With regard to mammalian STCs, two analogous proteins, STC-1 [6] and [7] and STC-2 [8] and [9], have been recently isolated in humans and rodents. There are differences between these two proteins regarding molecular size, distribution and function [10]. STC-1 mimics the fish hormone in mineral homeostasis. Human STC-1 is 247 amino acids long, and shares 73% amino
10-Deacetylbaccatin sequence similarity with fish STC [7]. Functional studies with recombinant human STC (rhSTC) have demonstrated that STC-1 reduces renal phosphate excretion rates in rats [7] and [11], and increases phosphate reabsorption in rodent and swine intestinal preparations [12]. Mammalian STC-1 is expressed in a number of different tissues, but is not detectable in circulation under normal conditions [13]. STC-1 may play a paracrine/autocrine role rather than the classic endocrine role in mammals. Many studies using mammalian tissues or cell lines revealed that STC-1 might act on local cellular calcium/phosphate metabolism without affecting systemic homeostasis [14]. In skeletal system STC-1 stimulates osteoblast differentiation [15]. In neural system STC-1 is involved in the resistance to ischemic damage [16]. In cardiomyocytes STC-1 prevents hypercontracture through the regulation of cellular calcium content [17]. Influx of calcium is a common initiator of terminal cell damage. One of the diverse functions of mammalian STC-1 may be to guard the integrity of differentiated
cells through the regulation of local calcium/phosphate homeostasis [18].
