beta-Carotene promotes the development of NNK-induced small airway-derived lung adenocarcinoma

Abstract

Aim: beta-Carotene has shown cancer-preventive effects in preclinical studies while increasing lung cancer mortality in clinical trials. We have shown that beta-carotene stimulates cAMP signalling in vitro. Here, we have tested the hypothesis that beta-carotene promotes the development of pulmonary adenocarcinoma (PAC) in vivo via cAMP signalling.

Methods: PAC was induced in hamsters with the carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), followed by beta-carotene for 1.5 years. Incidence, multiplicity and size of lung tumours were recorded, and phosphorylated CREB and ERK1/2 in tumour cells were determined by Western blots. Cyclic AMP in blood cells was analysed by immunoassays, retinoids in serum and lungs by HPLC.

Results: beta-Carotene increased lung tumour multiplicity, lung tumour size, blood cell cAMP, serum and lung levels of retinoids and induced p-CREB and p-ERK1/2 in lung tumours.

Conclusions: Our data suggest that beta-carotene promotes the development of PAC via increased cAMP signalling.

Figure 1

Figure 1A. Modulation of retinoid levels in serum and lung tissue. Retinyl palmitate and ATRA were the predominating metabolites and their increase over controls was highly significant (p<0.001). Mean values and standard errors of samples from 5 hamsters per group expressed as fold increase over controls. Figure 1B. Modulation of systemic cAMP levels in blood cells. NNK significantly (p<0.001) increased systemic cAMP levels over controls. Animals treated with NNK+β-carotene showed an additional significant (p<0.001) increase in systemic cAMP. Mean values and standard errors from 5 hamsters/group expressed as fold increase over controls.

Figure 2

Tumor incidences in lungs, pancreas and nasal cavity (A) and lung tumor multiplicity (B). Beta-carotene significantly (p<0.001) increased lung tumor multiplicity. Data in both graphs are from 20 animals per group.

Figure 3

Modulation of NNK-induced lung tumor sizes by β-carotene. The graph in B shows the results of lung tumor area measurements. The lung tumors in animals treated with NNK+β-carotene were significantly (p<0.001) larger than in the hamsters treated with NNK alone.

Figure 4

Histopathology of lung tumor induced in a hamster by NNK alone (A) or by NNK+β-carotene (B). Hematoxylin/eosin stain; X 40.

Figure 5

Western blots (A) and densitometry values (B) showing levels of p-ERK1/2 and p-CREB in small airway epithelial cells and lung tumor cells. Protein induction by NNK (p<0.001) was significantly (p<0.001) increased by β-carotene. Bars in the graph are mean values and standard errors of five densitometric measurements per band expressed as ratio of p-ERK1/2 or p-CREB over ERK1/2.