Cbl-independent degradation of Met: ways to avoid agonism of bivalent Met-targeting antibody

Abstract

The Met receptor tyrosine kinase, found to be constitutively activated in many tumors, has become a leading target for cancer therapy. Disruptions in Met downregulation have been associated with aggressive tumor progression with several therapeutic strategies addressing this aspect of Met biology. Castias B-lineage lymphoma (Cbl) E3 ligase-mediated degradation, which attenuates Met signaling via ligand-dependent Met internalization, is a major negative regulator of Met expression. It is believed that one of the mechanisms by which the therapeutic anti-Met antibodies induce cancer cell death in Met overexpressing tumors is via internalization and subsequent degradation of Met from the cell surface. However, a previously reported Met-targeting antibody demonstrated intrinsic agonistic activity while being capable of inducing Cbl-mediated degradation of Met, suggesting that Cbl-mediated degradation requires receptor activation and impedes therapeutic application. We have developed a potent and selective bivalent Met-targeting antibody (SAIT301) that invokes Met degradation using an alternative regulator LRIG1. In this report, we demonstrate that LRIG1 mediates degradation of Met by SAIT301 and this degradation does not require Met activation. Furthermore, SAIT301 was able to downregulate Met and dramatically inhibit growth of tumors with low or no Cbl expression, as well as tumors with Met exon 14 deletion that prevents Met binding to Cbl. In summary, we demonstrate the enhanced therapeutic potential of a novel tumor-inhibiting anti-Met antibody, SAIT301, which utilizes a Cbl-independent, LRIG1-mediated Met degradation pathway and thereby avoids the agonism that limits the effectiveness of previously reported anti-Met antibodies.

Leucine-rich Repeat and Immunoglobulin Domain-containing Protein-1 (Lrig1) Negative Regulatory Action toward ErbB Receptor Tyrosine Kinases Is Opposed by Leucine-rich Repeat and Immunoglobulin Domain-containing Protein 3 (Lrig3)

“…However, subsequent studies from our group revealed that Cbl knockdown had no effect on Lrig1 activity toward the Met receptor (4), calling into question the role of Cbl and, by extension, the Lrig1 cytoplasmic domain in Lrig1 action. In agreement with our results, Lee et al (25) recently reported that the Met-targeting antibody SAIT301 promotes an Lrig1-dependent but c-Cbl-independent lysosomal degradation of the Met receptor. Our current results with ⌬-Cyto-Lrig1 demonstrate unequivocally that the cytoplasmic domain of Lrig1 is not required for the following: (a) complex formation with Lrig3 and (b) Lrig3 suppression, revising the model of Lrig1 function.…”

Microenvironment-Derived HGF Overcomes Genetically Determined Sensitivity to Anti-MET Drugs

“…However, HGF induced an IC 50 increase ranging from 5 to 24 times (Supplementary Table S1), a change that is in line with the IC 50 shifts induced by multiple rescuing cytokines in a recent major study on targeted drug resistance (23). In the case of antagonistic antibodies that reduce MET expression by inducing "shedding" or internalization (6,36,37), the mechanism responsible for HGF-induced rescue may rely on the fact that expression of MET in c-MET-amplified cells is way above the physiologic level, and antagonistic antibodies decrease MET expression down to normal levels rather than abrogating it (Fig. 4).…”

Activation of MET via Diverse Exon 14 Splicing Alterations Occurs in Multiple Tumor Types and Confers Clinical Sensitivity to MET Inhibitors

“…These MET ex14 alterations were shown to promote RNA-splicingbased skipping of MET exon 14, which results in activation of MET kinase activity through a unique mechanism. The portion of the protein encoded by exon 14, most prominently Y1003 in a DpYR motif, is required for effi cient recruitment of the ubiquitin ligase CBL, which targets MET for ubiquitinmediated degradation (16)(17)(18). Loss of MET exon 14 maintains the reading frame and leads to increased MET stability and prolonged signaling upon HGF stimulation, leading to increased oncogenic potential ( 19,20 ).…”