Dual Role for Transforming Growth Factor β-Dependent Signaling in Trypanosoma cruzi Infection of Mammalian Cells

Expression of functional transforming growth factor b (TGF-b) receptors (TbR) is required for the invasion of mammalian cells by the protozoan parasite Trypanosoma cruzi. However, the precise role of this host cell signaling complex in T. cruzi infection is unknown. To investigate the role of the TGF-b signaling pathway, infection levels were studied in the mink lung epithelial cell lines JD1, JM2, and JM3. These cells express inducible mutant TbR1 proteins that cannot induce growth arrest in response to TGF-b but still transmit the signal for TGF-b-dependent gene expression. In the absence of mutant receptor expression, trypomastigotes invaded the cells at a low level. Induction of the mutant receptors caused an increase in infection in all three cell lines, showing that the requirement for TGF-b signaling at invasion can be divorced from TGF-b-induced growth arrest. TGF-b pretreatment of mink lung cells expressing wild-type TbR1 caused a marked enhancement of infection, but no enhancement was seen in JD1, JM2, and JM3 cells, showing that the ability of TGF-b to stimulate infection is associated with growth arrest. Likewise, expression of SMAD7 or SMAD2SA, inhibitors of TGF-b signaling, did not block infection by T. cruzi but did block the enhancement of infection by TGF-b. Taken together, these results show that there is a dual role for TGF-b signaling in T. cruzi infection. The initial invasion of the host cell is independent of both TGF-b-dependent gene expression and growth arrest, but TGF-b stimulation of infection requires a fully functional TGF-b signaling pathway. Trypanosoma cruzi is the agent responsible for Chagas’ disease, a chronic infection widespread in Central and South America (4). T. cruzi is an intracellular parasite in the mammalian host. Infectious metacyclic trypomastigotes introduced in the feces of the hemophagic reduviid bug invade host cells. Initially the parasite is located in a lysosomal compartment, but it rapidly escapes into the cytoplasm. Amastigotes, the intracellular form of the parasite, divide in the host cell cytoplasm and then differentiate back into trypomastigotes, which are released, destroying the cell and allowing continuation of infection in new cells. The process of mammalian cell invasion by T. cruzi and the mechanisms of parasite survival within the host cell are poorly understood. Entry is accompanied by stimulation of several host cell signaling pathways by trypomastigotes. Calcium transients (16), tyrosine phosphorylation (20), MAP kinase activation (20), and transforming growth factor b (TGF-b) receptor (TbR) activation (10) are all important in invasion. The role of TGF-b signaling in T. cruzi infection is demonstrated by the fact that infection is very low in the absence of TbR1 or -2 and is restored by expression of the deficient receptor (10). Preincubation of host cells with TGF-b causes a marked enhancement of infection (10), but the mechanism by which TGF-b promotes T. cruzi infection is not known. TGF-b is a pleiotropic factor that regulates a number of cellular functions and can stimulate expression of extracellular matrix proteins, cell cycle arrest, cell growth, and apoptosis in