. 1C and D), which has been implicated in cell invasion. The level of Snail is significantly elevated in half an hour and got a peak at 1 hour. Of interest was 769P with over-expressed Snail at as early as 30 min.
BC040587 is required in acidosis-induced cell invasion
Long non-coding RNAs(LncRNAs) is of vital importance in cancer development including invasion and metastasis[8]. Thus, we focused on which LncRNAs could be implicated in cell invasion of renal cancer response to acidosis. As shown in supplemental table, Small scale LncRNA profiling which includes 90 crucial LncRNAs uncovered potential related LncRNAs to acidosis. For example, we found 40 down-regulated LncRNAs and 44 up-regulated LncRNAs (2 LncRNAs unavailable), among which BC040587 was increased significantly (Fig. 2A). To validate the up-regulated level of BC040587 upon acidosis in renal cancer cells, we then exposure the cells to acidosis. It is indicated that acidosis induced increased BC040587. In consistent with invasion assay, BC040587 got a peak at 1 hour in both 769P and 786-O cells. (Fig. 2B)
To figure out more about the role of BC040587 in cell invasion upon acidosis, we knocked out BC040587 by CRISPR/Cpf1 system (Fig. 2C).We then use the knockout clone (#17) for further characterization. We then asked whether BC040587 was required for cell invasion upon acidosis. Acidosis cannot induce increased invasion ability after BC040587 had been knockout in 769P cells(Fig. 2D), while cell invasion was inductive in original 769P cells. Moreover, as expected in Fig. 2E, Snail over-expression was almost abolished 1 hour after exposure to acidosis, demonstrating that BC040587 is required in acidosis-induced cell invasion.
Acidosis induces the activation of AKT/mTOR and MEK/ERK pathway in a BC040587-dependent manner
It was acknowledged that acidosis induced phosphorylation of AKT in breast cancers[5]. We previously found that ERK activates upon acidosis when there was elevated AKT background[6]. Hence, we asked whether acidosis induce activation of AKT and MEK in renal cancer cells. As Snail level and cell count in invasion assay reached a peak at 60 min, we exposed the cells at the same duration and determined some critical pathways. As shown in Fig. 3A, acidosis phosphorylated P90RSK and MEK1/2. The same trend was also found as for the AKT(Fig. 3A and B). Moreover, phosphorylation of AKT 1 hour after exposure to acidosis was abolished after knockout of BC040587. Activated MEK directly connected to ERKs through its N-terminal region, catalyzing phosphorylation of ERK and activating ERK. Therefore, we determined the activation of ERK in 769P and 769PBC040587(−). As indicated in Fig. 3C, we found ERK activation in 769P cells in response to acidosis. Of interest is the 769PBC040587(−)cells, in which ERK inactivation upon acidosis were confirmed. This data suggests that BC040587 is of vital importance in activation of MEK/ERK pathway. On the other hand, acidosis can also phosphorylate the crucial factor mTOR in the AKT pathway. The role of BC040587 in activation of AKT/mTOR pathway is as some as that in MEK/ERK pathway (Fig. 3D), which suggests the BC040587-dependent manner in the both pathways in response to acidosis. However, the activation of AKT is earlier than that of ERK, which might explain the potential crosstalk between the two pathways.
MDM2 is involved in the BC040587-required cell invasion upon acidosis
Given BC040587 is essential in cell invasion upon acidosis; we asked what was the mechanism and what might be regulated by BC040587? We took advantage of the database cBioPortal[19] to search for further upstream or downstream signaling molecules. Data of 538 RCC cases from cBioPortal indicated that BC040587 was closely related to those genes including CDKN2A, MDM2, CDK4, TP53, RB1, and CCNE1(Fig. 4A). Among them, MDM2 was the most attractive one. MDM2 forms a complex with oligomeric p53, HERC2, and NEURL4[14]. Activation of either AKT or ERK can induce both phosphorylation of MDM2 and enhancement of the fundamental interaction between MDM2 and p53[20]. Of interest, MDM2 phosphorylation was induced by acidosis and was abolished by BC040587 knockout(Fig. 4B). To study the feature of MDM2 in cell invasion upon acidosis, we then knocked down MDM2 with siRNA(Fig. 4C). Acidosis-induced over-expression of MDM2 was abolished by siRNA-MDM2(Fig. 4C). Additionally, average invasive 769P cells number were not increased as long as we knocked down MDM2(Fig. 4D). The same trend was also found as for expression of Snail, which is considered as a bio-marker of cell invasion (Fig. 4E).
BC040587 indicated poorer prognosis in RCC
As BC040587 has been shown to be required in the cell invasion of RCC, we then studied the role of BC040587 in clinical cases. In consistent with findings in vitro, as expected in Fig. 5A, the level of BC040587 was elevated in stage III/IV compared with that in stage I/II (Fig. 5A). This data indicated that over-expressed BC040587 led to invasion and metastasis, which was very common in higher stage. In addition, patients were divided into two categories based on the level of BC040587. As shown in Fig. 5B, significant differences between the two groups were found. Patients with over-expressed BC040587 had poorer prognosis when overall survival was compared, which suggests BC040587 might be novel bio-marker of prognosis in RCC.