Kindlin-1 and -2 Have Overlapping Functions in Epithelial Cells

Abstract

Kindlins are a novel family of intracellular adaptor proteins in integrin-containing focal adhesions. Kindlin-1 and -2 are expressed in the skin, but whether and how they cooperate in adult epithelial cells have remained elusive. We uncovered the overlapping roles of kindlin-1 and -2 in maintaining epithelial integrity and show that the phenotype of kindlin-1-deficient cells can be modulated by regulating kindlin-2 gene expression and vice versa. The experimental evidence is provided by use of human keratinocyte cell lines that express both kindlins, just kindlin-1 or kindlin-2, or none of them. Double deficiency of kindlin-1 and -2 had significant negative effects on focal adhesion formation and actin cytoskeleton organization, cell adhesion, survival, directional migration, and activation of ␤ 1 integrin, whereas deficiency of one kindlin only showed variable perturbation of these functions. Cell motility and formation of cell-cell contacts were particularly affected by lack of kindlin-2. These results predict that kindlin-1 and -2 can functionally compensate for each other, at least in part. The high physiologic and pathologic significance of the compensation was emphasized by the discovery of environmental regulation of kindlin-2 expression. UV-B irradiation induced loss of kindlin-2 in keratinocytes. This first example of environmental regulation of kindlin expression has implications for phenotype modulation in Kindler syndrome, a skin disorder caused by kindlin-1 deficiency.

Uncovering functional differences between kindlin-1 and kindlin-2 in keratinocytes

“…Likewise, kindlin-1-KO mice display reduced keratinocyte proliferation and some skin atrophy , although they do not develop skin blistering, and their basement membrane is normal, suggesting that kindlin-2 partially compensates in the kindlin-1-KO skin. Consistent with this, both kindlin-1 and kindlin-2 localize to the focal adhesions in keratinocytes and have overlapping functions in human keratinocytes (He et al, 2011a). Nonetheless, the fact that endogenous kindlin-2 is unable to fully compensate for kindlin-1 deficiency in human disease or mouse knockouts, suggests that kindlins might have functionally significant, as yet uncharacterized, differences in their interaction partners.…”

“…Keratinocytes express both kindlin-1 and kindlin-2. These proteins are highly conserved, exhibit overlapping functions and can, at least in part, functionally compensation for each other (He et al, 2011a). Here, using b1-integrin-deficient keratinocytes that adhere to fibronectin through avb6 integrins, we have uncovered functional differences between kindlin-1 and kindlin-2, demonstrated kindlin-1 binding to b6 integrins, revealed specificity in integrin b-tail binding of kindlins, and shown the importance of kindlin-2 targeting to focal adhesions for cell spreading.…”

Kindlin-1 regulates mitotic spindle formation by interacting with integrins and Plk-1

“…Because of their similarity, Kin-1 and -2 likely share at least some redundant roles 11,12 ; hence, we studied the cellular role of Kin-1 in MDA-MB-231 breast cancer cells as they express readily detectable levels of Kin-1 and no obvious Kin-2. This was used as the breast cancer cell line of choice as other breast cancer cell lines express both isoforms (Supplementary Fig.…”

Focal adhesion complex proteins in epidermis and squamous cell carcinoma

“…88 Kindlin-1 and Kindlin-2 were shown to have some overlapping functions in skin; however, loss of Kindlin-2 alone in cultured keratinocytes led to reduction of cell motility and formation of cell-cell adhesions. 89 Irradiation of keratinocytes led to loss of Kindlin-2 protein expression, which could explain why patients with KS experience enhanced UV sensitivity. 89 The function of Kindlin-1 or -2 has not been directly assessed in mouse models of SCC; however, there is a higher incidence of skin cancer in patients affected with KS.…”

“…89 Irradiation of keratinocytes led to loss of Kindlin-2 protein expression, which could explain why patients with KS experience enhanced UV sensitivity. 89 The function of Kindlin-1 or -2 has not been directly assessed in mouse models of SCC; however, there is a higher incidence of skin cancer in patients affected with KS. 90 Given the relative fitness disadvantage of nontumorigenic Kindlin-1 mutant cells, this increased cancer susceptibility seems somewhat surprising.…”