A High-Throughput Assay for Collagen Secretion Suggests an Unanticipated Role for Hsp90 in Collagen Production

Collagen
overproduction is a feature of fibrosis and cancer, while
insufficient deposition of functional collagen molecules and/or the
secretion of malformed collagen is common in genetic disorders like
osteogenesis imperfecta. Collagen secretion is an appealing therapeutic
target in these and other diseases, as secretion directly connects
intracellular biosynthesis to collagen deposition and biological function
in the extracellular matrix. However, small molecule and biological
methods to tune collagen secretion are severely lacking. Their discovery
could prove useful not only in the treatment of disease, but also
in providing tools for better elucidating mechanisms of collagen biosynthesis.
We developed a cell-based, high-throughput luminescent assay of collagen
type I secretion and used it to screen for small molecules that selectively
enhance or inhibit that process. Among several validated hits, the
Hsp90 inhibitor 17-allylaminogeldanamycin (17-AAG) robustly decreases
the secretion of collagen-I by our model cell line and by human primary
cells. In these systems, 17-AAG and other pan-isoform Hsp90 inhibitors
reduce collagen-I secretion post-translationally and are not global
inhibitors of protein secretion. Surprisingly, the consequences of
Hsp90 inhibitors cannot be attributed to inhibition of the endoplasmic
reticulum’s Hsp90 isoform, Grp94. Instead, collagen-I secretion
likely depends on the activity of cytosolic Hsp90 chaperones, even
though such chaperones cannot directly engage nascent collagen molecules.
Our results highlight the value of a cell-based high-throughput screen
for selective modulators of collagen secretion and suggest an unanticipated
role for cytosolic Hsp90 in collagen secretion.