The Erbb2 receptor is activated by UV irradiation, the primary cause of non-melanoma skin cancer. We hypothesized that Erbb2 activation contributes to UV-induced skin tumorigenesis by suppressing cell cycle arrest. Consistent with this hypothesis, inhibition of Erbb2 in v-ras(Ha) transgenic mice before UV exposure resulted in both 56% fewer skin tumors and tumors that were 70% smaller. Inhibition of the UV-induced activation of Erbb2 also resulted in milder epidermal hyperplasia, S-phase accumulation, and decreased levels of the cell cycle regulator Cdc25a, suggesting altered cell cycle regulation on inhibition of Erbb2. Further investigation using inhibition or genetic deletion of Erbb2 in vitro revealed reduced Cdc25a levels and increased S-phase arrest in UV-irradiated cells lacking Erbb2 activity. Ectopic expression of Cdc25a prevented UV-induced S-phase arrest in keratinocytes lacking Erbb2 activity, demonstrating that maintenance of Cdc25a by Erbb2 suppresses cell cycle arrest. Examination of checkpoint pathway activation upstream of Cdc25a revealed Erbb2 activation did not alter Ataxia Telangiectasia and Rad3-related/Ataxia Telangiectasia Mutated activity but increased inhibitory phosphorylation of Chk1-Ser(280). Since Akt phosphorylates Chk1-Ser(280), the effect of Erbb2 on phosphatidyl inositol-3-kinase (PI3K)/Akt signaling during UV-induced cell cycle arrest was determined. Erbb2 ablation reduced the UV-induced activation of PI3K while inhibition of PI3K/Akt increased UV-induced S-phase arrest. Thus, UV-induced Erbb2 activation increases skin tumorigenesis through inhibitory phosphorylation of Chk1, Cdc25a maintenance, and suppression of S-phase arrest via a PI3K/Akt-dependent mechanism.

Ras is frequently activated in cutaneous squamous cell carcinoma, a prevalent form of skin cancer. However, the pathways that contribute to Ras-induced transformation have not been entirely elucidated. We have previously demonstrated that in transgenic mice, overexpression of the Ras activator RasGRP1 promotes the formation of spontaneous skin tumors and enhances malignant progression in the multistage carcinogenesis skin model that relies on the oncogenic activation of H-Ras. Utilizing a RasGRP1 knockout mouse model (RasGRP1 KO), we now show that lack of RasGRP1 reduced the susceptibility to skin tumorigenesis. The dependency on RasGRP1 was associated with a diminished response to the phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Specifically, we found impairment of epidermal hyperplasia induced by TPA through keratinocyte proliferation. Using a keratinocyte cell line that carries a ras oncogenic mutation, we also demonstrated that RasGRP1 could further activate Ras in response to TPA. Thus, we propose that RasGRP1 upregulates signaling from Ras and contributes to epidermal tumorigenesis by increasing the total dosage of active Ras.

Solar ultraviolet (UV) radiation can cause severe damage to the skin and is the primary cause of most skin cancer. UV radiation causes DNA damage leading to mutations and also activates the Erbb2/HER2 receptor through indirect mechanisms involving reactive oxygen species. We hypothesized that Erbb2 activation accelerates the malignant progression of UV-induced skin cancer. Following the induction of benign squamous papillomas by UV exposure of v-ras(Ha) transgenic Tg.AC mice, mice were treated topically with the Erbb2 inhibitor AG825 and tumor progression monitored. AG825 treatment reduced tumor volume, increased tumor regression, and delayed the development of malignant squamous cell carcinoma (SCC). Progression to malignancy was associated with increased Erbb2 and ADAM12 (A Disintegin And Metalloproteinase 12) transcripts and protein, while inhibition of Erbb2 blocked the increase in ADAM12 message upon malignant progression. Similarly, human SCC and SCC cell lines had increased ADAM12 protein and transcripts when compared to normal controls. To determine whether Erbb2 up-regulation of ADAM12 contributed to malignant progression of skin cancer, Erbb2 expression was modulated in cultured SCC cells using forced over-expression or siRNA targeting, demonstrating up-regulation of ADAM12 by Erbb2. Furthermore, ADAM12 transfection or siRNA targeting revealed that ADAM12 increased both the migration and invasion of cutaneous SCC cells. Collectively, these results suggest Erbb2 up-regulation of ADAM12 as a novel mechanism contributing to the malignant progression of UV-induced skin cancer. Inhibition of Erbb2/HER2 reduced tumor burden, increased tumor regression, and delayed the progression of benign skin tumors to malignant SCC in UV-exposed mice. Inhibition of Erbb2 suppressed the increase in metalloproteinase ADAM12 expression in skin tumors, which in turn increased migration and tumor cell invasiveness.

Cell division cycle 25A (CDC25A) is a dual-specificity phosphatase that removes inhibitory phosphates from cyclin-dependent kinases, allowing cell-cycle progression. Activation of cell-cycle checkpoints following DNA damage results in the degradation of CDC25A, leading to cell-cycle arrest. Ultraviolet (UV) irradiation, which causes most skin cancer, results in both DNA damage and CDC25A degradation. We hypothesized that ablation of CDC25A in the skin would increase cell-cycle arrest following UV irradiation, allowing for improved repair of DNA damage and decreased tumorigenesis. Cdc25a(fl/fl) /Krt14-Cre recombinase mice, with decreased CDC25A in the epithelium of the skin, were generated and exposed to UV. UV-induced DNA damage, in the form of cyclopyrimidine dimers and 8-oxo-deoxyguanosine adducts, was eliminated earlier from CDC25A-deficient epidermis. Surprisingly, loss of CDC25A did not alter epidermal proliferation or cell cycle after UV exposure. However, the UV-induced apoptotic response was prolonged in CDC25A-deficient skin. Double labeling of cleaved caspase-3 and the DNA damage marker γH2A.X revealed many of the apoptotic cells in UV-exposed Cdc25a mutant skin had high levels of DNA damage. Induction of skin tumors by UV irradiation of Cdc25a mutant and control mice on a skin tumor susceptible to v-ras(Ha) Tg.AC mouse background revealed UV-induced papillomas in Cdc25a mutants were significantly smaller than in controls in the first 6 weeks following UV exposure, although there was no difference in tumor multiplicity or incidence. Thus, deletion of Cdc25a increased apoptosis and accelerated the elimination of DNA damage following UV but did not substantially alter cell-cycle regulation or tumorigenesis.