We identify a novel alternative TrkA splice variant, TrkAIII, with deletion of exons 6, 7, and 9 and functional extracellular IG-C1 and N-glycosylation domains, that exhibits expression restricted to undifferentiated early neural progenitors, human neuroblastomas (NBs), and a subset of other neural crest-derived tumors. This NGF-unresponsive isoform is oncogenic in NIH3T3 cells and promotes tumorigenic NB cell behavior in vitro and in vivo (cell survival, xenograft growth, angiogenesis) resulting from spontaneous tyrosine kinase activity and IP3K/Akt/NF-kappaB but not Ras/MAPK signaling. TrkAIII antagonizes NGF/TrkAI signaling, which is responsible for NB growth arrest and differentiation through Ras/MAPK, and its expression is promoted by hypoxia at the expense of NGF-responsive receptors, providing a mechanism for converting NGF/TrkA/Ras/MAPK antioncogenic signals to TrkAIII/IP3K/Akt/NF-kappaB tumor-promoting signals during tumor progression.

An association between elevated tyrosine kinase receptor (Trk)-A expression and better prognosis; the absence of mutation-activated TrkA oncogenes; the induction of apoptosis, growth arrest, morphological differentiation and inhibition of xenograft growth; and angiogenesis by TrkA gene transduction, provide the basis for the current concept of an exclusively tumor-suppressor role for TrkA in the aggressive pediatric tumor, neuroblastoma. This concept, however, has recently been challenged by the discovery of a novel hypoxia-regulated alternative TrkAIII splice variant, initial data for which suggest predominant expression in advanced-stage neuroblastoma. TrkAIII exhibits neuroblastoma xenograft tumor-promoting activity associated with the induction of a more angiogenic and stress-resistant neuroblastoma phenotype and antagonises nerve growth factor/TrkAI antioncogenic signaling. In this short review, the authors integrate this novel information into a modified concept that places alternative TrkA splicing as a potential pivotal regulator of neuroblastoma behavior and identifies the TrkAIII alternative splice variant as a potential biomarker of patient prognosis and novel therapeutic target.

Since its original identification as a leukocyte gelatinase/type V collagenase and tumour type IV collagenase, gelatinase B/matrix metalloproteinase (MMP)-9 is now recognised as playing a central role in many aspects of tumour progression. In this review, we relate current concepts concerning the many ways in which gelatinase B/MMP-9 influences tumour biology. Following a brief outline of the gelatinase B/MMP-9 gene and protein, we analyse the role(s) of gelatinase B/MMP-9 in different phases of the tumorigenic process, and compare the importance of gelatinase B/MMP-9 source in the carcinogenic process. What becomes apparent is the importance of inflammatory cell-derived gelatinase B/MMP-9 in tumour promotion, early progression and triggering of the “angiogenic switch”, the integral relationship between inflammatory, stromal and tumour components with respect to gelatinase B/MMP-9 production and activation, and the fundamental role for gelatinase B/MMP-9 in the formation and maintenance of tumour stem cell and metastatic niches. It is also apparent that gelatinase B/MMP-9 plays important tumour suppressing functions, producing endogenous angiogenesis inhibitors, promoting inflammatory anti-tumour activity, and inducing apoptosis. The fundamental roles of gelatinase B/MMP-9 in cancer biology underpins the need for specific therapeutic inhibitors of gelatinase B/MMP-9 function, the use of which must take into account and substitute for tumour-suppressing gelatinase B/MMP-9 activity and also limit inhibition of physiological gelatinase B/MMP-9 function.

