Interestingly, both receptors are less commonly inactivated in MSS colon cancers, which tend to have a worse prognosis than MSI-H colon cancers [9], and both pathways may be targeted independently. To date, little is known about the distinct selleck chemical Seliciclib contribution of activin signaling to colon cancer development and metastasis and specifically, how TGF�� and activin signaling effects differ despite identical intracellular SMAD signaling. p21 (also known as p21cip1/waf1) is a cyclin-dependent kinase inhibitor controlling cell cycle arrest via cdk1 and 2 inhibition and is a master regulator of multiple tumor suppressor pathways via both p53-dependent and independent mechanisms [10].

It is a known target gene of TGF�� in colon cancer [11], and has been associated with activin-induced growth arrest in plasmacytic and breast cancer cells [12], [13], but effects of activin on p21 in colon cancer cells as well as downstream consequences have not been assessed. In this study, we explored the mechanisms of TGF�� and activin on p21 regulation and the ensuing functional effects thereof in colon cancers. We found that despite identical intracellular SMAD signaling, TGF�� and activin regulate p21 via diverse mechanisms that are functionally relevant in colon cancer leading to more apoptosis or reduction in growth suppression dependent on the activin/TGF�� signaling status with p21 as a differentially regulated target. Results In the Presence of SMAD4, TGF�� is a more Potent Inducer of Growth Suppression While Activin is a more Potent Inducer of Apoptosis To test and compare the effects of activin and TGF�� on cell growth, we used colon cancer cell lines with differing SMAD4 status as described elsewhere [22], [23] in addition to SMAD4 knockdown.

The ACVR2/TGFBR2/SMAD4 wild type microsatellite stable colon cancer cell line FET and ACVR2/TGFBR2 wild type/SMAD4-null SW480 colon cancer cells were treated with either activin or TGF�� and cellular growth was assessed. As an additional control, SMAD4 was knocked down via siRNA in FET cells which were treated and analyzed accordingly. While both activin and TGF�� treatment led to significant growth suppression in SMAD4 wild type FETs, the effect was more pronounced following TGF�� treatment. In contrast, neither ligand was growth suppressive in the absence of SMAD4 in ACVR2/TGFBR2 wild type/SMAD4-null SW480 colon cancer cells or following SMAD4 knockdown in SMAD4 wild type FET cells (Figure 1A).

Figure 1 In the presence of SMAD4, TGF�� is a more potent inducer of growth suppression and activin a more potent inducer of apoptosis. We then compared apoptosis induction of either ligand in the presence and absence of SMAD4 or p21. Activin induced apoptosis to a greater extent than TGF��, and apoptosis only occurred in the presence of SMAD4 (Figure Brefeldin_A 1B, C).