LOXL4 (lysyl oxidase-like 4) is a member of the lysyl oxidase (LOX) family. Its C-terminal region is conserved in all five members of this copper-dependent amine oxidase family and includes a copper-binding site, lysyl and tyrosine residues that form the lysyltyrosine-quinone cofactor (LTQ) and a cytokine receptor-like domain. The N-terminal region of LOXL4 contains four scavenger receptor cysteine-rich (SRCR) domains and has homology with LOXL2 and LOXL3, but not with LOX or LOXL that do not contain these domains. The second SRCR domain contains a 13 amino acid insertion that is unique to LOXL4.

The LOXL4 genomic sequence is 14.095 kb with an open reading frame of 2.278 kb and a 5 UTR of 384 bases. The TATA-box is at -25 and the TRE sequence, TGACTCA (TPA-responsive element), is at -75. The GATA domain is located at -113 and -669, and the RFX1 transactivator binding site, GGAA, is found at -149. Sp1 transcription factor consensus sequence, GGCGGC, is at -181, CCAAT (CP1-factor) at -257 and -892 and the repetitive sequence motif GAAA at -310 and 329. No GC box is present in the promoter region.

The 14 exons of the LOXL4 gene make up an mRNA of 3.877 kb with a coding region of 2.278 kb. Transcriptional start site is at +384 upstream of the translation initiation codon (ATG). The first stop codon (TGA) is at position 14.480. In the 3 UTR there is a 1230 bp untranslated trailer sequence and two consensus polyadenylation signals have been located 30 bp and 322 upstream of the poly-A tail.

Western blot analysis of HT-1080 cells detected the recombinant and secreted LOXL4 form as slightly larger than the cellular 85 kDa form probably due to glycosylation or other modifications, some of which may be cell type dependent.

The predicted LOXL4 protein is 756 amino acids including a 24 amino acid signal peptide, with a molecular mass of approximately 84.5 kDa.

In tissues the LOXL4 mRNA is expressed in the placenta, trachea, lung, kidney, pancreas, testis, aorta liver, fetal liver and at lover levels in several other tissues that include the heart, skeletal muscle, spleen, prostate, ovary, small intestine, colon, bladder, and thyroid, adrenal, salivary and mammary glands. LOXL4 mRNA was also reported in vocal cord, laryngeal, hypopharyngeal, parotid and oropharyngeal carcinoma tumor biopsies. The LOXL4 mRNA was reported in cultured fibroblasts, smooth muscle cells, MDA-MB-231 breast cancer and osteosarcoma (OHS) cells. Both mRNA and immunohistochemistry analyses demonstrated LOXL4 expression in head and neck squamous cell carcinoma (HNSSC) tissues and cultured cell lines. No mRNA expression was detected in HCT-116 and DLD-1 colon, MCF7, T47D and Hs578T breast and DU-145 prostate cancer lines.
The mouse homologue of LOXL4, reported as LOXC, was identified as a mRNA expressed in calcified ATDC5 cells, MC3T3-E1 cells, C3H10T1/2 embryonic fibroblast and myoblastic C2C12 cells. Up-regulation of the LOXL4 mRNA (EER-7) was reported 100-fold in human umbilical vein endothelial cells (HUVECs) transfected with estrogen receptor (alpha and beta) in response to treatment with 17beta-estrogen.

The recombinant LOXL4 protein in human HT-1080 fibrosarcoma cell line localized both intra- and extracellularly. In HNSCC, LOXL4 immunohistochemical staining was prevalently cytoplasmic in poorly differentiated cases with increasing perinuclear staining in well-differentiated areas. In HTB-43 pharyngeal SCC cells LOXL4 staining revealed a punctate pattern within cells with perinuclear enrichment. This pattern suggested containment of LOXL4 in the endomembrane sytem of cells at sites of synthesis and vesicular transport for secretion at the cell surface. Flow cytometry (FACS) analysis demonstrated that approximately 15% of these cells had LOXL4 localized on their surface. In contrast, LOXL4 expression was not detected (or only to a very low extent) in cultured normal epithelial cells derived from oral mucosa samples.

LOXL4 may function as an active amine oxidase, as betaAPN (beta-aminopropionitrile) inhibitable enzymatically active recombinant human LOXL4 was generated in an E. coli expression system and positively evaluated for its catalytic activity with a diamine substrate. The mouse homologue of LOXL4, LOXC, showed similar betaAPN inhibitable catalytic activity when tested using a collagen substrate.

LOXL4 has homology with the C-terminal domains to LOX, LOXL1, LOXL2 and LOXL3 and homology with the four N-terminal SRCR domains of LOXL2 and LOXL3.