Introduction
Cutaneous squamous cell carcinoma (cSCC) is the second most common cancer in the United States, behind basal cell carcinoma (BCC).1-4 cSCC is also the second most likely skin cancer to metastasize, following melanoma.1-4Globally, 2.4 million cases and 56,100 deaths were attributable to cutaneous cSCC in 2019.5 Actinic keratoses (AKs) are precancerous skin lesions that can potentially progress to SCC. AKs and cSCC represent aberrant keratinocytes, with AKs characterized by a less aggressive phenotype. AKs are the most commonly diagnosed pathology in dermatology and have an estimated prevalence of 11.5 to 26% in the United States.6,7 Treating AKs is important as progression to SCC represents a significant public health risk.1-4,8 From 2007 to 2011, the cost of treating nonmelanoma skin cancer, including SCC and BCC, totaled over 4.5 billion US dollars.8
5-Aminolevulinic acid (5-ALA) photodynamic therapy (PDT) is a Federal Drug Administration (FDA) approved treatment for AK and has been studied in the management of non-invasive SCC and BCC tumors.9-12 During PDT, 5-ALA is either applied to single lesions or to a field of skin that includes aberrant and surrounding normal cells. 5-ALA is then metabolized to photosensitive protoporphyrin IX (PP-IX).13,14 Cancerous cell types have decreased metabolization of PP-IX compared to normal cells, resulting in preferential accumulation.13-16 PP-IX absorbs blue light (~417 nm) from 1000 seconds of phototherapy, producing reactive oxygen species and subsequent cell death.14
PDT leads to the targeted destruction of cSCC cells by inducing apoptosis and necrosis.13,14,17 However, a residual population of cells may not undergo cell death.18 In these cells, 5-ALA blue light PDT may induce favorable changes in gene expression to a more benign profile. Previous research has shown that PDT treatment of AKs with methyl-aminolevulinic acid (MAL) and red light may beneficially alter the expression of cancer genes.19-22 MAL PDT has been shown to downregulate genes and pathways related to cellular proliferation while upregulating keratinocyte differentiation markers and autophagy.19-22 The present study investigates whether 5-ALA blue light PDT similarly modulates gene expression and pathways related to proliferation and cancer pathogenesis in cSCC cells that survived PDT. Differential gene expression and pathway analysis of cSCC and human dermal fibroblasts (HDFs) were compared before and after 5-ALA blue light PDT using RNA-Sequencing (RNA-Seq).