R Spondin in Cancer: Inducer or Impeder?

Player in the Game

Being the one of the key trigger factors for Wnt/β-Catenin pathway, R-Spondins (RSPOs) are actively involved in cancer cell proliferation and pathogenesis. There is accumulating evidence suggesting that RSPOs are interrelated to cancer prognosis. RSPOs are involved in vital aspects such as therapeutics, drug targets and immune mechanisms. This raised interested in scientists to explore RSPOs more and probe in the angle of RSPOs mediated alternations in diverse cancers.^1,2

Besides possessing a pivotal role in development, RSPO family have been identified for being instrumental in cancer. ¹ Oncogenic capability of R-spondins especially RSPO3 has been implicated in the intestinal tract related cancers especially colon cancer patients. RSPO gene fusions have been noted in diverse carcinomas making it a possible target in cancer therapeutics.

The current editorial is aimed to enlist the dichotomous nature of RSPOs and how that might influence cancer progression. As evident in the Figure 1, RSPOs expression varies from cancer-cancer. ² While it might be observed that RSPO1, RSPO2 and RSPO3 are down regulated in lung cancer, however some RSPOs such as RSPO2 and RSPO4 are over expressed in breast cancer patients.^3,4

Figure 1.

Median expression of RSPOs in diverse tumors ² The color solidness denotes the mean expression of specific RSPOs in the specific tissue. The expression has been normalized by the highest median expression.

RSPO gained importance subsequent to the work by Seshagiri and team where they have identified recurring R-spondin fusions in colon cancer. ⁵ The evident key findings were: the presence of R-spondin fusions in 10% of colon cancer tumor samples and R-spondin might be an activator of Wnt signaling. It was also noteworthy through this study that, RSPO positive fusion tumors switched on R-spondin receptors such as LGR4, LGR5 and LGR6. R-spondin receptors have been positively associated with poor prognosis and metastasis.^6,7 The primary linkage and crosstalk between the RSPO and Wnt were initiated from this study. RSPO blockade has been correlated with suppression of stem cell signaling in colon cancer. ⁸ We therefore thought to cover the means how RSPO can dictate and orchestrate signaling pathways of both po-cancer and anti-cancer mechanisms.

Partners of RSPOs and their Nature as Double-Edged Sword

Besides the synergistic relationship during mouse development, there has been an overlap between RSPOs and Wnt. RSPO members were found to orchestrate WNT pathway by a certain mechanism in which the expression pattern might determine biological activity in conditions like cancer. ⁹ Under the conditioning of anti-RSPO3, Wnt target genes were impeded in colorectal cancer (CRC) models. ⁸ Additionally, anti-RSPO treatment aided in sensitizing the tumor cells via down regulation of WNT which signifies the RSPO-WNT axis and crosstalk. In high grade serous ovarian cancer, WNT signaling was fueled by RSPO1 and its partner LGR6. This study involved testing clinical pattern, where WNT pathway was found to have a significant impact on patient survival. ¹⁰ Aberrant expression of RSPOs were evidently increased by fusions in CRC. RSPO-fusion tumors were observed to be WNT dependent via WNT inhibitor LGK974 in a study by Han and colleagues. ¹¹ This interrelationship has been well reviewed in a neoteric opinion by Srivastava and associates. ¹² However, on the contrary, a recent study reported that RSPO3 stimulates advancement of breast cancer independently regardless of WNT pathway which was validated using organoids where WNT inhibitor C59 had no effect on RSPO3. RSPO3 aided in proliferation as well and once again independent of WNT. ¹³ RSPO-BRAF axis was evaluated wherein, subsequent to a screening cohort of papillary thyroid cancer, it was found that the metastatic papillary thyroid cancer (V600E) displayed an increased RSPO4, which enumerated a novel angle for drug targeting since BRAF mutation V600E was regarded as an aggressive phenotype and difficult to target. ¹⁴

In the β-catenin-dependent pathway, FZD7 and the co-receptor LRP5/6 can bind to several Wnt proteins to trigger downstream signaling.

RSPOs as Inhibitors

On the other side of the coin, RSPOs exhibit cancer suppression potential utilizing the same axis, for example RSPO2 impedes Wnt25a signaling thereby preventing the binding of Wnt5 to Fzd7 (Figure 2) which led to repression of migration and invasion. ¹⁵ RSPO1 was independently found to initiate TGF-β signaling in an independent study of colon cancer. Not only does RSPO1 suppresses the Colon cancer progression in vivo, but also it activates TGF-β signaling via complexes leading to Smad 2 formation. ¹⁶ This expression correlation was also applicable to clinical scenario where tumors with phosphorylated Smad2 also expressed LGR5 which demonstrates the RSPO1/LGR5 mediated TGF-β signaling. Furthermore, to support this work, recent research underpins the statement that RSPO1 spikes the LRP6 expression with CK1€ and Axin1. RSPO1 teams up with Wnt3A to pull down Axin 1. This might be a beneficial anti-cancer potential as Axin communicates with β-catenin. ¹⁷ Here, Wnt activation brings in Axin and GSK3β leading to laminin receptor precursor (LRP) signalosome initiation and then the β-catenin translocates to the nucleus and triggers Wnt target genes where it was found to be as a prime target in numerous cancer treatments. ¹⁸ Female sex hormones such as progesterone and estrogen are vital not in developmental phases but also in cancer development and progression such as breast. A study referring to endocrinal development demonstrated that RSPO1 acts independent of Wnt and switches on Erα expression via G-protein coupled receptor cAMP/PKA pathway. ¹⁹ Why can this be a novel angle of usage of RSPOs as anti-cancer might be because high Erα expression is positive in female cancers treatment. The increased expression of Erα might impede tumor growth and also reduce vascular endothelial growth factor (VEGF) as observed in the previous study. ²⁰

Figure 2.

RSPO association with Wnt.

A compendium of RSPOs role in cancer suggests its duality in cancer treatment. A better understanding of its nature is necessary for a more precise usage and drug targeting. Further synergistic pathways and signaling mechanisms exploration is indispensable. Although the expression pattern of RSPOs has been primarily or majorly explored in the alimentary canal cancers or colon cancers, further exploration will lead to unraveling a complex RSPO mediated tumorogenic cascade in diverse cancers and how this will open the pandora box of cancer pathogenesis.