Lysyl oxidase initiates the enzymatic stage of collagen and elastin cross-linking. Among five isoforms comprising the lysyl oxidase family, LOX is the better studied. LOX is associated to an antitumor activity in ras-transformed fibroblasts, and its expression is down-regulated in many carcinomas. The aim of this work was to shed light on LOX functions within the epidermis by studying its expression in human basal and squamous cell carcinomas and analyzing the effect of its enzymatic activity inhibition and protein absence on human keratinocytes behavior in a skin equivalent. In both carcinomas, LOX expression by epidermal tumor cells was lacking, while it was up-regulated around invading tumor cells in association with the stromal reaction. Lysyl oxidase activity inhibition using h-aminoproprionitrile in a skin equivalent model prepared with both primary human keratinocytes and HaCaT cell line affected keratin 10 and filaggrin expression and disorganized the collagen network and the basement membrane. In spite of all these changes, no invasion phenotype was observed. Modelization of the invasive phenotype was only noticed in the skin equivalent developed with LOX antisense HaCaT cell line, where the protein LOX is specifically absent. Our results clearly indicate that lysyl oxidase enzymatic activity is essential not only for the integrity maintenance of the dermis but also for the homeostasis of the epidermis. Moreover, LOX protein plays a role in the skin carcinomas and invasion but not through its enzymatic activity.Lysyl oxidase gene family comprises five members acting as extracellular modulating enzymes: LOX, LOXL, LOXL2, LOXL3, and LOXL4 (1). The first identified and the better-studied isoform of this family is LOX. It is a copper-dependent amine oxidase that initiates the covalent cross-linking of collagens and elastin in extracellular matrices (ECM; refs. 2,3). LOX is secreted as a glycosylated proenzyme, processed by procollagen C-proteinase into a mature active form. LOX activity modulation, due to either an increase or a decrease in its expression, induces multiple effects on the structure and major characteristics of the ECM. LOX is essential in maintaining the characteristics of blood vessels and arteries, where its activity modulation is correlated to atherosclerosis (4), aneurism (5), and human arterial dissection (6) as well as to Alzheimer disease (7). LOX down-regulation is correlated to many connective tissue disorders seen in Ehler-Danlos syndrome, cutis laxa, and Menke's syndrome (8 -10). In tumors, LOX upregulation is found in the stromal reaction observed around tumor foci in ductal breast carcinomas (11) and in bronchopulmonary carcinomas (12).LOX might also have a role on nonmesenchymal cells. Indeed, LOX expression has been recently associated to differentiated keratinocytes in a skin equivalent model (13). Besides, LOX, which is expressed in healthy skin (13, 14), has not been detected in tumor cells from invading ductal breast carcinomas (11) in primary and metastatic prost...