To test the two opposing mechanisms of action performed by RAF inhibitors are highly dependent on cell context. RAF inhibitors selectively inhibit and induce the cell growth.
Experiment 1 and result
RAF inhibitors: GDC-0879/PLX4720
In BRAF (V600E) tumours—inhibit cell growth
In KRAS mutant and RAS/RAF WT tumours—activation of cell growth (Proliferation in a subset of BRAF-WT cell line --marked with asterisk)
MEK inhibitors (unlike RAF inhibitor)--eg. PD0325901
In BRAF (V600E) tumours, KRAS mutant and RAS/RAF WT tumours—inhibit all cell growth
Experiment 2 and result
GDC-0879 inhibits BRAF (V600E) tumour growth, but induce the growth rate of KRAS-MT lung xenografts.
Experiment 3 and result
Histopathological examination of mice treated with GDC-0879:
Hyperkeratosis and acanthosis of epidermis
Inflammation in the dermis
Controlled mice: no these symptoms.
Other types of cancer after inhibitor treatment:
Erythema grows in mice treated with chemically unrelated RAF inhibitors.
Experiment 4 and result
Immunohistochemistry of skin:
With inhibitor, staining intensity of proliferation marker Ki67 and level of phospho-ERK increased in the keratinocyte.
However,
Vehicle treated animal stays the original.
However,
Vehicle treated animal stays the original.
Experiment 5 and result
Effects on different tumour cell lines after treatment of RAF inhibitors:
Phospho-MEK and phospho-ERK level:
RAS/RAF-WT (MeWo) and KRAS-MT (H2122) cell lines—induction
BRAF (V600E) cell lines—inhibiton
Conclusion:
- In BRAF (V600E) tumour, RAF inhibitors effectively block MAPK signalling pathway and decease tumour growth.
- In KRAS mutant and RAS/RAF WT tumours, RAF inhibitors activate the MARK signalling pathway in a RAS-dependent manner, thus enhance tumour growth.
- RAF and MEK inhibitors –excellent preclinical activity in tumour models with BRAF (V600E)
(examples of RAF inhibitor as an activator)
