TGF-β is produced in a latent form. Activation of the latent form is initiated by regulatory T cells with a transmembrane protein, glycoprotein A repetitions predominant (GARP), which binds and cleaves pro-TGF-β to produce latent TGF-β. The latent form is activated by integrins ( Figure 1A ) (17, 18). BMPs, GDFs, and Lefty A and B are produced and processed similarly, with inactive precursors being cleaved and activated by proteases ( Table 1 ) (19–21). The activins and inhibins are composed of common subunits and are formed by cleavage of dimerized subunits; inhibins are αβ heterodimers, and activins are ββ homodimers ( Table 1 ) (16). The TGF-β superfamily is essential in physiologic functions, including tissue development and differentiation, regulation of immunologic responses, and tissue healing (13). These members activate physiologic activities through canonical and non-canonical signaling (22). Canonical signaling occurs through the SMAD pathway, where receptor-activated (R) SMAD1/5/8 and SMAD2/3 are phosphorylated by receptors following ligand binding ( Figure 1B ). These phosphorylated SMADs then complex with SMAD4 and translocate to the nucleus to regulate the expression of the target genes ( Figure 1C ) (13). Non-canonical pathways can be activated upon the ligand binding to the receptors, such as Erk, involved in epithelial-mesenchymal transition (EMT), and JNK/p38, involved in EMT and apoptosis (23).

These signaling pathways play an essential role in proliferation and in controlling the growth of specific cell types, including epithelial cells, endothelial cells, immune cells, and neuronal cells, through growth inhibition and induction of apoptosis (24). BMPs are specifically involved in developing and maintaining skeletal tissues and are regulated extracellularly by antagonists, including Noggin, Chordin, and Gremlin1 (Grem1) ( Figure 1A ) (14). Additionally, activin A is a critical mesoderm-inducing factor (25), and GDFs are primarily involved in developmental processes (26). Inhibins antagonize activin signaling, and lefty inhibits phosphorylation of SMAD2 and subsequently regulates downstream signaling ( Table 1 ) (16, 27).

TGF-β also plays a role in immunoregulation by inhibiting T-lymphocyte proliferation and promoting T-cell differentiation (28–30). Additionally, these proteins play an essential role in fibroblast activation and are involved in routine wound healing; TGF-β1 is secreted by the platelets forming the hemostatic plug and is a chemoattractant for monocytes and fibroblasts essential to tissue repair (31–34).

In addition to the essential role of TGF-β in physiologic mechanisms, aberrantly increased TGF-β has been shown to contribute to excess fibrosis (35). Administration of exogenous TGF-β leads to fibrosis in subcutaneous tissues, lung parenchyma, and hepatic tissue (36–38). Furthermore, dysregulation of the TGF-β signaling pathway contributes to carcinogenesis (39). For example, tumor cells have been shown to evade the growth regulation of TGF-β through mutations in the TGF-β receptors and SMAD family (40, 41).

PDAC is currently the third leading cause of cancer-related death (42). Poor outcomes can be attributed to both late diagnosis and a fibrotic tumor microenvironment that surrounds the cancer cells, creating a chemo-resistant barrier. Further understanding of the role of the TGF-β superfamily may elucidate potential targets for novel therapies that could improve patient outcomes.

AP results from injury to the pancreatic acinar cells, leading to premature activation of pancreatic enzymes and causing pancreatic autodigestion and tissue inflammation (43). Apoptosis and suppression of proliferation have been reported to limit the inflammatory cascade in response to the insult in AP (44). TGF-β is released by various cells at the site of injury and induces apoptosis and suppress the proliferation of pancreatic acinar cells (45). However, this was contradicted by a study showing the induction of apoptosis upon suppressing the TGF-β signaling pathway in pancreatic epithelial cells (46). Additionally, our group has demonstrated that BMP signaling is upregulated in AP and causes dysregulation of autophagic processes. Administration of a BMP antagonist Noggin in vivo in a mouse model attenuated AP inflammation, suggesting a proinflammatory role of BMP signaling in AP (47).

In addition to involvement with apoptosis, TGF-β mediates the inflammatory response in AP through T-cell activation. Specifically, TGF-β induces the differentiation of both Th9 and Th17 cells, which are proinflammatory (48, 49). Th17 cells are known to secrete IL-17, associated with increased inflammatory markers and severity of AP (50).

CP results from repeated injury to the pancreas from recurrent bouts of AP, which leads to the replacement of normal pancreatic tissue with fibrotic scarring (51). This is primarily facilitated by the activation of PSCs, which secrete growth factors and chemokines such as TGF-β and produce excess ECM (52). The TGF-β secreted by activated PSCs is directly related to the characteristic fibrosis of CP (53). This fibrosis results from increased ECM production by PSCs (54) and inhibition of matrix metalloproteinases, which are involved in ECM degradation (55). Our group has shown that BMPs oppose the fibrogenic function of TGF-β on PSCs in CP by activating the SMAD1/5/8 pathway, which inhibits SMAD2 (56, 57).

