Society: AGA

We have shown that Activin A (activin), a TGF-β superfamily member, has pro-metastatic effects on colon cancer cells. Recent evidence indicates activin activates pro-metastatic pathways to enhance tumor cell survival and migration while augmenting CD4+ to CD8+ communications to promote cytotoxicity. We recently provided evidence that activin signaling is required for TGF-β stimulation of metastasis indicating significant cross-talk in the tumor microenvironment (TME). Understanding the link between stimulation of metastasis and inflammation via activin and/or TGF-β may provide a novel opportunity for targeted therapy in colorectal cancer (CRC) via anti-activin treatment. Here, we hypothesized that activin exerts cell-specific effects in the TME to both promote anti-tumoral activity of immune cells and pro-metastatic behavior of tumor cells in a cell-specific and context-dependent manner. Western blot and transwell migration assays with and without activin were performed in human microsatellite instability (MSI) epithelial colon cancer cells with varying expression levels of ACVR2. The influence of the canonical Smad4 pathway was elucidated in a Ts4-Cre;cApc^flox;Smad4^flox mouse model for CRC. We stained and analyzed IHC images from the TMA of 1055 stage II and III CRC patients from the QUASAR2 cohort for CD4, TGF-β, and activin. TMA samples were also analyzed via Digital Spatial Profiling (DSP, NanoString) to determine the immune cell heterogeneity and cell signaling patterns within the TME relative to activin. We found that activin leads to activation of the non-canonical pAkt pathway in ACVR2 restored HCT116+chr2 colon cancer cells, but not in ACVR2 mutated HCT116 cells and leads to tumor cell migration in a PI3K-dependent manner. IHC analysis of the TMA samples indicated that increased levels of CD4, activin, or TGF-β were associated with increased survival time. Interestingly, low levels of both activin and TGF-β expression was associated with the worst survival time suggesting critical cross-talk and highly specific cellular signaling outcomes. In vivo, ablation of canonical SMAD signaling is associated with elevated activin, α-SMA and pAkt and increases dysplasia, mortality and stromal disorganization in intestinal tissue. DSP analysis identified activin co-localization in the stroma was coupled to increases in T-cell exhaustion markers while activin-negative regions of the tissue were associated with increases in markers for antigen presenting cells (APCs). Activin co-localization in the tumoral compartment of the tissue was associated with increases in markers of the MAPK and PI3K/Akt pathways. Taken together, these results suggest that the effects of activin in CRC are highly context-dependent and cellular-specific to promote a tumor-tolerant TME while enhancing tumor cell survival via PI3K/Akt and MAPK.

Colorectal cancer (CRC) development is modulated by the crosstalk between tumor cells and extrinsic factors, such as immune cells. CD4+ T cells, in particular, are important players in CRC, due to their dual role in activating the cytotoxic CD8+ T cells, while participating in tumor killing as well. To protect against extensive DNA damage due to constant exposure to tumor antigens, T cells can activate p21, which induces cell cycle arrest. However, the functional role of p21 in CD4+ T cells during the antitumor response agaings CRC has not yet been evaluated.
p21 expression in CD4+ T cells was evaluated in murine CRC tumor models, as well as in tumor biopsy samples from patients with CRC. To evaluate effector function of p21-expressing CD4+ T cells, in vivo orthotopic MC38 tumor models and in vitro co-culture models were employed.
Evaluation of tumor microenvironment in murine and human CRC samples revealed that p21 is upregulated in both nucleus and cytoplasm of CD4+ T cells. Interestingly, p21 expression in CD4+ T cells was negatively regulated with stage in tumor samples from patients with CRC. In in vivo murine CRC models, p21-deficiency in CD4+ T cells resulted in increased tumor growth compared to wildtype mice. Further evaluation of the tumor microenvironment revealed that p21-/- CD4+ T cells have a reduced cytotoxic potential, as shown by the low relative numbers of Tbet+ T cells and decreased IFNγ production. Co-culture experiments using MC38 tumor cells and in vitro polarized Th1 cells confirmed that the p21-deficient CD4+ T cells have a reduced effector function compared to the B6/J ones, therefore leading to decreased tumor cell death. Furthermore, persistent proliferation of p21-/- Th1 T cells led to accumulation of effector / effector memory T cells and loss of CD27/CD28 co-stimulatory molecules. These changes together with increased expression of DNA repair gene BRCA1 and checkpoint molecule CTLA4 suggest that p21-deficiency in CD4+ T cells promotes their exhaustion.
All in all, our data describe the previously unknown role of p21 in regulating CD4+ T cell exhaustion during the antitumor response against colorectal cancer. A better understanding of these cells could help improve the therapies used for treatment of CRC patients.

Maximilian Waldner

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