Assessment of the Role of the JAK-STAT Pathway in the Pathogenesis of Male Androgenetic Alopecia

Background and rationale Androgenetic alopecia occurs in men and women,and is characterised by the loss of hair from the scalp in a defined pattern. Determining factors appear to be genetic predisposition coupled with the presence of sufficient circulating androgen.

The transformation of testosterone into dihydrotestosterone(DHT) by type 2 5-alpha reductase, which causes hair miniaturization,is universally accepted as the main player in the disease's pathogenesis. Nonetheless,how DHT causes hair thinning is not well understood. New studies revealed that a lymphocytic microfolliculitis targeting the bulge epithelium along with deposits of epithelial basement membrane zone immunoreactants are frequent findings in androgenetic alopecia and could point toward an immunologically driven trigger.

Tyrosine kinases (TKs) are enzymes involved in intracellular signaling that catalyze the phosphorylation of tyrosine residues on protein substrates. They are key components of signaling pathways that drive any array of cellular responses including proliferation, differentiation, migration and survival. Janus kinases (JAKs) are specific TKs.

Signal transducer and activator of transcription (STAT) proteins are transcription factorsprimarily phosphorylated and activated by JAKs.The JAK-STAT pathway is utilized by cytokines including interleukins (ILs), interferons (IFNs), and other molecules to transmit signals from the cell membrane to the nucleus. Growing evidence suggests that JAK inhibitors are efficacious in atopic dermatitis, alopecia areata, psoriasis and vitiligo.

It is a well known fact that the JAK-STAT pathway plays a pivotal role in the pathogenesis of alopecia areata. Both phosphorylated STAT 1 and 3 have been found to be upregulated in the disease. However, whether this pathway plays a role in other hair loss disorders remains unclear. A study showedthat topical treatment of mouse and human skin with small molecule inhibitors of the JAK-STATpathway resulted in rapid onset of anagen and subsequent hair growth. It was shown that JAK inhibition regulates the activation of key hair follicle populations such as the hair germ. These findings indicate that the JAK-STAT pathway may be involved, not only in immune-mediated hair loss (alopecia areata), but also in the normal hair cycle.