The authors generated doxycycline (DOX)-inducible transgenic mice to express a constitutively active form of MEK1 (HA-CA-MEK1) under control of the myeloid-selective MRP8 promoter. Two independent transgenic lines, referred to as line A and line B, were produced by co-injecting hMRP8/pTet-On and HA-CA-MEK1/pTRE-Tight constructs into C57BL/6 oocytes. DOX treatment was used to induce CA-MEK1 expression in adult mice and tissues were harvested for biochemical and histologic analyses.
Following DOX administration, both transgenic lines exhibited induction of the HA-tagged CA-MEK1 transgene. Immunoblotting of tissue lysates demonstrated prominent induction in certain tissues. The experimental design allowed temporal control of CA-MEK1 expression to assess short-term (days to weeks) and longer-term (weeks to months) effects on epithelial and hematopoietic compartments.
Unexpectedly, both transgenic lines developed distinct epithelial abnormalities after DOX induction. Line A mice showed pervasive skin and epithelial thickening following DOX exposure. Line B mice developed papillomas, with lesions visible on the ear and tail after as little as two weeks of DOX treatment. Representative macroscopic images and hematoxylin & eosin (H&E) staining were used to document these phenotypes.
Biochemical fractionation and immunoblotting revealed that CA-MEK1 induction was prominent in the epidermal compartment but not in the dermis. Epidermis-enriched samples from DOX-treated transgenic mice showed detectable HA-CA-MEK1 by immunoblot, consistent with the localization of the epithelial phenotype. This compartment-specific expression correlated with the observed epithelial hyperplasia and papilloma formation.
In line B mice, DOX induction produced measurable CA-MEK1 expression and associated ERK1/2 phosphorylation in bone marrow and in circulating blood cells. Immunoblots from bone marrow and blood samples detected HA-CA-MEK1 and increased phospho-ERK1/2, indicating activation of the MEK/ERK signaling cascade in hematopoietic tissues in that transgenic line.
Despite evidence of MEK/ERK activation in bone marrow and blood in line B mice, analysis of peripheral white blood cell composition over an induction interval of 8–33 weeks did not reveal changes in the relative percentages of lymphocytes, monocytes, or granulocytes. The authors report that induction of CA-MEK1 over this range of durations failed to alter the frequency of monocytes and granulocytes in blood, indicating that MEK/ERK hyperactivation alone did not perturb myeloid differentiation or produce a myeloproliferative phenotype in these animals.
Histologic examination of skin and papilloma specimens demonstrated epidermal thickening and cellular hyperplasia in DOX-treated transgenic mice compared with controls. H&E-stained sections from back skin, tail, and ear tissues showed clear differences between untreated control samples and DOX-treated transgenic tissues, supporting the conclusion that epithelial MEK/ERK activation drives hyperplastic responses in vivo.
The findings indicate that the human MRP8 promoter used in these constructs is active not only in myeloid compartments but also in epithelial tissues, particularly epidermis, in the generated transgenic lines. Epithelial expression of CA-MEK1 and resultant ERK activation produced marked epithelial hyperplasia and papilloma formation, confirming previously reported links between MEK/ERK hyperactivation and epithelial proliferation.
In contrast, activation of MEK/ERK signaling by CA-MEK1 in hematopoietic compartments was not sufficient, by itself, to alter steady-state myeloid differentiation or to initiate leukemia within the observed time frames. The authors propose that MEK/ERK hyperactivation likely promotes proliferation or survival in leukemias where differentiation is already blocked by other cooperating genetic or epigenetic events, rather than acting as a sole initiating driver of myeloid transformation.
This transgenic study demonstrates that DOX-inducible, MRP8-driven expression of constitutively active MEK1 causes prominent epithelial hyperplasia and papilloma formation in mice, attributable to epidermal CA-MEK1 expression and ERK activation. Although MEK/ERK signaling was activated in bone marrow and blood in one transgenic line, prolonged CA-MEK1 induction did not change peripheral monocyte or granulocyte frequencies nor produce myeloproliferative disease under the reported conditions. The results support a model in which MEK/ERK hyperactivation can enhance proliferation or survival but is insufficient on its own to disrupt myeloid differentiation or initiate leukemia without additional cooperating lesions.
Note: Specific experimental details such as quantitative measures, exact DOX dosing, animal numbers per group, and statistical values beyond those summarized here were reported in the original article and are not repeated in full in this summary.