TGFB1 is secreted through an unconventional pathway dependent on the autophagic machinery and cytoskeletal regulators

Abstract

TGFB1 (transforming growth factor beta 1) is a potent cytokine playing a driving role in development, fibrosis and cancer. It is synthesized as prodomain-growth factor complex that requires tethering to LTBP (latent transforming growth factor beta binding protein) for efficient secretion into the extracellular space. Upon release, this large latent complex is sequestered by anchorage to extracellular matrix (ECM) networks, from which the mature growth factor needs to be activated in order to reach its receptors and initiate signaling. Here, we uncovered a novel intracellular secretion pathway by which the latent TGFB1 complex reaches the plasma membrane and is released from fibroblasts, the key effector cells during tissue repair, fibrosis and in the tumor stroma. We show that secretion of latent TGFB1, but not of other selected cytokines or of bulk cargo, is regulated by fibroblast-ECM communication through ILK (integrin linked kinase) that restricts RHOA activity by interacting with ARHGAP26/GRAF1. Latent TGFB1 interacts with GORASP2/GRASP55 and is detected inside MAP1LC3-positive autophagosomal intermediates that are secreted by a RAB8A-dependent pathway. Interestingly, TGFB1 secretion is fully abrogated in human and murine fibroblasts and macrophages that lack key components of the autophagic machinery. Our data demonstrate an unconventional secretion mode of TGFB1 adding another level of control of its bioavailability and activity in order to effectively orchestrate cellular programs prone to dysregulation as seen in fibrosis and cancer.

The GRASP domain in Golgi Reassembly and Stacking Proteins: differences and similarities between lower and higher Eukaryotes

“…A previous work has suggested a direct (and necessary) interaction of GRASPs with the protein to be secreted, especially in the case of type 4 UPS [22]. This direct interaction with the protein cargo has already been observed in other situations [23,24,25,26]. Some specific roles have also emerged, such as the participation in the formation of specific autophagosome-like structures (named compartments of unconventional protein secretion, or CUPS), but regardless their true nature and existence in yeast [27,28], so far there are no evidences that these structures exist in mammalian cells [29].…”

The GRASP domain in golgi reassembly and stacking proteins: differences and similarities between lower and higher Eukaryotes

“…A previous work has suggested a direct (and necessary) interaction of GRASPs with the protein to be secreted, especially in the case of type 4 UPS . This direct interaction with the protein cargo has already been observed in other situations . Some specific roles have also emerged, such as the participation in the formation of specific autophagosome‐like structures (named compartments of unconventional protein secretion, or CUPS), but regardless of their true nature and existence in yeast , so far there are no evidences that these structures exist in mammalian cells .…”

Identification of new therapeutic targets for osteoarthritis through genome-wide analyses of UK Biobank

“…Apparent causal associations of knee pain with osteoarthritis ( Supplementary Table 12 and 15) are potentially attributable to reverse causality (Supplementary Note). We estimate the proportion of the total narrow sense heritability explained by osteoarthritis loci to be 14.7 % for knee osteoarthritis, 51.9 % for hip osteoarthritis, 24.2% for osteoarthritis of the hip and/or knee, and 22.5% of osteoarthritis at any site ( Supplementary Table 16). We do not find evidence for a role of low-frequency or rare variation of large effect in osteoarthritis susceptibility, and have limited power to detect smaller effects at lower-frequency variants (Figure 2).…”