Other modulators of TGF-β superfamily pathways are also involved in CP pathophysiology. Our group has shown that Grem1, an endogenous BMP antagonist, is pro-fibrogenic in a CP mouse model (58). Additionally, SMAD7, a known inhibitory SMAD, suppressed TGF-β signaling and modulated CP fibrosis through decreased ECM deposition and decreased inflammatory cell response in an in vivo mouse model (59).

The TGF-β superfamily plays dual roles in PDAC, promoting tumorigenesis in some capacities while acting as an inhibitor in others (60). In the early stages of PDAC, TGF-β has been shown to suppress tumor progression by promoting apoptosis and regulation of the cell cycle and promoting the stroma’s development by activating PSCs and increasing stromal production (61). Additionally, BMP2 expression is increased in pancreatic cancer and has variable mitogenic effects on pancreatic cancer cell lines, with a greater capacity to stimulate growth in cell lines with SMAD mutations (62). BMP signaling has also been shown to play a role in EMT through canonical BMP signaling, mediated by Grem1 inhibitory feedback, resulting in a maintenance of heterogeneity (63). Our group has also shown that activated fibroblasts express Grem1 and that increased expression is associated with a more severe tumor stage (64).

SMAD4 mutations are common in PDAC and are identified in approximately 60% of cases (66). Interestingly, an isolated SMAD4 mutation does not independently cause cancer; it must be paired with another mutation, such as KRAS (67). SMAD4 is part of the intracellular signaling pathway that complexes with activated and phosphorylated SMAD2/3 and SMAD1/5/8 in response to TGF-β and BMP binding their respective receptors (68). Mutation of SMAD4 results in loss of the tumor suppressor function of canonical TGF-β signaling (69). Loss of SMAD4 results in decreased T-cell recruitment and a suppressed immune response (70). Additionally, knockout of Smad4 in a PDAC mouse model has increased tumor sensitivity to host immune control and induced DNA damage (71).

Mutations in TGF-β receptors have been identified as disruptions of TGF-β signaling pathways that result in the loss of TGF-β suppressive effects. Studies have shown that mutations in TGFBR1, which encodes TGF-β type I receptor (TGFBRI), occur in approximately 1% of cases, and mutations in TGFBR2, which encodes TGF-β Type II receptor (TGFBRII), appear in approximately 4% of patients (72). Type III TGF-β receptor mutations also occur and result in increased EMT-associated increased motility and invasiveness (73).

Disruption in the expression of other receptors has also been reported. Deletion of ACVR1B, which encodes the ALK-4 receptor for activin A, is associated with a more aggressive cancer phenotype (74). Interestingly, mutations in BMPRI are described in patients with hereditary juvenile polyposis. BMPRI and BMPRII mRNA levels are upregulated in pancreatic cancers, and cells with higher levels have been shown to have more significant metastatic potential (75, 76).

KRAS is frequently mutated in human carcinomas and approximately 85% of PDAC cases (77). KRAS mutations are often detectable early in disease progression (78). GREM1 is upregulated in hereditary mixed polyposis syndrome, where duplications of the gene result in increased antagonism of BMP signaling (79). Similar mutations are observed in sporadic intestinal polyps (80). However, mutations in GREM1 have not been reported in cases of PDAC.

Because of the complexity of the TGF-β signaling pathway, numerous potential targets are under investigation ( Figure 1 ). Therapeutic strategies include antisense oligonucleotides, neutralizing antibodies, ligand traps, and small molecule kinase inhibitors. Many of these therapies are being investigated in several cancers, including pancreatic cancer.

In addition to TGF-β, other members of the TGF-β superfamily are promising targets for cancer therapeutics. Interestingly, ginisortamab (UCB6114), an antibody that neutralizes Grem1 and blocks its antagonistic effects on BMP signaling, has been shown to restore BMP signaling pathways in human colorectal cancer cell lines and fibroblasts ( Figure 1B ) (110). It is currently being evaluated by a phase 1/2 clinical trial (NCT04393298) in advanced solid tumors, including pancreatic adenocarcinomas.

GDF-15 has also been identified as a potential target. While the exact signaling pathway has yet to be elucidated, recent studies have identified a unique GDF-15 receptor glial cell-derived neurotrophic factor family receptor α-like (GFRAL) and have shown that GDF-15 inhibits leukocyte integrin activation and T cell migration, which is reversed with neutralization of GDF-15 (111, 112). Visugromab (CTL-002), a neutralizing antibody of GDF-15, is currently under evaluation in phase 2a of the GDFATHER trial (NCT04725474) and is showing promising results in combination with nivolumab in advanced non-small cell lung cancer and urothelial cancer.