Anterior pituitary, sex hormones, and keratoconus: Beyond traditional targets

Abstract

"The Diseases of the Horny-coat of The Eye", known today as keratoconus, is a progressive, multifactorial, non-inflammatory ectatic corneal disorder that is characterized by steepening (bulging) and thinning of the cornea, irregular astigmatism, myopia, and scarring that can cause devastating vision loss. The significant socioeconomic impact of the disease is immeasurable, as patients with keratoconus can have difficulties securing certain jobs or even joining the military. Despite the introduction of corneal crosslinking and improvements in scleral contact lens designs, corneal transplants remain the main surgical intervention for treating keratoconus refractory to medical therapy and visual rehabilitation. To-date, the etiology and pathogenesis of keratoconus remains unclear. Research studies have increased exponentially over the years, highlighting the clinical significance and international interest in this disease. Hormonal imbalances have been linked to keratoconus, both clinically and experimentally, with both sexes affected. However, it is unclear how (molecular/cellular signaling) or when (age/disease stage(s)) those hormones affect the keratoconic cornea. Previous studies have categorized the human cornea as an extragonadal tissue, showing modulation of the gonadotropins, specifically luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Studies herein provide new data (both in vitro and in vivo) to further delineate the role of hormones/gonadotropins in the keratoconus pathobiology, and propose the existence of a new axis named the Hypothalamic-Pituitary-Adrenal-Corneal (HPAC) axis.

Keywords: Biomarkers; Eye; Gonadotropins; Keratoconus; Sex hormones.

Figure 1:

Raised subepithelial nodular scars may develop over the apex of the cone in some patients with KCN (left panel). ETM (middle panel) often demonstrates epithelial thinning directly underneath the scar with a concentric region of epithelial thickening around the scar. AS-OCT (left panel) highlights the extent, location and depth of the scar. These scars may preclude the use of soft toric and rigid gas permeable contact lenses. A scleral contact lens (as shown in the AS-OCT) allows for vaulting of the lens over the scar to improve visual acuity.

Figure 2:

Protein expression of DHEA-S (A), Estrone (B) and Estriol (C) before and 3-months after CXL treatment in two keratoconus patients. Estrone and Estriol show higher levels after treatment.

Figure 3:

Deep anterior keratoplasty (DALK) snapshots. (A) After partial trephination, a cannula is inserted deep in the corneal stroma; (B) Air injection through the cannula separates Descemets membrane from stroma; (C) Manual dissection ensures that recipient’s Descemets membrane is preserved; (D) Lamellar donor graft sutured in place. Source: (Karamichos et al., 2014)

Figure 4:

Penetrating keratoplasty snapshots. (A) Distorted cornea removed; (B) Corneal graft placed in recipient bed; (C) “Stay sutures” placed; (D) Single running suture in place at end of surgery. Source: (Karamichos et al., 2014)

Figure 5:

(A) Representative scheme of CIH treatment on rats. Protein levels of cFN (B), αSMA (C) and TSP1 (D) normalized to β-actin in rat corneas showing a significant increase after CIH treatment. N=4, ****p<0.0001.

Figure 6:

Representation of steroidogenesis of steroid hormones by cytochrome P450 enzyme in the (A) brain, and (B) cornea.

Figure 7:

Cytochrome P450 enzymes in corneal stromal cells from two (1 male and 1 female) healthy and two (1 male and 1 female) keratoconus donors. (A) Overall results for CYP19A1 in healthy and keratoconus corneal stromal cells. (B) Overall results for CYP20A1 in healthy and keratoconus corneal stromal cells. (C) Overall results for CYP2B6 in healthy and keratoconus corneal stromal cells. (D) Sex results for CY19A1 in healthy and keratoconus corneal stromal cells. (E) Sex results for CYP20A1 in healthy and keratoconus corneal stromal cells. (F) Sex results for CYP2B6 in healthy and keratoconus corneal stromal cells. N=3, *p<0.05, **p<0.01.

Figure 8:

The cornea expression androgen receptors, estrogen receptor alpha(a), estrogen receptor beta (b), which mediate sex hormone actions. These sex hormone receptors can have diverse cellular actions, depending on the location of the sex hormone receptor. Sex hormones are lipophilic molecules that can cross the plasma membrane of the cell and bind with intracellular/nuclear sex hormone receptor to have genomic actions. Once sex hormones binds to intracellular/nuclear receptor, the receptor dimerizes and translocates into the nucleus to bind to the hormone response element (HRE) of a gene to induce genomic transcription and translation. Sex hormones can also have non-genomic actions by binding to membrane sex hormone receptors that activate cellular signaling cascades within the cell.

Figure 9:

Representation of steroidogenesis by GnRH from the anterior pituitary acting on the gonads to release luteinizing hormone and follicle stimulating hormone.

Figure 10:

Expression of LH (A), LHR (B) and FSHR (C) in healthy (HCF) and keratoconus (HKC) corneal fibroblasts grown in vitro. *p<0.05, **p<0.01. Source: (Karamichos et al., 2019)

Figure 11:

Expression of FSHR, LHR, FSH, and LH in human corneal epithelial cells. FSHR was the only one expressed by the epithelial cells. Source: (Karamichos et al., 2019)

Figure 12:

Expression of LHR in corneal stromal cells from one healthy and one keratoconus donors. Cells were stimulated with two concentrations of FSH: 2.5 and 10 miU/mL and LH: 5 and 35 miU/mL. N=3, ^$p<0.05, ^$ $p<0.01.