The Selective Regulation of αVβ1 Integrin Expression Is Based on the Hierarchical Formation of αV-containing Heterodimers

Abstract

The integrin ␤ 1 subunit can form a heterodimer with 12 different ␣ subunits. According to the present model, the expression level of any ␣␤ complex is regulated by the availability of the specific ␣ subunit, whereas ␤ 1 subunit is constantly present in a large excess. The expression of several heterodimers containing the ␣ V subunit seems to be regulated by an identical mechanism. The fact that many cells express ␣ V ␤ 1 heterodimer, and that this fibronectin/vitronectin receptor may be selectively regulated, compromises the present model of the regulation of ␤ 1 and ␣ V integrins. We have tried to solve this problem by assuming that distinct ␣␤ heterodimers are formed with different tendency. To test the hypothesis, we analyzed WM-266-4 melanoma cells transfected with a cDNA construct coding for an intracellular single-chain anti-␣ V integrin antibody. We could see 70 -80% reduction in the cell surface expression of ␣ V subunit. However, the only one of the ␣ V integrins reduced on the cell surface was ␣ V ␤ 1 . This suggests that the cell surface expression level of ␣ V ␤ 1 is dependent on the number of ␣ V subunits available after the formation of other ␣ V -containing heterodimers. Thus, there seems to be a hierarchy in the complex formation between ␣ V and its different ␤-partners. These observations explain how ␣ V ␤ 1 can be specifically regulated without concomitant changes in the expression of other ␣ V or ␤ 1 integrins.The members of the integrin family that form receptors for various extracellular matrix proteins can be divided into two major subgroups according to the subunits present in the receptors. Integrin ␤ 1 can form a complex with 12 different ␣ subunits. The ␤ 1 -containing heterodimers are receptors for collagens, laminins, tenascins, and fibronectin. The other subset, the ␣ V integrins, are fibronectin and vitronectin receptors, some of which also have the ability to bind various other matrix and plasma proteins.In many cells the two promiscuous subunits, ␣ V and ␤ 1 , can form a heterodimer with each other. ␣ V ␤ 1 was originally described as a fibronectin or vitronectin receptor (1, 2). It may also have a capacity for binding to osteopontin and to the latent form of transforming growth factor-␤ (3, 4). Some viruses, including parechovirus 1, adenovirus, and foot-and-mouth disease virus, use ␣ V ␤ 1 as their cellular receptor (5-7). The tissue distribution of ␣ V ␤ 1 is mostly unknown because of the lack of a specific antibody against ␣ V ␤ 1 complex. For the same reason the function of ␣ V ␤ 1 integrin in many human cell types is unknown, or the published information is based on cell transfections or on the use of combinations of function blocking antibodies against different ␣ V and ␤ 1 integrins. In ␣ V -transfected Chinese hamster ovary cells, ␣ V ␤ 1 integrin has been found to function as a fibronectin receptor while not supporting cell migration on fibronectin (8). On the other hand, in squamous carcinoma cells derived from head and neck tumors, ␣ V ␤ 1 integrin contributes to migr...

αV integrin promotes in vitro and in vivo survival of cells in metastatic melanoma

“…In accordance with our previous observations, ␣V␤1 heterodimer seems to be the most abundant ␣V integrin in these melanoma cells. 27 In addition, we could see some coprecipitation of ␤3 and ␤5 subunits (Fig. 2d).…”

“…Both antibodies precipitated a 150 kDa protein band corresponding to the fully glycosylated form of the ␣V subunit. 22,27,34 When mAb L230 was used, the amount of precipitated ␣V decreased by about 82% in RAda-␣V-treated cells (Fig. 2d).…”

“…RNA was isolated by Rneasy kit (Qiagen, Valencia, CA), and RT-PCR was performed by the GeneAmp RNA PCR kit (Perkin-Elmer, Boston, MA) using primers A and D as described previously. 22,27 …”

“…MAb 1978 also precipitated an extra band with an approximate molecular size of 140 kDa, corresponding to the precursor form of ␣V integrin. 22,27,34 While the higher molecular weight form represents an ␣V integrin with complex-type N-linked oligosaccharides, the lighter precursor form represents an integrin with high mannose-type oligosaccharides that is still inside the endoplasmic reticulum. 30 Interestingly, L230 anti-␣V antibody could not precipitate the lighter precursor form.…”

“…We have previously used cDNA transfection of osteosarcoma and melanoma cells to generate stable cell lines. 22,27 However, in this study we have used a recombinant adenovirus vector in gene delivery. This approach provided the means for fast transient intracellular antibody expression, which made it possible to study the cell survival.…”