The
insulin-like growth factor (IGF) signaling pathway is involved in certain human
cancers, and the feasibility of directly targeting the IGF receptor has been actively investigated. However, recent evidence from clinical trials suggests that this approach can be problematic. We have developed an alternative strategy to indirectly inhibit the IGF signaling by targeting the
metalloproteinase,
pregnancy-associated plasma protein-A (
PAPP-A).
PAPP-A associated with the cell surface cleaves IGF binding protein-4 (IGFBP-4), when IGF is bound to
IGFBP-4, and thereby increases IGF bioavailability for receptor activation in an autocrine/paracrine manner. We hypothesized that inhibition of
PAPP-A would suppress excessive local IGF signaling in tissues where this is caused by increased
PAPP-A proteolytic activity. To test this hypothesis, we developed an inhibitory
monoclonal antibody, mAb 1/41, which targets a unique substrate-binding exosite of
PAPP-A. This inhibitor selectively and specifically inhibits proteolytic cleavage of
IGFBP-4 with an inhibitory constant (Ki) of 135 pM. In addition, it inhibited intracellular signaling of the IGF receptor (AKT phosphorylation) in monolayers of A549 cells, an IGF-responsive
lung cancer-derived cell line found to express high levels of
PAPP-A. We further showed that mAb 1/41 is effective towards
PAPP-A bound to cell surfaces, and that it is capable of inhibiting
PAPP-A activity in vivo. Using a murine xenograft model of A549 cells, we demonstrated that mAb 1/41 administered intraperitoneally significantly inhibited
tumor growth. Analysis of xenograft
tumor tissue recovered from treated mice showed penetration of mAb 1/41, reduced
IGFBP-4 proteolysis, and reduced AKT phosphorylation. Our study provides proof of concept that IGF signaling can be selectively reduced by targeting a regulatory
proteinase that functions extracellularly, upstream of the IGF receptor.
PAPP-A targeting thus represents an alternative therapeutic strategy for inhibiting IGF receptor signaling.