Transforming growth factor‐beta (TGFβ1) enhances human MDA‐MB‐231 breast tumour cell invasion of reconstituted basement membrane in vitro but does not inhibit proliferation of this cell line. In contrast to basal invasion, which is plasmin‐, urokinase (uPA)‐, tissue‐type plasminogen activator (t‐PA)‐, matrix metalloproteinase (MMP)‐9‐ and TIMP‐1‐inhibitable MMP‐dependent, TGFβ1 enhanced‐invasion is dependent upon plasmin and uPA activity but does not appear to involve t‐PA‐, MMP9‐ or TIMP‐1‐inhibitable MMPs, as judged by inhibitor studies. Enhanced invasion is associated with increased u‐PA, UPAR, PAI‐1, MT‐MMP‐1, MMP‐9 and TIMP‐1 expression; with reduced t‐PA, MMP‐1 and MMP‐3 expression; and with the induction of membrane MMP‐9 association. The net result of these changes includes increased secreted, but not membrane‐associated, uPA levels and activity and reduced secreted levels of plasmin and APMA‐activatable gelatinolytic, collagenolytic and caseinolytic MMP activity but no change in membrane‐associated gelatinolytic activity, despite increased MT‐MMP‐1 expression and MMP‐9 membrane association. TGFβ1 does not induce MMP‐2 expression. Our data indicate that TGFβ1 can promote the malignant behaviour of MDA‐MB‐231 cells refractory to TGFβ1‐mediated proliferation control by enhancing their invasive capacity. We suggest that this results from the action of a uPA/plasmin‐dependent mechanism resulting from stimulation of uPA expression, secretion and subsequent activity, despite elevated PAI‐1 inhibitor levels. Int. J. Cancer 75:721–730, 1998.© 1998 Wiley‐Liss, Inc.

The hypoxia-regulated alternative TrkAIII splice variant expressed by human neuroblastomas exhibits oncogenic potential, driven by in-frame exon 6 and 7 alternative splicing, leading to omission of the receptor extracellular immunoglobulin C 1 domain and several N-glycosylation sites. Here, we show that the TrkAIII oncogene promotes genetic instability by interacting with and exhibiting catalytic activity at the centrosome. This function depends upon intracellular TrkAIII accumulation and spontaneous interphase-restricted activation, in cytoplasmic tyrosine kinase (tk) domain orientation, predominantly within structures that closely associate with the fully assembled endoplasmic reticulum intermediate compartment and Golgi network. This facilitates TrkAIII tk-mediated binding of ␥-tubulin, which is regulated by endogenous protein tyrosine phosphatases and geldanamycin-sensitive interaction with Hsp90, paving the way for TrkAIII recruitment to the centrosome. At the centrosome, TrkAIII differentially phosphorylates several centrosome-associated components, increases centrosome interaction with polo kinase 4, and decreases centrosome interaction with separase, the net results of which are centrosome amplification and increased genetic instability. The data characterize TrkAIII as a novel internal membrane-associated centrosome kinase, unveiling an important alternative mechanism to "classical" cell surface oncogenic receptor tk signaling through which stressregulated alternative TrkAIII splicing influences the oncogenic process.Alternative splicing is fundamental for differential protein expression from the same gene and not only increases the proteomic complexity of higher organisms (29) but is also involved in cancer pathogenesis, activating several oncogenes and inactivating several oncosuppressors (17).The neurotrophin receptor tropomyosin-related kinase A (TrkA) is among the proto-oncogenes activated by alternative splicing, with a novel hypoxia-regulated oncogenic alternative TrkAIII splice variant recently identified in advanced-stage human neuroblastomas (NB) and primary glioblastomas (44,45). In contrast to wild-type TrkAI/TrkAII, the expression of which is associated with better prognosis for NB, induces NB cell differentiation, exhibits a tumor suppressor function in NB models in vivo (9,19,22,30,44,45), and may regulate both spontaneous and therapy-induced NB regression (30), TrkAIII is expressed by more-advanced-stage NB and exhibits oncogenic activity in NB models (44,45). This has challenged the hypothesis of an exclusively tumor-suppressing function for TrkA in NB by providing a way through which tumor-suppressing signals from TrkA can be converted to oncogenic signals from TrkAIII during tumor progression.The oncogenic potential of TrkAIII, characterized by NIH 3T3 cell transforming and NB xenograft primary and metastatic tumor-promoting activity (44,45), is driven by in-frame alternative splicing of exons 6 and 7. This results in the omission of the receptor extracellular immunoglobulin C 1 (Ig C...