C4.4A as a biomarker in pulmonary adenocarcinoma and squamous cell carcinoma

Abstract

The high prevalence and mortality of lung cancer, together with a poor 5-year survival of only approximately 15%, emphasize the need for prognostic and predictive factors to improve patient treatment. C4.4A, a member of the Ly6/uPAR family of membrane proteins, qualifies as such a potential informative biomarker in non-small cell lung cancer. Under normal physiological conditions, it is primarily expressed in suprabasal layers of stratified squamous epithelia. Consequently, it is absent from healthy bronchial and alveolar tissue, but nevertheless appears at early stages in the progression to invasive carcinomas of the lung, i.e., in bronchial hyperplasia/metaplasia and atypical adenomatous hyperplasia. In the stages leading to pulmonary squamous cell carcinoma, expression is sustained in dysplasia, carcinoma in situ and invasive carcinomas, and this pertains to the normal presence of C4.4A in squamous epithelium. In pulmonary adenocarcinomas, a fraction of cases is positive for C4.4A, which is surprising, given the origin of these carcinomas from mucin-producing and not squamous epithelium. Interestingly, this correlates with a highly compromised patient survival and a predominant solid tumor growth pattern. Circumstantial evidence suggests an inverse relationship between C4.4A and the tumor suppressor LKB1. This might provide a link to the prognostic impact of C4.4A in patients with adenocarcinomas of the lung and could potentially be exploited for predicting the efficacy of treatment targeting components of the LKB1 pathway.

Keywords: Atypical adenomatous hyperplasia; LYPD3; Liver kinase B1; Ly6/Urokinase-type plasminogen activator receptor; Metaplasia; Non-small cell lung cancer; Precursor lesions; Prognosis; Solid growth pattern; Squamous differentiation.

Figure 1

Structure and expression of the protein C4.4A. A: Schematic outline of the gene cluster encompassing multidomain members of the LU protein family, highlighting the gene encoding C4.4A (LYPD3) and its exon composition. Modified from Kriegbaum et al^[20,24], 2011 and Jacobsen et al^[20,24]; B: Cartoon of the structure of C4.4A. The insert represents the three-finger fold characteristic of LU domains (made in PyMOL, DeLano Scientific, using PDB coordinates 1NEA). Modified from Hansen et al^[19]; C and D: C4.4A expression in suprabasal layers of squamous epithelium, exemplified by tissue-engineered human epidermis (C) and the transition between the glandular and non-glandular portions of the rodent stomach (D). C4.4A is detected by a polyclonal C4.4A antibody and visualized by NovaRED chromogen. D is reproduced with permission from BestPractice Onkologi, Denmark^[89].

Figure 2

C4.4A in pulmonary squamous cell carcinoma. Panels A-E: C4.4A expression as detected by immunohistochemistry with a polyclonal antibody in normal bronchial epithelium (A), hyperplasia (B), metaplasia (C), dysplasia (D) and invasive squamous cell carcinoma (SCC) (E). A, B, D and C, E are reproduced with permission from Jacobsen et al^[34], 2012 and BestPractice Onkologi, Denmark^[89], respectively. Panel F: Kaplan-Meier curves for the survival of SCC patients, which is independent of C4.4A scores, here stratified by tertiles (red: Lowest level of C4.4A; blue: Intermediate level of C4.4A; green: Highest level of C4.4A). Modified from Jacobsen et al^[33], 2013.

Figure 3

C4.4A in pulmonary adenocarcinoma. Panels A-E: C4.4A expression as detected by immunohistochemistry with a polyclonal antibody in normal alveoli (A; reproduced with permission from Jacobsen et al[34], 2012), atypical adenomatous hyperplasia (B), invasive AC with predominant mucinous lepidic (C), non-mucinous lepidic (D), acinar (E), papillary (F) and solid (G) pattern. Panel H: Kaplan-Meier estimates for the survival of AC patients, which is correlated with C4.4A scores, here stratified by tertiles (red: Lowest level of C4.4A; blue: Intermediate level of C4.4A; green: Highest level of C4.4A). Modified from Jacobsen et al^[33], 2013.