Adoptive cell therapy of solid tumors with reprogrammed T cells can be considered the “next generation” of cancer hallmarks. CAR-T cells fail to be as effective as in liquid tumors for the inability to reach and survive in the microenvironment surrounding the neoplastic foci. The intricate net of cross-interactions occurring between tumor components, stromal and immune cells leads to an ineffective anergic status favoring the evasion from the host’s defenses. Our goal is hereby to trace the road imposed by solid tumors to CAR-T cells, highlighting pitfalls and strategies to be developed and refined to possibly overcome these hurdles.

The developmental protein Numb is a major determinant of binary cell fates. It is also required for the differentiation of cerebellar granule cell progenitors (GCPs) at a stage of development responsive to the morphogenic glycoprotein Hedehog. Hedgehog signalling is crucial for the physiological maintenance and self-renewal of neural stem cells and its deregulation is responsible for their progression towards tumorigenesis. The mechanisms that inhibit this pathway during the differentiation stage are poorly understood. Here, we identify Numb as a Hedgehog-pathway inhibitor that is downregulated in early GCPs and GCP-derived cancer cells. We demonstrate that the Hedgehog transcription factor Gli1 is targeted by Numb for Itch-dependent ubiquitination, which suppresses Hedgehog signals, thus arresting growth and promoting cell differentiation. This novel Numb-dependent regulatory loop may limit the extent and duration of Hedgehog signalling during neural-progenitor differentiation, and its subversion may be a relevant event in brain tumorigenesis.

The ErbB-2 interacting protein receptor-associated late transducer (RALT) was previously identified as a feedback inhibitor of ErbB-2 mitogenic signals. We now report that RALT binds to ligand-activated epidermal growth factor receptor (EGFR), ErbB-4 and ErbB-2.ErbB-3 dimers. When ectopically expressed in 32D cells reconstituted with the above ErbB receptor tyrosine kinases (RTKs) RALT behaved as a pan-ErbB inhibitor. Importantly, when tested in either cell proliferation assays or biochemical experiments measuring activation of ERK and AKT, RALT affected the signalling activity of distinct ErbB dimers with different relative potencies. RALT DEBR, a mutant unable to bind to ErbB RTKs, did not inhibit ErbB-dependent activation of ERK and AKT, consistent with RALT exerting its suppressive activity towards these pathways at a receptor-proximal level. Remarkably, RALT DEBR retained the ability to suppress largely the proliferative activity of ErbB-2.ErbB-3 dimers over a wide range of ligand concentrations, indicating that RALT can intercept ErbB-2.ErbB-3 mitogenic signals also at a receptor-distal level. A suppressive function of RALT DEBR towards the mitogenic activity of EGFR and ErbB-4 was detected at low levels of receptor occupancy, but was completely overcome by saturating concentrations of ligand. We propose that quantitative and qualitative aspects of RALT signalling concur in defining identity, strength and duration of signals generated by the ErbB network.

The ErbB signaling network consists of four transmembrane receptor tyrosine kinases and more than a dozen ligands sharing an epidermal growth factor (EGF) motif. The multiplicity of ErbB-specific ligands is incompletely understood in terms of signal specificity because all ErbB molecules signal through partially overlapping pathways. Here we addressed the action of epiregulin, a recently isolated ligand of ErbB-1. By employing a set of factor-dependent cell lines engineered to express individual ErbBs or their combinations, we found that epiregulin is the broadest specificity EGF-like ligand so far characterized: not only does it stimulate homodimers of both ErbB-1 and ErbB-4, it also activates all possible heterodimeric ErbB complexes. Consistent with its relaxed selectivity, epiregulin binds the various receptor combinations with an affinity that is approximately 100-fold lower than the affinity of ligands with more stringent selectivity, including EGF. Nevertheless, epiregulin's action upon most receptor combinations transmits a more potent mitogenic signal than does EGF. This remarkable discrepancy between binding affinity and bioactivity is permitted by a mechanism that prevents receptor down-regulation, and results in a weak, but prolonged, state of receptor activation.Various biological processes are controlled by intercellular interactions that are mediated by polypeptide growth factors. Examples include embryonic development, neuronal functions, hematopoiesis, and pathological situations, like wound healing and malignant transformation. The mechanism transmitting extracellular signals ultimately starts with binding of the growth factor to a cell surface receptor, that in many cases carries an intrinsic tyrosine kinase activity (1). These receptors fall into several subgroups sharing structural and functional characteristics. Each subgroup of receptors specifically recognizes a family of structurally homologous growth factors. Perhaps the most striking multiplicity of related growth factors is exemplified by the epidermal growth factor (EGF) 1 family of molecules (2). This six cysteine-containing motif of 45-60 amino acids is shared by all members of the family, and it functions as the receptor binding portion of the molecule. Currently there are four known receptors for EGF-like ligands, constituting the ErbB subgroup of receptor tyrosine kinases (also known as HER, or type I receptor tyrosine kinases (3)). Whereas ErbB-1 binds many ligands, including EGF, transforming growth factor ␣ (TGF␣), and amphiregulin, both ErbB-3 and ErbB-4 bind to a family of isoforms, collectively known as neuregulins (also called Neu differentiation factors, heregulins, glial growth factors, and acetylcholine receptor inducing activity) (4). A related group of molecules, termed NRG2, binds to the same two receptors (5-7), and a third molecule, NRG3, exclusively binds to ErbB-4 (8). Two other ligands, betacellulin (9), and the heparin-binding EGF-like growth factor (10, 11) bind to both ErbB-1 and ErbB-4. Interestingly, the m...

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