Kallikrein

Kallikreins belong to the S1 family of serine proteases. In humans, plasma kallikrein (KLKB1) has no known paralogue, while tissue kallikreins are encoded by 15 structurally similar genes (KLKs). Human kallikreins (hKs) are widely expressed in diverse tissues and implicated in a range of normal physiologic functions, such as regulation of blood pressure, prohormone processing, electrolyte balance to tissue remodeling, neural plasticity, and skin desquamation. Several evidences show that kallikreins may be involved in cascade reactions and that cross-talk may exist with proteases of other catalytic classes. The proteolytic activity of kallikreins is regulated by zymogen activation, endogenous inhibitors, and internal cleavage. Dysregulated kallikrein expression is associated with multiple diseases.

Tissue Expression

KLK1 gene expression is highest in the pancreas, kidney, and salivary glands. KLK3 and KLK2 gene are expressed in diverse tissues but highest in the prostate. It has been found that hK3 and hK2 proteins and mRNA are in significant amounts in the female breast and at lower levels in many other tissues. KLK4 also appears to have prostatic-restricted expression, but it was demonstrated that it is also expressed in breast and other tissues. None of the remaining kallikreins is tissue-specific, although certain genes are preferentially expressed in breast, skin, central nervous system, salivary glands, etc. It is clear that there is frequent co-expression of many kallikreins in the same tissues, and this may point to a functional relationship. For example, it has been shown that hK3 and hK2 are regulated by similar mechanisms and that they are frequently co-expressed in tissues and body fluids.

Structure

The human tissue kallikrein gene locus spans a region of 261,558 bp on chromosome 19q13.4 and is formed of 15 kallikrein genes with no intervening non-kallikreins. More recently, a potential kallikrein processed pseudogene has been cloned, and the possibility exists for the presence of at least another two pseudogenes. The kallikrein genes are clustered together, and the distances between two adjacent genes range from 1.5 (KLK1 and KLK15) to 32.5 kb (KLK4 and KLK5).

Extensive analyses over the last few years have led to the identification of many common structural features of kallikreins. Some of these features are shared with other members of the S1 family of serine proteases. Other features, however, are unique to certain kallikreins. The common structural features of kallikreins can be summarized as follows:

1. All genes possess five coding exons (except for a KLK4 variant, which has four exons), and most of them have one or two extra 50 untranslated exons.
2. Exon sizes are very similar or identical.
3. The intron phases of the coding exons are conserved in all genes. The pattern of the intron phase is always I-II-I-0.
4. The positions of the residues of the catalytic triad of serine proteases are conserved, with the histidine always occurring near the end of the second coding exon, the aspartate at the middle of the third coding exon, and the serine residue at the beginning of the fifth coding exon.
5. All kallikrein proteins are predicted to be synthesized as pre-pro-peptides, with a signal peptide of 16–57 amino acids at the N terminus, followed by an activation peptide of about three to nine amino acids, followed by the enzymatically active (mature) protein (223–252 amino acids).
6. The amino acid of the substrate-binding pocket is either aspartate or glutamate, indicating trypsin-like specificity, or another amino acid, as is the case with hK3 (serine), hK7 (asparagine), and hK9 (glycine).
7. In addition to the conservation of the catalytic amino acid triad, seven additional protein motifs were also found to be highly conserved in kallikreins.
8. Most kallikrein genes are under steroid hormone regulation.
9. All proteins contain 10–12 cysteine residues, which will form five or six disulphide bonds. The positions of the cysteine residues are also fully conserved.
10. Classical or variant polyadenylation signals have been found 10–20 bases away from the poly-A tail of all kallikrein mRNAs. All three classical kallikreins have the same variant polyadenylation signal AGTAAA. Multiple alignments of all kallikrein proteins have been published previously.

Figure 1. Gene, mRNA and protein structure of kallikreins. (Kalinska M, et al. 2016)

Regulation

Several reports confirmed that many kallikreins are under steroid hormone regulation in endocrine-related tissues and cell lines. In different tissues, kallikrein genes are under hormonal regulation with a tissue-specific pattern. For example, KLK4 is upregulated by androgens in prostate and breast cancer cell lines and by estrogens in endometrial cancer cell lines. KLK12 is upregulated by androgens and progestins in prostate cancer cell lines and by estrogens and progestins in breast cancer cell lines. KLK14 and KLK15 are mainly regulated by androgens. In general, it can be concluded that most, if not all, kallikrein genes are regulated by steroid hormones, either predominantly by androgens or by estrogens/progestins/glucocorticoids.

Diseases

It has been confirmed that kallikreins are associated with the pathogenesis of many human diseases. For example, the KLK1 gene is involved in inflammation, hypertension, renal nephritis, and diabetic renal disease. The relationships between hK5 and hK7 and skin diseases have already been reported, including pathological keratinization and psoriasis. In addition, many kallikreins seem to play important physiological roles in the central nervous system (CNS). Kallikrein expression may also be involved in the pathogenesis of several skin diseases, such as psoriasis, ichthyoses, squamoproliferative, epidermal thickness, hyperkeratosis, psoriasis vulgaris, seborrheic keratosis, lichen planus, and squamous cell carcinoma. The association of kallikreins with cancer is well established. hK3 and hK2 are useful biomarkers for prostate cancer. KLK6 and KLK10 were originally isolated by differential display from breast cancer libraries. hK6 and hK10 are emerging diagnostic markers for ovarian cancer. Also, hK11 was shown to be a potential marker for ovarian and prostate cancer.

References

1. Borgoño C A, Michael I P, Diamandis E P. Human tissue kallikreins: physiologic roles and applications in cancer. [J]. Molecular Cancer Research Mcr, 2004, 2(5):257.
2. Yousef G M, Diamandis E P. The new human tissue kallikrein gene family: structure, function, and association to disease. [J]. Endocrine Reviews, 2001, 22(2):184-204.
3. Yousef G M, Obiezu C V, Luo L, et al. Human Tissue Kallikreins: From Gene Structure to Function and Clinical Applications [J]. Advances in Clinical Chemistry, 2005, 39:11-79.
4. Kalinska M, et al. Kallikreins - the melting pot of activity and function [J]. Biochimie, 2016, 122:270-282.