Chronic cadmium exposure in vitro induces cancer cell characteristics in human lung cells

Abstract

Cadmium is a known human lung carcinogen. Here, we attempt to develop an in vitro model of cadmium-induced human lung carcinogenesis by chronically exposing the peripheral lung epithelia cell line, HPL-1D, to a low level of cadmium. Cells were chronically exposed to 5 μM cadmium, a noncytotoxic level, and monitored for acquired cancer characteristics. By 20 weeks of continuous cadmium exposure, these chronic cadmium treated lung (CCT-LC) cells showed marked increases in secreted MMP-2 activity (3.5-fold), invasion (3.4-fold), and colony formation in soft agar (2-fold). CCT-LC cells were hyperproliferative, grew well in serum-free media, and overexpressed cyclin D1. The CCT-LC cells also showed decreased expression of the tumor suppressor genes p16 and SLC38A3 at the protein levels. Also consistent with an acquired cancer cell phenotype, CCT-LC cells showed increased expression of the oncoproteins K-RAS and N-RAS as well as the epithelial-to-mesenchymal transition marker protein Vimentin. Metallothionein (MT) expression is increased by cadmium, and is typically overexpressed in human lung cancers. The major MT isoforms, MT-1A and MT-2A were elevated in CCT-LC cells. Oxidant adaptive response genes HO-1 and HIF-1A were also activated in CCT-LC cells. Expression of the metal transport genes ZNT-1, ZNT-5, and ZIP-8 increased in CCT-LC cells culminating in reduced cadmium accumulation, suggesting adaptation to the metal. Overall, these data suggest that exposure of human lung epithelial cells to cadmium causes acquisition of cancer cell characteristics. Furthermore, transformation occurs despite the cell’s ability to adapt to chronic cadmium exposure.

A Review of Molecular Events of Cadmium-Induced Carcinogenesis

“…The overexpression of major metallothionein (MT) isoforms is associated with both Cd-induced and lung cancers (85, 86). Specifically, MT-1A and MT-2A levels were shown to be significantly higher in CCT-LC as compared to control (78), suggesting that MT may increase its expression in the lung to provide protection from Cd-induced toxicity.…”

“…Chronic inhalation of Cd causes pulmonary adenocarcinomas (20, 77) in animal and in exposed human cell lines (78). Prolonged exposure of normal bronchial epithelial cells to Cd in vitro causes them to transform to a malignant state, and a similar observation was noted in a nude mouse model (79).…”

“…Although the mechanisms of Cd-induced pulmonary carcinogenesis are still incompletely defined, the development of specific cell lines has greatly aided in defining the mode of Cd-mediated lung cancer development. Cd-induced, transformed human lung epithelial cells called chronic cadmium treated lung cells (CCT-LC) exhibit increased MMP-2 activity, colony formation, invasion, autonomous growth, EMT, and hyperproliferation (78). Malignant transformed lung cells as well displayed altered pro-survival, apoptotic and DNA regulatory gene expression following Cd treatment (Table 3).…”

“…Oxidative stress conditions are induced by antioxidative proteins Heme Oxygenase-1 (HO-1) and hypoxia inducible factor-1A (HIF-1α) (78). HIF-1 expression also occurs through Cd-induced reactive oxygen species formation associated with transformation of bronchial epithelial cells (87).…”

2D-DIGE and MALDI TOF/TOF MS analysis reveal that small GTPase signaling pathways may play an important role in cadmium-induced colon cell malignant transformation

“…Cadmium accumulation in the environment is a worldwide concern due to its extremely long half-life (Filipic et al, 2006;Garcia-Esquinas et al, 2014). Recent population-based studies reveal that environmental exposure to cadmium increases the risk of cancer (Garcia-Esquinas et al, 2014;Nawrot et al, 2006;Person et al, 2013;Luckett et al, 2012;Akesson et al, 2008;Stayner et al, 1992). Cadmium affects cell proliferation, differentiation, apoptosis and other cellular activities and causes numerous molecular lesions relevant to carcinogenesis (Zhang et al, 2010;Son et al, 2010Son et al, , 2012.…”