Pak1 regulates multiple c-Kit mediated Ras-MAPK gain-in-function phenotypes in Nf1+/- mast cells

Abstract

Neurofibromatosis type 1 (NF1) is a common genetic disorder caused by mutations in the NF1 locus, which encodes neurofibromin, a negative regulator of Ras. Patients with NF1 develop numerous neurofibromas, which contain many inflammatory mast cells that contribute to tumor formation. Subsequent to c-Kit stimulation, signaling from Ras to Rac1/2 to the MAPK pathway appears to be responsible for multiple hyperactive mast cell phenotypes; however, the specific effectors that mediate these functions remain uncertain. p21-activated kinase 1 (Pak1) is a downstream mediator of Rac1/2 that has been implicated as a positive regulator of MAPK pathway members and is a modulator of cell growth and cytoskeletal dynamics. Using an intercross of Pak 1(-/-) mice with Nf1(+/-) mice, we determined that Pak1 regulates hyperactive Ras-dependent proliferation via a Pak1/Erk pathway, whereas a Pak1/p38 pathway is required for the increased migration in Nf1(+/-) mast cells. Furthermore, we confirmed that loss of Pak1 corrects the dermal accumulation of Nf1(+/-) mast cells in vivo to levels found in wild-type mice. Thus, Pak1 is a novel mast cell mediator that functions as a key node in the MAPK signaling network and potential therapeutic target in NF1 patients.

Figure 1

Loss of Pak1 does not affect expression of important mast cell maturation markers. Mast cells were cultured for 4 weeks, and expression of c-kit and FcϵRI was measured by incubation with anti-IgE followed by FITC-conjugated anti–mouse IgG, as well as PE-conjugated anti–c-kit antibodies. Double-positive cells (upper right quadrant) are mature mast cells, expressing both c-kit and FcϵRI. Data shown are representative of 6 independent lines from each genotype.

Figure 2

A Pak/MAPK pathway regulates Nf1 haploinsufficient mast cell hyperproliferation. (A) Mast cells were starved overnight in RPMI and plated in a 24-well plate at 3 × 10⁵ per well in triplicate samples after treatment with DMSO (control; ■), 10 μM of selective p38 inhibitor SB203580 (), or 10 μM of selective Mek1 inhibitor PD98059 (□). Cells were then stimulated with 25 ng of SCF for 72 hours, and viable cells were measured by trypan blue exclusion. Results are expressed as percentage of input number of cells at 72 hours after stimulation. Each value represents the mean, and the error bars represent the SEM of 6 independent experiments. *P < .05 compared with WT control. **P < .05 compared with Nf1^+/− control. #P < .05 compared with DMSO treated cells within a genotype using Student unpaired t test. (B-D) Mast cells were serum starved overnight, stimulated with SCF, and cell lysates isolated at 0 and 2 minutes after stimulation. A total of 100 μg of protein was used for each time point. Levels of active Erk1 (B) and Mek1 (C,D) were determined by Western blotting using phospho-specific antibodies. Levels of total Erk1 and Mek1 are shown as loading controls. Western blot of the results is shown and is a representative of 3 independent experiments. Vertical lines in panel D have been inserted to indicate repositioned gel lanes for consistency with other blots.

Figure 3

Increased migration of Nf1^+/− mast cells is mediated through a Pak/p38 pathway. (A) Mast cells were starved overnight in RPMI without serum and plated in the upper well of a transwell chamber at 10⁵ per well in triplicate samples after treatment with DMSO (control; ■), 10 μM of selective p38 inhibitor SB203580 (), or 10 μM of selective Mek1 inhibitor PD98059 (□). Cells were then stimulated with 25 ng of SCF in the lower chamber for 4 hours, and mast cells that had migrated to the bottom surface of the CH296-coated membrane in response to SCF were counted after staining the cells with crystal violet. Results are expressed as cells per 20× high-power field. Each value represents the mean; error bars represent the SEM of 6 independent experiments. *P < .05 compared with WT control. **P < .05 compared with Nf1^+/− control. #P < .05 compared with DMSO-treated cells within a genotype using Student unpaired t test. (B) Mast cells were serum starved overnight, stimulated with SCF, and cell lysates isolated at 0 and 5 minutes after stimulation. A total of 100 μg of protein was used for each time point. Levels of active p38 were determined by Western blotting using phospho-specific antibodies. Level of total p38 is shown as a loading control. Western blot of the results is shown and is representative of 3 independent experiments.

Figure 4

Pak1 and p38 cooperate to regulate activation and organization of the F-actin cytoskeleton. Mast cells were starved overnight in RPMI and plated in the upper well of a transwell chamber at 10⁵ per well in triplicate samples after treatment with DMSO or 10 μM of selective p38 inhibitor SB203580. Cells were then stimulated with 25 ng of SCF in the lower chamber for 30 minutes, and mast cells were removed from the upper chamber for phalloidin staining of the F-actin cytoskeleton. (A-L) Representative micrographs of phalloidin-stained mast cells analyzed with the Zeiss UV LSM-510 confocal microscope system equipped with a UV Argon laser (351, 364 nm excitation), a visible Argon laser (458, 488 nm excitation) and two Helium-Neon lasers (543, 633 nm excitation). The microscope was equipped with 4 epifluorescence detectors and 1 transillumination detector. The system was mounted on a Zeiss Axiovert 100 inverted microscope and software for image analysis was Zeiss LSM browser R 4.0 (all Carl Zeiss, Thornwood, NY). Green indicates phalloidin stain; blue, DAPI nuclear stain. Original magnification ×400. (M) Fluorescence intensity of phalloidin-stained mast cells, determined by fluorescence cytometry. Data are expressed as fold increases over WT levels; each value represents the mean, and error bars represent the SEM of 6 independent experiments. *P < .05 compared with WT control. **P < .05 compared with Nf1^+/− control. #P < .05 compared with DMSO-treated cells within a genotype using Student unpaired t test.

Figure 5

Effect of genetic inactivation of Pak1 on accumulation of cutaneous Nf1^+/− mast cells in response to local administration of SCF in vivo. SCF was delivered in vivo via a micro-osmotic pump on the middorsum at 10 μg/kg per day. Skin sections at the site of SCF administration were fixed and stained with hematoxylin and eosin to assess routine histology along with Giemsa to identify mast cells. (A) Cutaneous mast cells were quantitated in a blinded fashion by counting 2-mm² sections. (B) The percentage of degranulating mast cells present per 2-mm² section was calculated. Representative sections are displayed in panels C to F. Resting mast cells in panels C to F are marked with ■; degranulating mast cells in panels C to F are marked with an open arrow. Values in panels A and B represent the mean of 3 independent experiments each using 3 mice per genotype, and error bars represent SEM. *P < .05 compared with WT control. **P < .05 compared with Nf1^+/− control using Student unpaired t test. Images in panels C through F were obtained using a Nikon Eclipse 80i microscope (Tokyo, Japan) using a 10×/0.30 DIC L/N1 magnification and lens in an air medium with a QCapture 2.90.1 camera (QImaging, Surrey, BC).

Figure 6

Schematic representation of hypothesized pathways explaining how Nf1 haploinsufficient-associated increases in proliferation and migration are dependent on Pak1 signaling. Hatched arrows represent potential downstream effectors of Pak1.