RAF inhibitor resistance is mediated by dimerization of aberrantly spliced BRAF(V600E)

Activated RAS promotes dimerization of members of the RAF kinase family. ATP-competitive RAF inhibitors activate ERK signalling by transactivating RAF dimers. In melanomas with mutant BRAF(V600E), levels of RAS activation are low and these drugs bind to BRAF(V600E) monomers and inhibit their activity. This tumour-specific inhibition of ERK signalling results in a broad therapeutic index and RAF inhibitors have remarkable clinical activity in patients with melanomas that harbour mutant BRAF(V600E). However, resistance invariably develops. Here, we identify a new resistance mechanism. We find that a subset of cells resistant to vemurafenib (PLX4032, RG7204) express a 61-kDa variant form of BRAF(V600E), p61BRAF(V600E), which lacks exons 4-8, a region that encompasses the RAS-binding domain. p61BRAF(V600E) shows enhanced dimerization in cells with low levels of RAS activation, as compared to full-length BRAF(V600E). In cells in which p61BRAF(V600E) is expressed endogenously or ectopically, ERK signalling is resistant to the RAF inhibitor. Moreover, a mutation that abolishes the dimerization of p61BRAF(V600E) restores its sensitivity to vemurafenib. Finally, we identified BRAF(V600E) splicing variants lacking the RAS-binding domain in the tumours of six of nineteen patients with acquired resistance to vemurafenib. These data support the model that inhibition of ERK signalling by RAF inhibitors is dependent on levels of RAS-GTP too low to support RAF dimerization and identify a novel mechanism of acquired resistance in patients: expression of splicing isoforms of BRAF(V600E) that dimerize in a RAS-independent manner.     

Latent TGF-β structure and activation

Transforming growth factor (TGF)-β is stored in the extracellular matrix as a latent complex with its prodomain. Activation of TGF-β1 requires the binding of α(v) integrin to an RGD sequence in the prodomain and exertion of force on this domain, which is held in the extracellular matrix by latent TGF-β binding proteins. Crystals of dimeric porcine proTGF-β1 reveal a ring-shaped complex, a novel fold for the prodomain, and show how the prodomain shields the growth factor from recognition by receptors and alters its conformation. Complex formation between α(v)β(6) integrin and the prodomain is insufficient for TGF-β1 release. Force-dependent activation requires unfastening of a 'straitjacket' that encircles each growth-factor monomer at a position that can be locked by a disulphide bond. Sequences of all 33 TGF-β family members indicate a similar prodomain fold. The structure provides insights into the regulation of a family of growth and differentiation factors of fundamental importance in morphogenesis and homeostasis.     

Synchronicity of Antarctic temperatures and local solar insolation on orbital timescales

The Milankovitch theory states that global climate variability on orbital timescales from tens to hundreds of thousands of years is dominated by the summer insolation at high northern latitudes. The supporting evidence includes reconstructed air temperatures in Antarctica that are nearly in phase with boreal summer insolation and out of phase with local summer insolation. Antarctic climate is therefore thought to be driven by northern summer insolation. A clear mechanism that links the two hemispheres on orbital timescales is, however, missing. We propose that key Antarctic temperature records derived from ice cores are biased towards austral winter because of a seasonal cycle in snow accumulation. Using present-day estimates of this bias in the 'recorder' system, here we show that the local insolation can explain the orbital component of the temperature record without having to invoke a link to the Northern Hemisphere. Therefore, the Antarctic ice-core-derived temperature record, one of the best-dated records of the late Pleistocene temperature evolution, cannot be used to support or contradict the Milankovitch hypothesis that global climate changes are driven by Northern Hemisphere summer insolation variations.     

Functional complementation between FADD and RIP1 in embryos and lymphocytes

FADD is a common adaptor shared by several death receptors for signalling apoptosis through recruitment and activation of caspase 8 (refs 1-3). Death receptors are essential for immune homeostasis, but dispensable during embryogenesis. Surprisingly, Fadd(-/-) mice die in utero and conditional deletion of FADD leads to impaired lymphocyte proliferation. How FADD regulates embryogenesis and lymphocyte responses has been a long-standing enigma. FADD could directly bind to RIP1 (also known as RIPK1), a serine/threonine kinase that mediates both necrosis and NF-κB activation. Here we show that Fadd(-/-) embryos contain raised levels of RIP1 and exhibit massive necrosis. To investigate a potential in vivo functional interaction between RIP1 and FADD, null alleles of RIP1 were crossed into Fadd(-/-) mice. Notably, RIP1 deficiency allowed normal embryogenesis of Fadd(-/-) mice. Conversely, the developmental defect of Rip1(-/-) lymphocytes was partially corrected by FADD deletion. Furthermore, RIP1 deficiency fully restored normal proliferation in Fadd(-/-) T cells but not in Fadd(-/-) B cells. Fadd(-/-)Rip1(-/-) double-knockout T cells are resistant to death induced by Fas or TNF-α and show reduced NF-κB activity. Therefore, our data demonstrate an unexpected cell-type-specific interplay between FADD and RIP1, which is critical for the regulation of apoptosis and necrosis during embryogenesis and lymphocyte function.     

American swallow bug, Oeciacus vicarius Horvath (Hemiptera: Cimicidae), in Hirundo rustica and Petrochelidon pyrrhonota nests in west central Colorado

Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Oeciacus vicarious bed bugs were collected from 32% of Hirundo rustica nests and 83% of Petrochelidon pyrrhonota nests on bridges in western Colorado in December 1984. A total of 409 bugs (158 adults and 251 juveniles) were counted in 47 nests, two months after the hosts had departed for the winter.     

First record of pygmy rabbits ( Brachylagus idahoensis ) in Wyoming

Pygmy rabbits are reported for the first time in southwestern Wyoming. The range for this species is thus extended 240 km and 145 km from the nearest records in Idaho and Utah, respectively.